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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.

1. If the frozen issue is intermittent, please update to the latest MagArrow embedded software and survey manager, which can be found in the download section of MagArrow. The compass status can now be monitored on the browser interface. If the compass fails, power cycle the MagArrow.2. The frozen issue often occurs during the startup when the MagArrow faces exactly north/south/west/east. The compass initialization sometimes fails when one of the compass readings is close to 0. The latest embedded software greatly improves the robustness of the startup procedure. However, if the frozen issue persists, please power up your MagArrow facing north-west.3. If the above two steps do NOT fix the issue, the MagArrow has to come back for repair.

The Gammas2.exe is a Windows program that can be used to estimate the amplitude of the magnetic anomaly produced by a steel object that has not been magnetized. In other words, the program assumes that the magnetic anomaly is solely induced by the earth’s field according to the object’s susceptibility. The program uses a susceptibility of 10 cgs units to compute the anomaly amplitude. The estimate produced by Gammas2.exe can be used as a tool to help design magnetic surveys and help interpret survey results. Geometrics no longer updates the Gammas2.exe software, and we offer no guarantee that it will work on your computer.

This is for customers of the ATOM 1C and ATOM 3C seismographs (“ATOMs”) and is intended to provide guidance on ways to improve the download behavior via Wi-Fi. The ATOMs should be removed from any enclosure and positioned with the metal base plate down and relatively close to the Access Point (AP), which will increase the signal strength available to both the Atom’s and the AP. The RF environment during data download can impact whether the ATOMs can connect and on how fast they download data. Other RF devices in the vicinity (i.e., Bluetooth, wireless cameras, other APs, etc.), can cause slower download time. If there is a microwave oven operating nearby, this can disrupt communication between the ATOMs and the AP. If you are having trouble getting the ATOMs to connect and download, try setting up the AP and laptop and then turning on the ATOMs one-by-one waiting for it to connect before turning on the next unit. Another thing that will improve download performance is connecting the laptop directly to the AP with an Ethernet cable. This eliminates all of the RF traffic between the laptop and AP. Summary: *Choose a location with the least amount of RF traffic. *Remove all ATOMs from any enclosure and place each ATOM on its aluminum bottom base plate. This is the best position for the internal antenna. *When possible, everything should at least be on the same level, off the ground, on a desk or table. The AP can even be placed higher than the ATOMs. *Each ATOM should have at least a foot of space around it and be within 10 to 12 ft. of the Access Point. Being too close to the AP is also not ideal. *The ATOMs and AP should be in the same space or room with the AP being centrally located. *Connect the Access Point directly to the Laptop with an Ethernet cable rather than using a WiFi connection between the laptop and AP. *The Access Point can connect up to 30 devices at once, but it is advisable to connect 24 or fewer. The time it takes to connect will vary from ATOM to ATOM. If after 1.5 minutes an ATOM is not found, restart the ATOM. If after another 1.5 minutes an ATOM is still not connected, it could be a hardware issue. Contact support@geometrics.com Download time may also vary from ATOM to ATOM. If an ATOM disconnects or takes a much longer time than other ATOMs to download (>30 minutes) it could be a hardware issue. Contact support@geometrics.com

Hello everyone, my name is Devan, and I would like to open a discussion about finding a lost MagArrow. To give you some background, I lost my MagArrow II while it was mounted below the DJI M400 during a flight mission in a highly dense forest. We have the drone flight log, which indicates that it was stuck in a tree within a 50 m radius of the last known location. We have searched the whole area, but due to a highly dense forest and steep terrain, it was very difficult to find the MagArrow II and the drone on foot. For more than a month, I presume it was still perched within the tree canopy. And now we have bought a new MagArrow II to continue our survey. In this case, I desperately want to find and retrieve the lost MagArrow using the new device. I have an idea that if I conduct a 5 m spacing grid in both East-West and North-South directions within a 100-meter radius of the last known location, we could eventually narrow down our perimeter by finding an anomaly that indicates the lost device. Therefore, I have a few questions:1. How magnetic is the drone and the MagArrow II?2. Is it feasible to find the old MagArrow with the new MagArrow with the stated method? Alternatively, do you have any effective suggestions for a different approach? I appreciate any insights you can provide. Thank you!

Magnetic contamination can be a problem, though rarely. Most often this is caused when a “permanent” magnetic component was accidentally attached to or close to one of the sensors. This can cause big shifts randomly in the data from one or both sensors. Check by removing the sensor door and visually inspect the two MFAM sensors and their surrounding areas for anything unusual. If you have another magnetometer, like the G-864, you can also measure the magnetic signature of the sensor part of the MagArrow with the following instructions. Turn on the magnetometer Wave the sensor part of the MagArrow above the magnetometer (as close as possible but not touching) Wave in both West and east directions Check whether this is any magnetometer reading change when the MagArrow passes by If reading changes are observed, there must be some contamination.

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:

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.

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.

Raw Data MagArrow data is imported into Survey Manager in the form of .MAGDATA files, downloaded from the MagArrow. The .MAGDATA file contains measurements from different sensors inside the MagArrow: 1000Hz magnetometer readings; accelerometer, gyro, compass, temperature readings; and GPS info. MFAM assigns a fiducial number, or “FID” to each magnetometer reading, in a cycle from 1 to 1000 that repeats every second. In the instrument, the magnetometer readings and the GPS sentence data are synchronized so that the “FID-1” magnetometer record is matched with the GPS location and timing information. Exports to CSV and Geosoft file formats 1000Hz un-filtered export The 1000Hz export provides the original raw magnetometer data plus some simple interpolations: • Magnetometer reading: The raw magnetic field values are exported without application of a filter. [While these raw measurements are the output of a filter inside the MFAM sensor: a 9-pole Butterworth low pass filter with a -3dB Point at 400Hz, that filter is considered part of the sensor for this description.] • Auxiliary sensors: Gyro, accelerometer, and temperature are acquired once per every 5 magnetometer readings, and are reported only when acquired. Compass readings are acquired one per every 10 magnetometer readings and are reported only when acquired. • GPS NMEA sentence: Reported with the associated FID-1 mag record. A few individual fields from the GPS are also broken out from the GPS sentence and reported separately, without interpolation. • Interpolated GPS fields: Time, date, latitude, longitude, and track (course over ground) are linearly interpolated between GPS readings. Decimated exports Each of the exports at frequencies from 10 Hz to 100 Hz is a decimation – data are filtered by a low-pass filter and then down-sampled to the target sample rate. Each low-pass filter (a different one for each decimation) is a symmetric finite impulse response (or FIR) filter, with the following design goals: • -3dB attenuation at 0.75 * Nyquist frequency (e.g., the -3dB Point for the 10Hz decimation is 3.75Hz) • Significant attenuation of 50Hz and 60Hz signals. • Reasonably flat response in the pass band. Linear phase (or zero phase, or constant group delay) filters. These filters are not Kalman filters. The filters are applied to fields in the decimations as follows: • Magnetometer readings: Magnetometer readings are decimated: the FIR is applied, then the data are down-sampled to the target rate. • Aux sensors: Aux sensors are first up-sampled to 1000Hz by linear interpolation of values between individual readings (which occur once every 5 mag readings for gyro, accelerometer, and temperature, and once every 10 mag readings for compass). Then these 1000Hz values are decimated in the same process as the magnetometer readings. • Latitude and longitude are first up-sampled to 1000Hz by linear interpolation of values between successive GPS data (once per second), then these 1000 Hz values are decimated in the same process as the magnetometer readings. • Time, date, and track are linearly interpolated as in the raw, unfiltered 1000Hz export. Merging filtered and unfiltered data. Some of the values in an individual line of data are filtered: mag readings, aux sensors, etc. Other measurements are not filtered: time and date, GPS sentences, record counters, and the simple interpolated fields. These two sets of values – filtered and unfiltered, must be reported in individual lines that contain values of both types. The question “How should the two sets of values be matched?” is addressed as follows: A decimation filter has a center. For example, a single filter result that weighs 499 individual measurements running from record number 752 to record number 1250 (in DSP terms, it is the result of the convolution of 499 input values with 499 filter weights), is centered on record number 1001. The result is the “filtered value of record 1001”. In a single line along with this value should be the other filtered results centered on record number 1001 plus the unfiltered raw and interpolated values that were recorded as part of the original, raw record 1001. The down-sampling part of decimation involves keeping some results and discarding others; down-sampling from 1000Hz to 100Hz includes discarding 9 out of 10 results. During exports, Survey Manager keeps the “FID 1” record, because it includes the original GPS information, then discards the next 9 records (if it’s a 100Hz decimation), and then repeats the pattern, each time starting with FID 1. If you have questions about the MagArrow decimations, please contact your Geometrics account manager.

Geometrics instruments, such as G-864, MagEx, MagArrow, MagStation et al, communicate with a mobile device via WIFI. But because the instrument WIFI has NO internet access, some Android device, after connecting to the instrument for a while, will automatically start searching for another WIFI access Point. To prevent this from happening, when the No Internet Access warning window pops up after first WiFi connection, choose "Tap for options". On the following window, check the “Don’t ask again for this network” box and press “Yes” .

The answer is NO. MagArrow or MagEx operate in the so-called combined sensor mode. The single reading is NOT the averaged reading from the 2 sensors inside. Instead, the signals from the 2 sensors are combined and then data processed to produce one reading. To us, there is no Point to produce 2 readings if 2 sensors are right next to each other. We are open to discussions!

Hello, guys.I need to ask some questions about grounding technique for Geode seismographs. Current territory that we need to investigate has strong electromagnetic pollution from nearbuy high voltage power line. When using 48 to 96 geophones many channels experiencing strong EMI interference in wide spectrum. So I want to ask the forum society for some advices on the grounding technique of Geode modules. Because in documentation there is only a mention of "grounding plug" on the side of boxes. But there is no advices on how to properly ground 4 or more modules, distanced 240 meters from one another. Even more when soil has different properties along streamer line (moisture etc). On geometrics.com could not find any instructions on rightful grounding technique. Maybe there is exist some sort of special grounding schemes for large number of modules in use. As I understand the better way is to have star topology for grounding (with one common rode Point for several devices) when using spreaded device system. Because in other cases ground potential for different modules will vary. Maybe I asking a silly quastion but EMI problem anyway exist on long receivers arrays. That ruins noise/signal ratio and etc. Thanks.

Hello Dear Rui, I have questions regarding MagArrow Magnetic data compensation, if you can answer please, 1- Does the heading error compensation that applied by surveying data processing program remove the heading effect due to direction only or calibrates the maneuver effect due to the swinging of the sensor as well? 2- Regarding the heading compensation flight, I am confused about the best way to do this. In the manual, it is mentioned that we can apply a cloverleaf pattern (this is usually used in airborne survey) or a survey pattern or we can do a 360 degree turn at a certain Point. However, it is written that we can use the calibration data measured in the turn, can I use the data from the turn to apply the heading error correction separately for each flight instead of doing a heading flight. Regarding the cloverleaf pattern application, is the data also processed in the same way using surveying data processing program? 3-What is the importance of the base station magnetometer data in this correction as mentioned in the manual, I think we can apply it without base magnetometer? 4- Finally, is there a specific flight that can be done to make maneuver correction (FOM) (roll, pitch and yaw) that we apply in conventional magnetic airborne survey? or We just resort to the LP filter. Sorry for the long post Kind Regards, Ahmed