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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.

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.

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

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). 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. 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.

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.

There are basically three types of "gold": low concentration disseminated gold in ore, placer gold deposits and solid gold such as that associated with treasure. Magnetometers are used to find disseminated gold by its association with mineralized zones which also contain magnetite or other magnetic minerals. Magnetometers are often used in conjunction with airborne electromagnetic surveys to find the conductive ore bodies. Placer gold is the type found in buried stream channels such as the gold which sparked the California gold-rush in 1849. Gold dust and magnetic minerals have been concentrated in river banks over thousands of years. Where there is gold there is often magnetite and therefore the magnetometer can be used to locate placer gold deposits. Gold treasure is a different story and being non-magnetic gold, silver, and other precious minerals are not directly detectable by the magnetometer. The magnetometer can only detect ferrous (iron or steel) objects. If the gold is stored in an iron box or has iron materials next to the gold (such as colonial ship ballast stones in the marine environment), there is the possibility of detecting the iron material. This is true for land and marine (sunken galleon) gold bullion. The vast majority of target search surveys are performed on a grid in a "lawn mower" back and forth manner to cover the area of interest. Lane spacing is dependent on target size (magnetic mass). At a sensor to target distance of 2 to 3 meters there will need to be at least 1-2 kilograms of iron. This can produce a 1-2 nT anomaly that is detectable in a magnetically clean environment. The ideal environment would be in a plowed farm field or the bottom of the ocean away from human activity i.e., away from a port or harbor. You will probably not be able to detect this small of an anomaly in a city or port location. The more iron mass there is, the better the chance of detecting it. Training to use the magnetometer can take 1-2 days depending on experience with setting up computerized survey equipment and a GPS. Processing the magnetic data requires several days of training and would require a geophysical background to interpret the final maps. We provide free software to make maps and estimate the target depth of burial (inversion). If you are unfamiliar with this procedure, we would recommend that you find a local geotechnical firm to look at the data to determine if there are anomalies that should be investigated further. Remembering that non-ferrous materials do not cause anomalies (gold, silver, copper, brass, Aluminum, gems) you will be looking for anomalies either associated with the container OR associated with ground disturbance (i.e., gravesite). In this way some anomalies can be detected where there has been an excavation such as a gravesite. In order to understand the process more fully, we strongly suggest that you download and read the Applications Manual for Portable Magnetometers. Other additional resources are available. Understanding how the magnetometer functions and how the earth’s field responds to distortions due to ferrous materials will help you make good decisions about how to interpret and use the data to direct recovery or exploration efforts.