Geometrics requires a Return Material Authorization / Case # included with your shipment back to our office in San Jose, California, USA. You can acquire an RMA / CASE# by going to our RMA Generator Webpage.
You will then receive an email reply with the RMA/CASE# and shipping instructions. You must have the serial number of the instrument to complete the request.
You will be redirected here after a successful RMA request.
Our cesium-vapor magnetometers do not require periodic calibration in order to maintain the accuracy as described in our published specification when the instrument is operated within specified environmental ranges. Geometrics cesium-vapor magnetometers are manufactured and tested based on the discoveries and the basic designs of Carian Associates (U.S. Patent 3,071,721). This method of total magnetic field measurement relies upon the measurement of the optical absorption of a particular cesium spectral frequency by the cesium vapor enclosed in a small glass cell. This method is similar to those used in the measurement of atomic emission and absorption frequencies using spectroscopic reference cells. The technique thus relies on well-known fundamental quantum mechanical constants for accurate and precise measurement of the magnetic field. As a result, no adjustments to the sensor are needed in order to correct or maintain its accuracy and Geometrics sensor and sensor driver electronics are designed to either work correctly or to not work at all and to report both the strength of the magnetic field as well as the strength of the electrical signal produced by the working sensor. In this way, the signal strength measurement provides a direct indication of the operational state of the magnetometer while it is running and serves to alert the operator if the magnetometer encounters environmental conditions that are outside of its operating range.
Occasional maintenance of the instrument at Geometrics facility should be performed when the instrument's internal diagnostics indicate substandard performance as described in the operator's manual.
Please contact Geometrics Support for technical advice and additional information pertaining to your specific model.
Degaussing is a method by which magnetic domains in metals or magnetic inclusions in other materials are randomized so that net magnetization is minimized. One tool do accomplish this is the “Bulk Tape Eraser” designed to erase data tapes.
The method works because the “Bulk Tape Eraser” generates an alternating electromagnetic field, which flips the magnetization of the magnetic domains in the material at 100 or 120 reversals per second (50 or 60 hertz). As the operator slowly removes the “Eraser” from the vicinity of the magnetized material, the magnetic domains of the material individually freeze in one orientation or the other, leaving the domains in a randomized orientation with minimal net magnetic effect.
Degaussing with a Bulk Tape Eraser
The procedure is straight forward. Plug the Eraser into an extension cord or wall socket (the Eraser cord is usually short). Holding the object to be degaussed in one hand, depress the Eraser start button and move it towards the object. Once close to the object or section of material, begin moving the Eraser with a small circular motion and then increase the radius of the circle as you draw the Eraser away from the object. DO NOT STOP the Eraser closer than three feet from the object being degaussed or it will become strongly magnetized in one direction! If this happens accidentally, just redo the degaussing procedure over again starting from the beginning.
For larger objects, run the Eraser along tubing or struts in a circular motion to “bathe” the objects in an oscillating field. Be sure to cover the entire surface area of the object being degaussed. Then slowly withdraw the eraser (while still running) until it is at least 3 feet away. Then release the power switch.
The magnetometer can be used to check the sufficiency of the degaussing procedure. After degaussing, rotate the object close to an operating magnetometer to see if there is a response from the magnetometer. This is best done with a cesium magnetometer operated in gradient mode, but it can be done with a single sensor with one person watching the result and another moving the object near the sensor.
Degaussing Sensor Mount
Degaussing Pack Frame
Degaussing GPS Antenna
Limitations of Degaussing with a Bulk Eraser
Depth of penetration: The Bulk Tape Eraser can only randomize materials to a certain depth. This is due to the size of the gap in the degaussing unit. A small gap makes for a very large degaussing field at the gap (about 2000 gauss, or 200 million nanoteslas), but also for a very rapid falloff away from the gap. Bulk tape erasers are optimized to penetrate through the thickness of a typical video tape. This gives a typical depth of an inch (2.5 cm). Deeper objects may need to be degaussed using stronger degaussing fields.
Degaussing through a conductive chassis: An additional problem occurs when the object being degaussed is covered by a conductive surface (such as a sheet of aluminum). The degaussing field will generate huge eddy currents in the conductive surface which will generate its own opposing magnetic field. This will be evident to the operator because the opposing field will cause the degausser to buzz loudly. This doesn’t hurt anything, but be aware that the degaussing field on the other side of the conductive surface will be attenuated by some amount, so it may take a longer amount of time or multiple passes to degauss the object.
The Bulk Tape Eraser is a short duty cycle device. It varies a little from manufacturer to manufacturer, but typically it is rated for 1 minute on and 5 to 10 minutes off. Most have an internal thermal cutout that will shut it off if it overheats, and if tripped may take 20 minutes or more to cool down enough to reset.
Frequently Asked Questions
Why is degaussing needed? Degaussing misaligns magnetic domains so that there is no net permanent magnetization that would give an offset or heading error to magnetic field readings. Sensitive magnetometers such as those manufactured by Geometrics can be effected by nearby materials that are not sufficiently magnetically randomized. Degaussing does not alter the induced magnetic moment of any material. A piece of steel, when degaussed, is still magnetic because it draws and concentrates the earth’s field through it. However, a degaussed piece of steel is much less magnetic than a permanently magnetized piece.
How much effect does it have on magnetic signatures? Depending on the distance from the sensor to the magnetic object and the amount of magnetization, the effects can be very large -10’s of nanoTeslas. Many materials including brass, aluminum, fiberglass and other non-ferrous materials may have some ferrous materials in them naturally or acquired during the manufacturing process. Other materials such as ‘non-magnetic’ stainless steel are hugely magnetic when compared to the sensitivity of our magnetometers. Degaussing can decrease the magnetic effect of these materials by a factor of 10 or more.
What should I degauss? The operator should degauss any metallic object that is near the sensor. By “near”, in general we mean within 1 meter but certainly those metallic and non-metallic materials within a few centimeters of the sensor must be considered (this also includes the sensor itself, which could have minute magnetic inclusions in the sensor materials). This could also include GPS antennas, magnetometer cart assemblies (including brass fittings, bolts, clamps), buckles, eyeglasses, boots and parts of backpacks. We would also do occasional degaussing of the G-858 console and batteries.
Will degaussing hurt anything? This is a tough question since it is impossible to imagine every conceivable system arrangement that could be subjected to degaussing. In all our experience we have never had any electronics device hurt by the degaussing process. This is because the induced voltages from the degausser are low, and the electronics components have a fairly high impedance at low voltages. It would be safer to degauss electronics while the power to the electronics is turned off in case the small induced voltages cause the device to operate incorrectly. It is always safe to degauss any of Geometrics’ manufactured equipment (including the sensor). On the other hand, here are some things to consider when degaussing some types of objects. Large conductive planes or rings will have large circulating currents induced in them by the degausser (but the voltages are still very small). This induced current will produce an opposing magnetic field that will fight the degaussing field – causing both the degausser and the conductive plane/loop to vibrate substantially. If the device being degaussed is sensitive to this vibration (intricate mechanical workings and the like) then this is a possible route for causing some damage. Also, sometimes objects being degaussed have embedded magnets that are necessary for the device to operate properly. A good example is a device with a permanent magnet speaker inside. Generally it is hard to degauss a magnetically hard permanent magnet, but the degausser is strong enough to at least partially do the job. A partially degaussed speaker (or other object that requires a magnet to work right) isn’t going to work the same as before – so be aware. [Things that have magnets in them shouldn’t be used near magnetometers anyway.]
When to degauss and how often? We recommend that parts close to the sensor be degaussed before every major survey event. In other words on a weekly or monthly basis or before a new survey. Remnant magnetism or “Perm” can be “picked up” (domains realigned) when the materials are static in the earth’s magnetic field for a period of time. The amount of time required to acquire a “Perm” can be from days to weeks or months depending on the magnetic “hardness” of the materials. This is also known as the materials “susceptibility”, that is, susceptibility to being magnetized. Also, magnets are everywhere, and they can easily and unknowingly ‘perm’ up parts on or near the sensor. Magnetic screwdrivers, for example, are great for holding steel screws on the end of the driver while starting them into a threaded hole, but they are bad news near any magnetometer sensors.
The Geode Seismograph uses 0.5 W/ch during acquisiton with a 0.25 ms sample rate. With that power consumption, a single 12 amp-hour battery is sufficient for a typical day of data acquisition.
In standby mode, power consumption is reduced by 70%. We recommend you run each Geode AU unit off of its own battery (2 is one and 1 is none), but you can run two Geode's off of one battery if the battery is able to deliver 2 amps per hour.
The smaller the battery, the more often it needs to be recharged, so keep that in mind when deciding on your power source.
1. Hook up Geode in normal configuration to computer Ethernet box.
2. Select Start New Survey.
3. Uncheck Line Tap.
4. Uncheck Aux.
5. Select YES to all pop up menus.
6. Locate lower left corner menu: Seismodule List Window.
7. Note what current loader version under LDVER column of table in Seismodule List Window. (Ex. 2.729)
8. In order to change the LDVER, you must first set up the table in column F from N/A to X by doing the following:
9. Select System pull down menu from the upper task bar.
10. Select Test.
11. Select Update System Board Bios.
12. Select I Agree.
13. Select Browse.
14. To set up table to enable loader version update (LDVER) select the file: GEODEFOR3D-1.0.exe.
15. Select Open.
16. Select Start Burning.
17. Select Yes.
18. Cycle power or shut down controller by using the software.
19. Restart the Geode.
20. Repeat necessary steps to get to Seismodule List Window.
21. Verify value in column is now X.
22. Select System pull down menu from the upper task bar.
23. Select Test.
24. Select Update System Board Bios.
25. Select I Agree.
26. Select Browse.
27. Select from Flash Update File Flash3_703&2_42.exe.
28. Select Start Burning.
29. Select Yes.
30. Verify Power LED light on Geode now blinks 3 seconds on 1 second off.
31. Select OK.
32. Cycle power or shut down controller by using the software.
Below is a series of diagrams that can act as analogies for impacts. If the impulse is enacted rigidly (hard tip hammer, steel plate, etc.), the impulse will look something like the far-left figure. High-amplitude (height of the curve), narrow wavelength (width of the curve). This is because the impacted materials respond rigidly to the impulse, i.e. the hammer rebounds from the plate almost instantaneously. Therefore, as a result of the narrow-wavelength impulse, the transmitted waves will have relatively high-frequency (short wavelength) content.
As you use softer and softer impact materials, applying impulses of equal force will appear like the diagrams to the right (smaller amplitude, longer wavelength). The impacted materials are responding less-rigidly to the impulse, so the hammer spends more time on the plate due to the more absorptive nature of the impact. The same amount of energy has been put in (area under the curve), but the amplitude of the input (height of the curve) decreases to compensate for the input duration (width of the curve) caused by the impact absorption of the softer materials. Therefore, as a result of the wide-wavelength impulse, the transmitted waves will have relatively low-frequency (long wavelength) content.
Using a more rigid striker plate (like one made of aluminum) on a hard surface can cause the generated wave frequency to be too high at times given the survey goals, so we suggest using a polyethylene plate on a relatively solid material like asphalt.
Remember: lower frequency -> deeper signal penetration -> decreased signal resolution.
That depends on a variety of factors such as the amount of explosives to be used per shot, your desired depth of exploration, etc. The answer is ideally that you drill the shot holes deep enough to avoid blowouts, but no deeper. If you are using explosives we recommend the use of the HVB-1 Blaster. We have HVB-1 Blasters available through our Rental Department as well.
Our seismographs do not require periodic calibrations in that they do not
have any adjustable parameters. With that being said, we do recommend
that they are checked for performance and routine maintenance to include
intensive analog tests of the acquisition circuitry. A reasonable interval
would be around every 5 years for normal usage.
During these tests we verify that the seismograph analog performance
meets our specifications by using a seismic test system. This system
incorporates a standard reference oscillator and precision resistor
networks to inject known signals into the seismograph. We then use
algorithms in our software to calculate the response and performance of
the analog circuits.
This "performance test" is run whenever we receive an instrument in for
repair or evaluation. Some of our customers do prefer to have the performance of their instrument checked on a periodic basis especially if they are
required by their clients, for example the NRC. We offer these nontraceable
recertification’s to include a calibration certificate and test results
for a fee of $300.
You can use the tape.exe application that was installed on your computer when you installed the Seismodule program. Read in the SEG-file, convert it to ASCII by selecting the File menu and Convert to ASCII. Check the box to convert to mV. Once you have converted the waveform to ASCII voltages, you can multiply by the sensitivity of the geophone to recover true ground amplitudes. Download Tape Reader. IMPORTANT - Be sure to back up your data files before running this program.
We get occasional calls asking how to use one of our seismographs as a vibration
monitor. The method for this is described below, but it should be noted that while true
amplitudes can be obtained, this method of measuring them would probably not stand up
in court. True vibration monitors – seismographs designed specifically for this task –
have a built-in geophone. The voltage output of the geophone per unit vibration is known to a very high degree of accuracy, and the system is calibrated by the manufacturer regularly (usually once a year). If you are measuring vibrations in a situation in which litigation might be involved, you should use a true vibration monitor. One of the more popular ones is the Blastmate by Instantel.
Vibrations are generally quantified in units of particle velocity, the first derivative of displacement. Geophones are particle velocity sensors – output is directly proportional to particle velocity. If you know the response function (sensitivity) of your geophone – the voltage output per unit velocity input – you can convert voltage (as measured by the seismograph) to mechanical vibration in terms of particle velocity. The sensitivity of your geophone can be obtained by the geophone manufacturer, and will be expressed as a function of frequency. A typical graph of geophone sensitivity is shown below:
It is best to used a geophone that has a natural frequency at or lower than the lowest frequency of interest.
Seismic data files are stored in a SEG format. The first step is to convert the SEG output of the seismograph to an ASCII columnar format.
If you are using an ES-3000 or Geode, your controller PC should have this icon for Tape Reader on the desktop:
Now, click on File>>Save Displayed Data to Ascii File:
After making your format choices (be sure to convert to mV), press Export. The record will be written in an ASCII format that can then be imported to Excel. From here you
can calculate the frequency spectra and particle velocities using the response function of the geophone.
1. When the transmitter is turned on, the red power light (or green light in later versions) comes on and stays on. The blue light will go into a rapid flashing pattern then settles into a three-flash sequence, for example short-long-short or short-long-long, or something like that. Is that what the transmitter is doing? If not, there are three possible causes of the problem and this will require require swapping parts:
Defective dipole cable or shorting plugs are two potential problems. The best test is to plug the shorting plugs directly into both ends of the Transmitter and turn on. If this works, then add one dipole cable and turn on again. Then add the second cable and power up. If failure occurs with just the shorting plugs then the most likely problem is a battery with a shorted internal cell. This will look like it is fully charged when you measure it with a volt meter, but will not be able to supply the current required to drive the transmitter. Swap out batteries to test. If swapping the batteries does not resolve the issue and you never get the blue light to start flashing you may have a bad Tx and it would need to be returned to Geometrics.
2. When the receiver is turned on the red power light will come on, then the blue light will flash rapidly, then the blue light will turn off waiting for the receiver to phase lock onto the Tx. Once it locks onto the transmitter the blue light will start flashing at once per measurement. Depending on how conductive the ground is and how far apart the Tx/Rx separation is you may have to wait up to a minute to get the lock. Try it with about a 5 meter separation between the end of the dipoles, i.e. the equivalent to having a 5-meter rope between them. The Rx should lock and start flashing within about 20 seconds. If it never locks on even though the Tx's blue light is flashing then there may be something wrong with the receiver and it would need to be sent back. Remember that the transmitter blue light has to be flashing first. If the Tx is not working the Rx will never detect it and start flashing.
3. With the Rx turned on, even if the blue light is not flashing, when you look at the OhmMapper Test screen on the console do you see the message: Setting Gain, Phase A, Phase B or something similar being updated on the screen every second (or twice per second with the old systems)? If so your console is communicating with the receiver. If not, you have no communication between the Rx and the console so you could have a bad dipole cable, bad optical wand, bad console cable, or a bad receiver. If you have spares of any of these items you can troubleshoot the problem. If you have no spares then you will need to send the system back here for evaluation by submitting an RMA request.
The dipole cables are a high failure item as they are subjected to considerable wear and tear. Here are some steps that you can take to prolong the life of the system:
Never lift the transmitter or receiver by the pulling on the Dipole cables.
While setting up the system, it is extremely important to remember that the transmitter and receiver can easily slip out of the white neoprene covers if you are not paying attention. This will cause them to hit the ground and break the yellow connector, or at the very least jam foreign debris into the connector. A good habit to get into is to carry the units with the nose (cone shape) pointed down, that way they cannot slip out of the neoprene cover.
Do not put undue stress by pulling on the system at angles greater than 45 degrees at any given point in the array. Always make broad turns or pick up the array to start a new line.
Make sure that the transmitter and receiver are always facing the direction of travel. (The cone pointing towards the console/operator) This will eliminate the possibility of turning the array into a "plow" and placing excessive strains on the connectors, not to mention communication problems.
By following these instructions you will find that the instrument's connectors will be able to withstand the strain incurred during normal usage. Of course there may be circumstances that make this difficult, but it is good to be aware of these issues.
It is easy for a healthy, fit person to tow the OhmMapper on flat dry ground or pavement. It is more difficult on grass or other surfaces where more friction is present. Towing the OhmMapper up hill is, of course, more difficult. The OhmMapper can be towed with a vehicle. However, a tow adaptor is required that will release the array in case a receiver cable is snagged on something in the tow path. A connector can be broken if it is snagged and towed by a vehicle but the tow adaptor is not required when it is manually towed because a person cannot exert enough force to break the connector under normal operating conditions.
The system is designed to collect GPS data along with the resistivity measurement during the survey. It requires a standard NMEA output from the GPS. The software uses the $GGA string. If the OhmMapper G-858 console is used a serial output from the external GPS is required. If it uses a Windows device an external GPS with any interface to the PC is acceptable. If the data logger is an Android device the internal cell phone or tablet GPS can be used.
Yes, SeisImager can process cross-hole tomography data.
SeisImager has a function to assume velocity is increasing with depth, so if we use this function, depth should be positive (SeisImager basically use a coordinate of X and elevation for modeling purposes). It is better to use elevation values that are decreasing with depth (i.e. elevation should decrease if we go down).
Stacking waveform data (SEG2) files using Pickwin
Make sure your dimension size is large enough. To start, select "option", then "Dimension size".
If the maximum traces is smaller than the total number of traces, increase the maximum traces, check “Change dimension size” and click “OK” to change dimension size.
Open one waveform file as usual.
Open another waveform file as usual.
Choose “Append to present data”.
If you want to change the color of traces depending on files, change component (2 to 10), check “Change” and click “OK”. Note that color does not affect stacking. Trace color is shown below.
If you uncheck the “Change”, all traces are shown black.
Confirm total number of traces.
Two waveform files are shown together. Make sure there is no time difference between shots.
After importing 3rd file.
After importing 4th file. Make sure there is no time difference among shots.
Confirm total number of traces..
All waveform files are shown together. Make sure there is no time difference among shots.
Select “Processing”, “Vertical stack”.
Select “a. Average” and click “OK”. You may select “Semblance” or “Semblance weighted stack” to emphasize coherent signal.
Stacked data is shown.
Geode/NZ acquisition boards are self-calibrating
to remove DC offset. This maintains the dynamic
range of the system and allows simultaneous
measurement of small and large signals.
Calibration takes approximately 2 seconds. There
are some situations when it is desirable to defeat
the calibration, for example if you are stacking
very quickly, or if you are doing marine surveys
where the inter-shot times need to be small.
By choosing to calibrate every nth save, you can
ensure periodic measurement appropriate to the
kind of survey you are undertaking. Also by
keeping the temperature of the instrument stable,
you can minimize the need for calibration.
The Geometry GUI provides a graphical representation of your survey, along with a wide range
of control capability. It is particularly useful when conducting reflection surveys, but can be
useful in a wide range of applications. It summarizes, in one simple view, the physical positions
and other attributes of the hardware on the ground, and allows graphical control of these.
Below is a typical display of a 96-channel, four-Geode layout. We will first describe the display
itself, and follow with a description of its control capabilities.
Example Geometry GUI
Channel remapping allows you to change:
the order of channels on each analog spread
cable that connects to the Geode
reorder the Geode boxes.
You would use this option if your cables were wired
opposite to the default order normally used in
Geometrics wiring, if you wished to turn your line
around to have the low channels at the opposite end, or if your cables had a wiring error. Channel
remapping is also often necessary when using more that a single network cable.
Default cable wiring of Geometrics seismographs
Default order is defined as the natural electrical order in which channels are oriented when the
system first powers up before remapping. Refer to Section 3 under Connector Wiring that
discusses standard wiring configurations. You may have requested a custom wiring configuration
from Geometrics. If you are confused about your wiring, contact the factory and refer to the serial
number and job number.
Geode cables are typically wired in a ‘high-side
configuration’, meaning that the Geode connects closest
to the highest numbered channel on the analog cable. The
figure above shows this configuration for a single box system, with 24 channels.
The following diagram shows a default single digital line (one network card) system with 3
Geodes. Note that Geode one is always closest to the controller in a default configuration.
Multiple Network Lines
The next diagram below shows a default configuration with two digital lines (two network cards)
with the controller positioned in the middle. Line 1 is on the left and line 2 is on the right. One
might use two lines to increase data throughput to reduce time between shots. Like the
configuration above, the Geodes are numbered starting closest to the controller. The seismic
controller software labels all of the channels contiguously even though they are on two separate
digital lines. However, if the lines are collinear, the first line will have the channels ordered
backwards. This can be easily rectified with the remapping feature.
There are two ways of remapping channels: automatic mode and manual mode. Automatic mode
settings are listed on the top of the remapping dialog box, and manual mode on the bottom.
Automatic Channel Remapping
Automatic channel remapping allows you to reverse
either the order of the Geodes on the line, or reverse the order of the channels on the spread cable.
The above diagram shows the result after both channels and Geodes have been reversed,
renumbering the line so that low channels start on the left hand side and increase towards the
right. In the dialog box, the automatic remapping boxes referencing line 2 remain unchecked,
since the default orientation on line two was correct.
Manual Channel Remapping
Channels can be remapped on an
individual basis using the Manual Map
Mode. Select the appropriate check box,
and enter the order in which you would
like the channels that differs from the
default order. You can specify individual
channels separated by a comma (1, 3, 4, 6
etc) or a range of channels (1-13, 24-14
For example, if you wanted the channels
ordered backwards on a 24-channel
system, you would enter 24-1. If you
wished to reverse the order of channels 1-
12 in a 24 channel system, you would type
12-1, 13-24. Other examples are shown opposite, and are available by pressing the See Examples
button on the remapping menu.
GPS Clocks, used in Continuous Recording systems to provide 1 pps signal to trigger seismograph. Also provides GPZDA serial string to stamp records with UTC for Continuous Recording and Self-Triggering systems. Includes cable set to connect clock to PC, seismograph, and 12V DC power.
There are three GPS clock options for the Geode we sell. They are:
GS-101B GPS clock from Orca, 1 pps accurate to within 100 ns of the precise time. Includes waterproof antenna with separate electronics module, with display of time and other indicators. Provides time base during loss of satellite lock. Options to provide 1 pps referenced to an IRIG-B generating source and to enclose electronics module in hardened aluminum case (P/N 25374-59).
P/Ns 20-700-101, 25374-0X....................$4,170
A101 Smart Antenna GPS clock from Hemisphere GPS, 1 pps accurate to within 20 ns of the precise time. Includes waterproof antenna with integrated electronics, with indicator of satellite lock, no display of time. Provides time base during loss of satellite lock. P/Ns 25374-84, 25374-86.......................$3,740
GC200 GPS clock from San Jose GPS, 1 pps accurate to within 1 μs of the precise time. Includes waterproof antenna with integrated electronics, no display of time or other indicators. No time base during loss of satellite lock.
The level of precision needed determines which GPS clock is best suited for the job. Typically the GC200 fulfills the needs of 95% of our clients.
Further Elaboration: You might ask this question to try and understand the low frequency response to determine if the Geode amplifiers effectively have a flat response from DC up. Does the Geode go down to DC for example?
Answer: We apply anti-alias filters to the data to prevent out of band noise from being introduced into the data.
The filters are set with a corner frequency at ¾ of the Nyquist frequency (1/2 the sampling frequency) for almost all of the sampling frequencies. When we sample at 1.5625 uS, the Nyquist frequency is 32kHz and the filters are set at 20 kHz, because of limitations of our electronics.
There are two single-pole filters: one analog and one digital. The analog filter is a simple RC filter with 1uF +/-5% and 100kOhm +/- 1%. We short out the capacitor in the conversion process. The digital filter is software controlled when the option is registered, so it can be switched in or out in the field. It is an IIR Butterworth with a -3dB corner at 0.9Hz for 48ksps sampling, and 0.6Hz at all the lower sample rates.
Also, we offer software and hardware options to modify the low end frequency response of the Geode.
Low-end bandwidth modification:
1.5 Hz, P/N 28311-37.......................$1,000
0.6 Hz, P/N 28147-01.............$400/8-ch
DC, P/Ns 28147-02, 28311-37..................... $1,000 per system plus $400/8-ch
The time associated with each data point in a SEG-2 data file generated by a Geode is related to the time of the
“trigger” event which was instrumental in the production of the file and its content.
The Trigger Master and Trigger Distribution
The trigger event occurs at the Geode designated within the Controller software as the Trigger Master.
Although all Geodes are capable of being Trigger Masters, there must be one and only one Trigger Master in
any properly functioning Geode system. The Controller automatically takes care of this requirement when the
designation is made by a user, and when the system is established at the time of Controller start-up based on a
previous designation (or a default setting in the case of a “new survey”). All other Geodes in the system will
have their Trigger Master circuit disabled. A trigger event can be initiated by an external electrical pulse
provided to the trigger input connector of the Trigger Master Geode, or by a command sent via Ethernet from
the Controller to the Trigger Master (usually for test purposes), but only when all conditions are satisfied to
allow data recording. There is also a special trigger initiation situation, called “self-triggering” which will not
be discussed further here.
Upon acceptance of a trigger event, the Trigger Master will distribute the trigger signal to all Geodes in the
system, itself included, via an RS-485 network that resides within the digital interconnect cabling. (Proper
termination of this RS-485 network is automatically taken care of by the Controller.) The trigger signal is
propagated through the cabling and Geodes at the nominal speed of 70% of the speed of light, or approximately
2.1x10^8 m/sec. The maximum distance of successful propagation depends on a number of factors such as the
number of Geodes involved, the noise environment, the quality of the cables, and the acceptable amount of
timing uncertainty for the particular application. Distances approaching or exceeding 1km should be given
careful attention in this regard. In a 3-D Geode system involving LTUs, each LTU, unlike a Geode, will
reconstruct the trigger signal before sending it on, effectively confining the maximum distance issue to each
sub-network separated by LTUs. The penalty is an additional delay of about 100nS for each LTU in the route.
The External Trigger Circuit
The external trigger input is capacitively coupled, with a 2mS time constant, to the midpoint of a resistive
voltage divider. The voltage difference between the two ends of the divider constitute a voltage "window",
which size is set by the trigger sensitivity parameter and can range from essentially zero at the highest
sensitivity, to about +/- 2.5V at the lowest sensitivity. The Geode will trigger (if enabled) if and when the
coupled signal exceeds the window, in either direction (i.e., positive or negative going). The signal, after the
capacitor, is clamped by diodes to the range between the trigger signal ground and +5VDC.
The trigger detector output is disabled when the system is disarmed, during a parameter change, and during a
shot, up to the trigger hold-off time after the end of the shot. The trigger hold-off time is a parameter set by the
Preceding the coupling capacitor (i.e., essentially the node accessible at pin A of the external connector), there
is a 3.3K-Ohm pull-up resistor to +5VDC (relative to pin B). Also a fast transient suppressor clamps the input
at about +/-14VDC. It is advised that the DC + AC level of any voltage applied to pin A relative to pin B be
kept within the range of +/-7V, giving some margin of safety.
If a DC voltage somewhat less than +5VDC is applied when the connector is first mated, the instrument may
trigger at that moment. But, subsequently, because of the capacitive coupling, it will trigger on the next positive
or negative going pulse that exceeds the window level. If the duration of the applied voltage pulse is less than
the record length + delay time + hold-off time, then the Geode will effectively be ready to trigger on the same
edge of another similar pulse.
The Geode supports a sub-sample timing synchronization feature used for synchronizing the data acquisition
after a trigger event to the distributed trigger signal, so that subsequent time points will be known to within 1/32
(~1/20 at the fastest two sampling rates) sample interval. It does this by increasing the sample interval at the
trigger time by 0 to 31/32 of a sample interval in increments of 1/32, so that the first sample after the trigger
would represent a time of one sample interval after the trigger event, with a tolerance within 1/32 of a sample
interval. The following samples continue from there at the expected intervals. For example, with a selected
sampling interval of ¼ mS and a recording delay of 0mS, the first sample in the recorded file for each channel
would represent data at 250 to 258uS after the trigger event.
This of course potentially introduces a small discontinuity at the time of the trigger, observable depending on
the nature of the channel waveform(s). (The zero-phase anti-alias filter will smear the discontinuity into the
nearby samples both before and after, consistent with the bandwidth of the filter.) Sub-sample synchronization
can be disabled if it is deemed to be detrimental for the particular application, at the expense of losing the 1/32
interval timing accuracy.
The principal errors in Geode timing are of two types: those associated with the trigger mechanism and which
are static over the duration of the record, and those associated with the time base and which change over the
duration of the record. Excluding the trigger propagation delay mentioned above, the trigger timing uncertainty
is about 1uS. The known fixed errors have been lumped together and are reported in the SEG-2 file trace
headers as channel SKEW. (The actual channel skew is zero, since all channels are effectively sampled
simultaneously, but the SKEW value in the header is used as the only place permitting small timing corrections.
Note that the SKEW value for every channel is identical.) If the size of this correction is important to the
application, the SKEW value should be added to the calculated time points when the data is being processed.
The Geode time base has a +/-15ppm stability over temperature (-20C to +70C) and component variations.
Thus time drift relative to absolute time and relative to other Geodes is possible. (However, all channels within
any Geode enclosure use the same time base, so there is no relative drift between channels in the same
enclosure.) Therefore timing uncertainty increases from that existing at the time of the trigger until the time of
the next trigger (or end of record).
Special Timing Issues Involved with “Continuous” Recording
“Continuous” recording is a method that allows unending 100% time coverage with recorded Geode data. It
produces a series of time-overlapped records created by the use of a negative time delay set equal to the record
length such that each record consists of completed history at the time of the trigger event. This technique
circumvents the problem of data transmission overrunning data acquisition. The principle constraint is that the
cycle time from trigger to trigger must always be less than the chosen record length. Otherwise, gaps rather
than overlap would result. Commonly it is used with GPSderived
triggering in order to provide time-stamping of each trigger event.
Upon consideration of the above, it will become clear that the time-stamp associated with a particular trigger
event will pertain to the data in the following record, not to the data in the record in which the time-stamp is
written. This comes about because the trigger event ends the record.
Because there is data overlap between records, the precise trigger point in the following record at which the
time-stamp applies can be found by comparison of the data values at the end of the former record with those
near the beginning of the subsequent record. The overlapping data will be exactly identical in both records
(since they are read from the same memory location, twice). The earliest data in the subsequent record that
goes beyond the data of the previous record is the data that is one sample interval (assuming sub-sample
synchronization is enabled) past the time-stamp.
Note well that this comparison must be made independently for at least one channel of each 8-channel Geode
board set, because the discrete time at which data values are written to the memory buffer, relative to the trigger
event, is a function of each individual board set in the Geode system.
Correct GPS Time-Stamping
There are differences between various GPS models that can affect accurate time stamping. The 1PPS signal
from a GPS has a “timing edge” and return edge, of which only the former is the true whole-second edge.
Some models use a rising edge as the timing edge, some the falling edge, and some have it selectable. Consult
the GPS manual to determine the definition of its timing edge. As indicated earlier, the Geode can be triggered
on either a rising or falling edge. It is important to insure that the Geode is being triggered on the proper edge
in order to avoid timing that may be a fraction of a second off. This is expanded upon below.
Some GPS units provide a very narrow timing pulse, others one that has a nearly 50/50 duty cycle. For the
narrow pulse units, almost certainly it is the leading edge (rising or falling) that is the “timing edge”. This case
can be easily handled by using the Geode Trigger Hold-off feature. If a 10-second cycle time is desired, set the
Trigger Hold-off time to about 9.5 seconds. In this case, there is a very small chance that the very first trigger
could occur on the wrong (trailing) edge, but from then on the leading edge will be used as the triggering edge.
If the GPS provides a 50/50 duty cycle edge, and it is not alterable, then the Geode by itself could as easily start
on the wrong edge as on the correct timing edge, and continue thusly until restarted. For this case, Geometrics
can provide a Trigger Timing Interface Box (TTIB) that will correct the situation. The TTIB can be
programmed to respond only to the correct edge (rising or falling), change the polarity if needed, and gate
through only one of every N 1PPS pulses, where N is programmable. (The TTIB also incorporates an alarm
system that can provide a remote alert if a record is missed.)
Another potential issue comes from the variations between GPS models of the time that the serial time string
(containing the time value of the associated 1PPS) is issued relative to the 1PPS itself. The Geode Controller
attempts to pick the correct serial string based on a calculation involving the known record length, the PC times,
and the trigger notification message from the Geodes. But if the GPS issues the serial string at an unusual time
(and the time has been seen to vary somewhat with a given GPS unit) then it could pick up the incorrect time,
off by 1 second. If rare, it can be subsequently detected and corrected during data processing, but if consistent
it may not be easily detected. Again, the TTIB can accommodate the situation by only gating through to the Controller PC the string belonging to the gated-through 1PPS pulse. The Controller Serial Input Time Window can then safely be widened to 2 seconds (assuming the
cycle time is more than 2 seconds) if need be, to expand the Controller’s search for the string around the calculated trigger time.
Theoretical and Practical Usage FAQ
A seismic wave is the transfer of energy through elastic earth materials by way of particle oscillation/vibration.
A seismic ray, or “wave front normal”, is an arrow drawn perpendicular to the seismic wave front to indicate the propagation direction at that point on the wave front. It is a convenient tool to help understand wave propagation through layered media; it is not something that exists in a physical sense.
Huygen's Principle can be stated in many ways. The simplest definition states that every point on a wave front can be thought of as a new point source for seismic waves.
There are four main types of seismic waves, each characterized by its specific particle motion:
Compressional Waves (“p” waves) are identical to sound waves – the particle motion is parallel to the propagation direction:
p-wave animation by L.W. Braille, Purdue University.
Shear Waves (“s” waves) are characterized by particle motion that is perpendicular to the propagation direction:
Taken collectively, p- and s-waves are known as “body” waves. The velocities of both can be measured via seismic refraction.
Surface Waves, as the name implies, travel primarily along the ground surface; amplitudes decrease rapidly with depth. There are two types of surface waves. Like body waves, they are characterized by particle motion.
Rayleigh waves are characterized by elliptical motion perpendicular to the surface:
In the near surface, this motion is “retrograde”, meaning that it is counter-clockwise when the propagation is left-to-right. At depth, the motion can reverse to prograde.
Love Waves are created when particles vibrate perpendicular to the propagation direction:
While the particle motion is similar to that of shear waves, Love wave amplitude is much higher and decreases rapidly with depth. Love waves are the most destructive waves in earthquakes because of their high amplitude and transverse particle motion.
While the various wave types shown above have been isolated for illustration purposes, all are present to some degree whenever seismic energy is traveling through a solid medium. Hence actual particle motion is extremely complex.
Different types of seismic waves travel at different velocities through any given material. In addition, different materials have different seismic properties, meaning that any one wave type can have a wide range of velocities, depending on the material properties. For instance, the p-wave velocity of shale can range from 800-3,700 m/s. Granite can range from 4,800-6,700 m/s. Because of this, by themselves, seismic velocities alone are not particularly diagnostic with regard to rock type.
Ultimately, seismic velocity depends on the density and elastic properties of the material, whatever its composition. Specifically,
Compressional-wave velocity depends on the “incompressibility” of the material, as embodied in the bulk modulus. The higher the bulk modulus, the less compressible the material, and the higher the p-wave velocity. Sound travels through water about four times faster than it does through air. Similarly, shear-wave velocity depends on the rigidity of the material, or the resistance to shear. The higher the shear modulus, the higher the s-wave velocity. Mathematically,
K = bulk modulus
µ = shear modulus
ρ = density
Note that Vp depends on both the bulk and shear modulus, while Vs depends only on the shear modulus. This observation implies two things:
Shear waves always travel slower than compressional waves through a given material.
Materials with zero rigidity – i.e., fluids – do not carry shear waves at all. Therefore, the absence or presence of groundwater has no effect on the shear wave velocity. It is interesting to note that, in general, seismic velocity increases with density – denser rocks tend to be much harder and faster. Yet in the above equations, density is in the denominator. This is known as the “velocity-density paradox”, the answer to which can be found in the fact that the elastic moduli tend to increase with density as well, and at a faster rate.
A seismic wave front emanates from an energy source like ripples on a pond, but in three dimensions. It is the surface connecting points of equal travel time from the source. In a homogeneous medium, a surface drawn through points of equal travel time is spherical, as depicted in the animation below.
The seismic trace, which is what is recorded by the seismograph, represents particle motion vs. time. In a homogeneous medium, the wave front can also be described as a surface of constant phase.
Snell’s Law describes quantitatively how wave fronts refract or "bend" at boundaries between contrasting velocities. You've seen it manifest in light waves by the apparent bend of the straw in your glass of water; light travels slower in water than it does in air. Refraction is well illustrated using Huygen's Principle. Consider a wave front (for our purposes, a seismic one) emanating from a point energy source, as shown in the animation above. For simplification, assume we are far enough from the energy source that the wave front is essentially planar, and is approaching an abrupt change in seismic velocity:
Applying Huygen's Principle, we see that after time t, the plane wave has advanced a distance d equal to the radii of a series of spherical wave fronts emanating from the plane wave:
The radii of the spheres, i.e., the distance the plane wave travels in time t, is equal to V1t. The tangent to the spherical wave fronts is the new position of the plane wave. The planar wavefront continues at velocity V1. Again applying Huygen, we see that "every point on the wave front" (see discussion of Huygen above) includes the points where the wave front intersects the velocity boundary:
As the planar wave front advances, the velocity boundary becomes a new source of spherical wave fronts expanding at V2. Hence, part of the plane wave (the tangent to the spherical wave fronts emanating from the velocity boundary) is now traveling at V2. Note that within V2, its direction of advance has changed. This is because in V2, which is higher than V1, Huygen's spheres grow faster during time t.
The refracted wave front continues in the new direction until another velocity boundary is encountered. Here is a simplified version at higher speed:
Zooming out, we see the effect of this on a spherical wave:
It should be obvious from the above that in order for a wave front to refract, it must strike the velocity boundary at an angle other than 90 degrees. It should also be obvious that in the case of V2 < V1, refraction will be in the opposite direction, and if V1 = V1, no refraction will occur.
Snell's Law quantifies refraction in terms of angle of incidence and velocity contrast. Combining the above diagrams and adding rays,
we can now describe Snell's Law:
In the figure above, i is the incident angle, and r is the refracted angle, measured between the ray and a line perpendicular to the refracting interface. In the example above, the velocity contrast is positive; V2 > V1. There are numerous derivations of Snell's Law on the web if you wish to understand the math.
From the equation, you can see that for any given positive velocity contrast, as i increases, r increases faster:
This is important; it is the property of refraction that allows us to use refracted energy to measure subsurface velocities.
Conversely, a negative velocity contrast results in refraction in the opposite direction:
All wave phenomena, the most familiar being visible light, are subject to refraction, or a change in propagation direction, at interfaces between materials of contrasting propagation velocities. The bigger the difference in velocity, the more the energy is refracted or "bent". As the name implies, seismic refraction uses the travel times of refracted seismic energy to determine the seismic velocity of the earth. A short practical discussion of seismic refraction can be found here.
The most important thing to keep in mind when learning how the seismic refraction method works is this:
When doing seismic refraction, we are only interested in first-arrival energy at each geophone. The rest of the wave train -- reflected energy, surface waves, etc. are discarded and ignored. Except in the exceedingly rare case where the near-surface is slower than the speed of sound in air, the first-arrival energy will always be either direct or critically-refracted energy.
Common applications include:
Estimating rippability prior to excavation
Mapping depth to bedrock/bedrock topography
Mapping depth to ground water
Measuring the thickness of the weathering zone
Calculation of elastic moduli/assessment of rock quality
Mapping thickness of landslides
Identification and mapping of faults
Fundamentally, there is no limit. Seismic refraction has been used to image the MOHO. The depth of investigation is governed by the size of the source you use and the length of the seismic spread, which needs to be 4-5 times the depth of investigation.
In practice, the most common use of seismic refraction is in shallow geotechnical, engineering, and groundwater investigations. Most of this is focused on the upper 20 meters or so. It is no coincidence that this happens to be around the upper limit of what can be accomplished with a hammer and plate. Every site is different – the specific geology and amount of cultural noise are important variables that ultimately govern depth of investigation for any given source – but 20 meters for a hammer and plate is a good rule of thumb.
In general, deeper work requires larger sources such as accelerated weight drops or light explosives. The only way to know for sure is to lay out your spread and see if you can get quality first breaks at the far geophones with 8-10 good swings with a sledgehammer.
Seismic refraction uses body waves, most commonly, p-waves. Shear-wave refraction can also be done, but it has become largely supplanted by MASW.
The crossover distance Xc is the distance from the source at which the critically-refracted energy from the next deepest layer overtakes the critically-refracted energy from the previous layer (in the two-layer case, the energy traveling through layer 1 is direct, not refracted energy, but the idea is the same). This is illustrated by the following animation:
The direct energy (red) is the first-arrival energy at the first six geophones. However, by the seventh geophone, the direct energy is overtaken by the critically-refracted energy (green). The reciprocal of the slope of each segment is equal to the apparent velocity of the material. If there were a third, even faster layer, a third slope and second crossover distance would eventually appear on the travel time graph. The crossover distance, along with the velocities indicated by the slopes of the segments, are used to determine the refractor depth.
This is the velocity that is measured directly from the travel time plot, uncorrected for any refractor dip relative to the surface. The apparent velocity is equal to the true velocity only when the refractor is parallel to the surface. If the refractor is dipping, the apparent velocity measured in the updip direction will be higher than the true velocity, and that measured in the downdip direction will be lower. The true velocity, especially for small dips, is close to the average of the updip and downdip velocities, although not mathematically so.
A velocity inversion refers to the case when seismic velocity decreases, rather than increases, with depth. When this happens, energy refracts away from the normal:
This means that no information from the refracting interface returns to the surface, hence no information is gained about that interface, even its very existence.
This is demonstrated by the animation below. There are three distinct velocity layers, but Layer 2 is lower in velocity in Layer 1. The result is a travel time curve that indicates the presence two layers -- layers 1 and 3. Layer 2 is not sampled, so there is no evidence of its presence in the record. However, the presence of Layer 2 does impact the travel time plot by moving the crossover distance out farther than it would be otherwise. The result is that not only is Layer 2 not detected, but the depth to Layer 3 is incorrect -- it is calculated to be deeper that it is.
The Principal of Reciprocity states that the time required for seismic energy to travel between two points is independent of the direction it is traveling. In other words, if it requires time T to travel from source to geophone, it will require the same time T if the source and geophone are interchanged. While this may seem trivial, it is an important concept in seismic refraction. The reciprocal time is used in the depth calculation and it is an important QC check in the field.
An air wave is simply the sound or acoustic wave created by the source, which by definition is what travels through the air rather than the ground. It is what you hear. In most cases, the seismic velocity of the near surface materials his higher than the velocity of sound in air. But in some cases the air wave is faster, and can arrive at the geophones before the seismic first break. This can make the first breaks difficult to pick.
An example of an air wave is shown below. The traveltime graph of an airwave is linear, and will be about 330m/s. It is generally lower in amplitude and higher in frequency than the first break, and can generally be dealt with in processing using a low-cut filter.
In general, two people can get the job done. If you are using explosives or a weight drop, a third person may be required.
10-14 Hz are the most common and the least expensive. Much more than 20-30 Hz and you start cutting into your signal. Less than 10 Hz and the phones become a somewhat fragile and less field-robust.
This is rarely necessary or helpful, even in high noise conditions. If you are having difficulty with air waves, burying the geophones might blunt their effect. If planted properly, wind is generally not a factor, and if it is, it is usually via vegetation vibrating around the geophones, which would not be mitigated by burial.
Can I do refraction on pavement? It is exceedingly difficult to do this, because the pavement is generally has a much higher velocity that the materials below it. It is generally not recommended.
Can I do refraction on a hillside? Yes. The slope of the surface is not relevant (at least in theory, although there may be obviously practical limits).
Can I survey in the rain? Rain won’t hurt the seismographs. If you are using a StrataVisor or SmartSeis and it starts raining, they should be fine as long as they are kept vertical, with the screen pointing up. But acquiring data in the rain should be avoided, especially if there is lightning. Even in the absence of lightening, rain can be a significant source of noise, especially those raindrops striking the geophones directly. Obviously, the harder the rain, the more noise. If waiting the rain out is impractical, shielding the geophones themselves can reduce the noise significantly. If for some reason the seismic system must be left on the ground during an electrical storm, it is advisable to disconnect the trigger and geophone cables from the seismograph.
The Reflection Coefficient R between two velocity layers is expressed as:
R = (ρ2V2 - ρ1V1) / (ρ2V2 + ρ1V1)
Where ρ = density and V = velocity. The quantity ρV is the seismic impedance of the material. The Reflection Coefficient is therefore the difference in seismic impedance over the sum of seismic impedance of two materials. From the above equation, it is apparent that R will be a positive number when V2 > V1, and a negative number when V2 < V1. A positive R means that the polarity of the reflected wave will be the same as that of the incident wave. A negative R means that the polarity of the reflected wave will be the opposite of the incident wave.
It should also be apparent that the larger the contrast in seismic impedance, the larger the amount of incident energy that is reflected (and the smaller the amount that is transmitted).
The above assumes normal incidence. For incident angles other than 90o, the equation is more complex.
Surveying using a randomized impulsive source (called "mini-Sosie), such as a Wacker, is supported by the StrataVisor NZ and the Geode along with the Urban Seismic software and hardware package. Read our Random Source Seismic Surveying Manual
In general, mini-Sosie surveying is useful in noisy, urban environments, particularly in close proximity to roads where near-field noise suppression can be used effectively.
the Geode Seismograph was designed specifically with reflection in mind. It includes hardware and software features and greatly facilitate reflection work, CDP reflection in particular.
The best source for seismic reflection is not always the most practical. Parameters such as cost, size, access, surface materials, and depth of penetration must all be taken into account. As always, a sledgehammer always supplies the most bang for the buck when practical. If the ground surface is too soft for a sledgehammer, you might consider a downhole seisgun. Small explosives are ideal in terms of portability and power, but for obvious reasons are often not feasible. Less portable sources like weight drops and vibrators should be considered when access allows and required depth of penetration exceeds that of a sledgehammer or seisgun. Although depth of penetration varies widely depending on the geology and cultural noise, you can expect to see somewhere in the range of 0.25 - 0.5 seconds with the latter two sources.
The de facto "standard" for high-resolution reflection is 40-Hz geophones. The 40-Hz cutoff will filter out much of the ground roll, allowing you to use the maximum gain even at near offsets, and taking advantage of the full dynamic range of the recording system.
A mechanical roll switch is not needed. You can use the electronic roll capability built in to the Geode acquisition software.
There is no one right answer to this question. The higher the channel count, the higher the fold (for any given shot interval), and the higher the signal/noise ratio. For land seismic, we generally recommend 24 or higher-fold data. If the shot spacing is equal to the geophone spacing (typical), this means that you must record on at least 48 channels with each shot. In addition, you must have extra channels for electronic rolling. Taken together, this means that for land CDP reflection, you should have a minimum of 72 channels, which will allow you to roll 48 live channels through a total of 72.
The MetalMapper 2x2 is a specialized instrument used for the characterization and discrimination of UXO (unexploded ordnance). The instrument uses an array of transmitter coils and receiver cubes to measure the EMI (electromagnetic induction) response of buried metal targets in the ground. Using software inversion algorithms, the data is used to discriminate targets of interest (TOI) from scrap metal.
Most metal UXO larger than a 37mm projectile can be discriminated from scrap metal. It can be difficult or impossible to discriminate smaller munitions like 20mm or small arms rounds. The MetalMapper 2×2 cannot be used for anti-tank or anti-personnel landmines, or for other UXO that is non-metallic.
The MetalMapper 2×2 can usually detect all UXO targets of interest to depths of 60cm, and larger TOI to depths of 1m or more.
The MetalMapper 2×2 measures in two distinct modes. A dynamic survey involves collecting data over the site while the instrument moves in a series of parallel swaths. The dynamic survey then identifies targets which need further characterization. These targets are then measured during a static or cued survey, where the instrument makes measurements while stationary over the previously identified targets.
In clear areas with a trained 2 man crew, the MetalMapper 2×2 can survey about ¾ of an acre per day in dynamic mode or collect 200 cued targets per day in static mode.
It is not practical to use the MetalMapper as a metal detector for applications like prospecting or utility location due to the specialized nature of the measurement and the instrument cost.