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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.

Here are a few additional details relating to the Measurement of elevation: GNSS AccuracyAccuracy depends on multiple aspects of the GNSS system: among them are clock accuracy, atmospheric effects, and satellite geometry. Satellite geometry"Satellite geometry" refers to how the currently visible satellites are distributed in the sky - close to each other or scattered around. The best satellite geometry includes satellites that are Near the axes on which you hope to locate your receiver; for example, to locate your receiver on the East/West axis, it's helpful to have good reception from satellites low in the sky in the East and in the West. If you also have satellites that are low in the sky Near the South and North horizons, you will have good accuracy on the horizontal (latitude/longitude) plane. It's best to have satellites scattered around the sky, overhead as well as Near the horizon all around. HDOPMagArrow records HDOP, a standard Measure of satellite geometry's effect on horizontal (or latitude/longitude) accuracy. A smaller number (less than 1.0 is very good) indicates that the visible satellites are in good positions to contribute to accuracy. Vertical AccuracyThe reason that GNSS systems aren't as accurate on the vertical axis as on the horizontal axes is that no satellites are visible in a full half of that axis: the half that is below the horizon. Consequently, vertical accuracy is on average about half that of horizontal accuracy; calculated offset from true elevation is on average about twice that as on the horizontal axes. While on average HDOP can therefore be used to estimate VDOP (the similar Measurement of the effect of satellite geometry on the vertical axis), that estimate is only a rule of thumb; it is possible to have an excellent HDOP, reflecting very good horizontal satellite geometry, while having poor vertical satellite geometry. In those cases, good HDOP does not indicate good VDOP. Keeping in mind that possibility, a combination of good HDOP and many satellites in view usually indicates good VDOP. Practical effects Some data processing techniques (upward continuation, for example) can include elevation as an input. Customers who are considering using GNSS elevation in those techniques should conduct a careful analysis of their data and develop test routines to verify that all their data Meet the requirements of the technique and its application to a particular survey. Some customers who require very accurate elevation data incorporate LIDAR data and drone elevation data into their analyses.

Regarding the deployment of magnetometers on conductive sleds or carts Near power lines: Depending on the proximity of the magnetometer to the sled, elevated field readings may be observe under power lines are a result of AC induction in the aluminum sledge you are using as the tow vehicle. The reason there can be a DC effect from an AC source is due to 1) the strength and proximity of the induced AC source and 2) the orientation of the induced AC field relative to the Earth's field (DC). Our cesium-vapor magnetometers Measure the total local field continuously but report these Measurements periodically, e.g at 10, 1000 times per second. For each reporting period, both the AC and DC components of the total field are integrated to produce the Measurement result as a time average over the Measurement cycle. If your Measurements are being reported 10 times per second (10 hz sample rate) and the AC component of the field is 50 hz, then each Measurement will include exactly 5 AC cycles. This AC component will add to the DC component as a vector sum and the magnetometer will Measure the magnitude of the resultant vector. Note that the vector component of the 50 hz AC field that is parallel to the DC component will not contribute to Measurement results: for half of each AC cycle this field is greater than the DC field and for the other half of the cycle it is less than the DC field by an equal value. This is not the case for the AC vector component that is perpendicular to the DC field: it will be adding magnitude to the DC field on each 1/2 cycle to produce a half-wave-rectified wave form. Specifically, this rectified field will add to the DC field by an amount equal to about 35% of its peak-to-peak field strength in the direction perpendicular to the DC component. The AC rectification described above is only seen on close approach to very strong AC sources (high tension power lines). An aluminum sled can act as an indirect source of the AC fields: the radiated 50 hz field from the power lines is inducing 50 hz eddy currents in the sled and, if a magnetometer is in close proximity of the sled's aluminum plates, it will detect large AC field values. Note that surveying Near other large, planar conductors under the high tension power line can produce a similar effect. These would include Metal buildings, Metal fences, and pipelines. We recommend constructing magnetometer sleds from non-conductive materials. If this is cannot be done, then conducive materials should be kept as far from the sensor as is practical and the sled's construction should not include sheets of conductive materials. Any joints between conductive structural elements should be insulated as well. You can use the magnetometer itself to Measure the effect of the sled.

A seismograph with an active trigger input like the Geode Seismograph or ES-3000 Seismograph can be triggered many different ways. The most commonly used Methods are with a trigger switch or a trigger geophone. Typically a trigger switch (known as a hammer switch) is attached to the handle of a sledgehammer Near the striking end, so when the sledgehammer is hit against a striker plate to create an active source of energy, the piezoelectric crystal in the hammer switch is activated and the seismograph is triggered to record data along the preset parameters. A trigger geophone does this too, but it is placed Near the source itself, and is more commonly used with larger energy sources like a propelled energy generator. If the seismograph isn't triggering with either a hammer switch or a trigger geophone, then the signal may be weak, so turning up the sensitivity could be a workable solution. If the sensitivity is set too high in SCS then false triggers might be encountered. In most situations having the sensitivity set to the middle works best. Depending on where the trigger geophone is it, there may be a difference between when it is triggered and when a hammer switch would have triggered. Especially in soft ground the trigger geophone signal may be delayed. In general the hammer trigger is a more reliable timing device. The differences in trigger time when using a trigger geophone could be due to things like not striking the center of the plate or differences in the strength of the impact. More trigger circuit information: The seismograph can be triggered by shorting the two input pins A and B on the trigger connector of the seismograph. In fact, that is what the hammer switch does (contact closure device) when it impacts a striker plate. The inertia of the impact causes a momentary closure in the device, which in turn, triggers the Geode. There are no internal components that need to be added. Externally, you could construct trigger device or switch, if that is what you desire. If you were to Measure the pins on the Trigger connector on your seismograph (pin A +, pin B -) you would see about 5VDC. The trigger circuit will sense a contact closure or a pulse. The Geode 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 triggers (if enabled) if and when the coupled signal exceeds the window, in either direction. 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 user. 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 Drawing Tool Menu provides functionality for integrating log file datasets—such as N-value, boring, soil, and DCP logs—with velocity profiles. Below is an example of a boring data log .txt file that includes a soil column: 0.45 8 1.45 8 2.45 1 3.45 5 4.45 5 5.45 31 6.45 21 7.45 23 8.45 31 9.45 22 10.45 8 11.45 11 12.45 12 13.45 8 14.45 3 15.45 32 16.45 33 17.45 42 4.9 C Clay 17.45 G Gravel Log files can be loaded with or without headers. To load a log file, navigate to the Drawing Tool Menu. After loading, use the Select Tool (represented by the arrow icon in the top toolbar) to interact with the objects in the plot. Attachment : image018.png The Select Tool allows you to select and edit objects such as logs. To modify a log, double-click Near the base of the comment line associated with the log. A configuration Menu will appear, where you can specify the data type—N-value, Line, or Soil Column—and adjust axis settings and labels as needed.

and also there is no light blinking in the Ethernet status Near the Ethernet port of MFAM dev kit.

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 has 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.

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 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.

Dead-Zones The MagArrow is a dual sensor magnetometer powered by MFAM sensors, but it is configured for use so it only has a single data output. The reason Geometrics has done this is so we could ensure the MagArrow encounters no "dead-zones". A dead-zone occurs when the orientation of a magnetometer results in the magnetometer producing poor or no Measurements. The dead-zone angle depends on the location of survey. Since we have the two MagArrow MFAM sensors in orthogonal orientations, the MagArrow Magnetometer has operability worldwide without affecting survey orientation, making it much easier to use for the customer. Heading Errors Heading errors are a type of noise magnetometers can experience. They come from three sources: Sensor Console Operator Magnetic materials in the sensor itself are the primary cause of heading errors. The physics of Cesium and Potassium magnetometers can contribute small amounts to the total heading error. Magnetic contamination Near the sensors, operating electronics, or operator can all contribute to heading error. Heading errors look like herringbone patterns in survey images. Alternate lines can also be corrugated. Dead-Zones vs Heading Errors while these two sources of error in magnetic data are different, there is overlap between them when operating a magnetometer like the MagArrow. Heading errors can be fixed relatively easily in software, where dead zones can be much harder to manage. If a line is completely ruined because of a dead zone then they will need to re-fly the line/mission which is time consuming. Even with advanced users, these sorts of mishaps can happen. Additionally, the closer a mag sensor operates to a dead-zone, the larger a heading error will be Measured. With compensation software and a pre-survey heading error flight, heading error can be reduced dramatically to around 1 nT for the MagArrow. Click to view the difference between Raw and Processed MagArrow Data The MagArrow is only outputting a single value as a Means to create a “no-dead-zone” system. Obviously each sensor has a dead zone themselves, but with the sensors orientated orthogonally at least one sensor at all times will have a magnetic Measurement. By combining the Measurements from both sensors it is possible to generate a constant magnetic field Measurement independent of orientation and location in the world. If the data from each MFAM sensor in the MagArrow was individually reported there would be gaps in the mag fields observed by either sensor as you fly, rotate, and swing.

If the sensor is in a very large gradient field such as inside a building Near some steel objects, then the high gradients (change of field with distance) causes different parts of the internal sensor components to respond to different fields, making the sum signal smaller. This only takes place when the gradients are very large such as several thousand nT per Meter and will not be seen under normal survey conditions.

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 of seismic refraction 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 Seismic refraction uses body waves, most commonly, p-waves. Shear-wave refraction can also be done, but it has become largely supplanted by MASW.

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. 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.

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 user. 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. Sub-sample Synchronization 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. Timing Errors 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.

Clarification regarding Geometrics standard magnetometers SX versions and the US Govt. export regulations In this brief review magnetometer specifications are given in terms of both nT/sq-rt-Hz RMS and in Peak-to-Peak (P-P) noise values as both forms are often used to describe instrument performance. The US Government specifies that an export license is required for magnetometers that have a sensitivity of better than (noise level less than) 0.02nT/sq-rt-Hz RMS. Obtaining an export license is not difficult but it does require approximately 6-8 weeks. Not all geophysical applications require export license sensitivity and so we offer SX models that have a noise floor of 0.02nT/sq-rt-Hz RMS. Compare this with our G-858 Magnetometer at 0.008nT/sq-rt-Hz RMS and our G-882 Marine Magnetometer at 0.004nT/sq-rt-Hz RMS. What does SX performance Mean in the survey results? When the sensor is deployed at some distance from the “source” such as in above the shoulder mounting for geological surveys (G-859SX) or at some distance (several Meters) from the seafloor for G-882SX surveys, the distance from the source provides some natural filtering of the Near surface response. This Means that surveys not focused on small target detection (20mm ordnance rounds) where the sensor is deployed very close to the ground (<1m), SX performance is more than adequate. Let us consider the G-858 man-portable model. Under low noise laboratory conditions at a sample rate of 10 samples per second, the G-858SX will show approximately 0.125 nT of noise (peak-to-peak) compared to a standard G-858 of about 0.05nT P-P. To understand the significance of this, the natural earth background noise due to geomagnetic micro-pulsations is about 0.02nT/sq-rt-Hz (about 0.125 nT peak-to-peak) at the quietest of times. Micro-pulsation amplitudes of 1 or 2 nT are common and, during active periods, they may be larger than 10 nT. Any magnetometer will produce a record of the combination of the background noise (micro-pulsations, diurnal drifts, etc) and its own internal noise. If the various noise components are not correlated with each other they will add as the square root of the sum of their squared amplitudes. In the case of the G-858SX, the combination of instrument noise and background micro-pulsations will be: √(0.125nT^2 + 0.125^2) = 0.18nT. For the standard G-858, this combination will be: √(0.05nT^2 + 0.125^2) = 0.13nT. That is, the SX model will exhibit about 30% more noise amplitude compared to the standard model if the atmospheric noise is typical. Unless the survey Measurements are referenced to a high performance base station magnetometer equipped with a very accurate clock, the user will not be able to detect any difference between SX and standard performance. If such base station data were available, the greatest difference that would be seen should be no greater than about 0.05nT P-P in the average peak-to-peak amplitude. Such small differences cannot be seen or even detected in the total field contour maps made for exploration surveys which are typically contoured at 1nT or more. It should be remembered that the amplitude of the geomagnetic micro-pulsations in the frequency range from 5hz to 10hz is not constant; i.e., at most times they will be greater than 0.125nT and occasionally less than this value. Their intensity is governed by the average intensity of the instantaneous global thunderstorm activity and sunspot activity.