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All of the 300 series Stainless (austenitic) Steel is considered to be weakly magnetic in its annealed state. These include the common 303, 304, and 316 alloys. 18-8 Stainless is another name for 304 Stainless (18% chromium and 8% nickel). Surprisingly it is the nickel content in the Stainless that makes the Steel less magnetic. Nickel usually makes things more magnetic in other metals. The 400 series are very magnetic (martensitic). They do not contain nickel. 17-4 Stainless is a different class of Stainless, and contains a little nickel (4%) but has other stuff in it that makes it magnetic. To make it even more confusing: All Stainless Steels, including the 300 series, become very magnetic when hardened. A soft ductile 316 Stainless cotter pin is not very magnetic, but a 316 Stainless spring is highly magnetic. If it is Stainless and "springy" it will most likely be very magnetic.

In general, avoid Stainless Steel parts. If you have to use Stainless Steel parts, make sure it is made of SS316. To make sure that the parts are magnetically clean, you can set up the MFAM Dev Kit in the gradiometer mode. 1. Have the two sensors separated by 20cm or more. 2. Turn on the gradient reading (green curve) in MagViewMFAM. 3. Place the part 10cm to one sensor and 30cm to the other. 4. Rotate the part while monitoring the gradient curve. Make sure you don't have any magnetic parts (such as keys, cell phone, watch) with you while doing this. 5. The peak-to-peak difference in the gradient curve (you may have to adjust the gradient scale if the curve wraps on screen) is the magnetic signature of the part at 10cm. 6. For the magnetic signature at different distances, scale the reading as 1/R^3. For example, if the part will be 20cm away, its magnetic signature should be 1/8 of of the reading at 10cm.

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.

If you perform a land magnetometer survey and you see random spikes in the data spread out over many cycles, these being real magnetic events, then the issue might not be the sensor but metal on your person. A baseball cap often has a little Steel button on the top, and for land magnetometers where the sensor is overhead, this Steel is very close to the sensor. With movement during walking, this can create strong magnetic spikes in the data. To test this, you can mark the location of the magnetic sensor in relation to the Steel button on a baseball cap and record the magnetic field standing still. Then lower the sensor 1 meter and you'll observe the anomaly increase by a factor of 16 or so. For magnetometer surveys, it is important that the operator is free of electronics and metal. This means no cell phone, no belt, no Steel toe boots, etc. Small quality checks can have a big impact on the data and save time in the field.

This is a very common question and for the best answer, please read page 45 of the Applications Manual for Portable Magnetometers. To give an idea, the rule of thumb is that 1 ton of Steel will give 1nT at 100 ft. The distortion caused by the Steel in the earth’s field falls off as the cube with distance and is linear with mass. Therefore, at 50 ft, 250 lbs will give 1nT, at 25 ft 30 lbs will give 1nT, at 12 ft 4 lbs will give 1nT. Cables and pipelines fall off at somewhat a different rate (inverse square) so can be seen further for a given mass.

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.

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.

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.

Total field magnetometers like the optically pumped cesium magnetometer are passive devices, they do not send out waves or pulses. They measure distortions in the earth’s normally homogenous magnetic field and can sense distortions due to ferrous objects at great distances. The basic rule of thumb is that one ton (1000 Kg) of Steel or iron will give us a 1nT anomaly at 100 ft. or 30m. Since the amount of distortion falls off as the cube with distance (compare a metal detector which falls off as the inverse 6th power!) and is linear with mass, every time we cut the distance in half, we can see 1/8th the mass. Therefore, we can sense 250 lbs. (100kg) at 50 feet (15m), or 30lbs (15kg) at 25 feet (8m), or 4lbs (2kg) at 12 feet (4m). However this is not the whole story. The factors given above are for induced magnetic fields only. Many targets also have remnant or permanent magnetic effects (meaning they have become magnetized either in production or by the earth’s field) and can therefore have larger anomalies by a factor of 3 or 5 or more. Also many hollow objects like barrels or other tubular structures appear as though they are solid due to self-shielding from the earth’s field, and thus have much larger anomalies than their mass would predict alone. Pipes fall off as the inverse square and are thus detectable at even greater distances. Please see our Applications Manual for Portable Magnetometers for more information.

A common question many have with magnetic surveys is "How wide of a survey swath does a single magnetometer sensor cover on a single pass?" The answer is it depends on what is being searched for. Magnetometers are passive instruments, meaning they don’t actively send out signals or have a limited swath or depth of exploration. When planning a magnetic survey the grid (line spacing and waypoint spacing) should be designed using the best possible model of the target. There are some general rules of thumb that can be used to determine typical detection ranges for common iron objects. For example, a 10lb sledgehammer has been lost and needs to be found, and assuming this is 10lbs of pure iron, it would be expected to see a 1nT anomaly when the magnetometer sensor passes 6 meters over the top of the tool. Knowing this, survey line spacings should not be any narrower than 6 meters. With a line spacing of 3 meters, the chances of getting a clear anomaly goes up 8 fold as the 10lb iron sledgehammer would be at a minimum a 8nT anomaly vs a 1nT anomaly. In a geological sense, let's say we have a mafic dike intrusion that we believe is running E-W and it extends at least 25 meters in the near-surface in a somewhat linear fashion. It's difficult to model the amount of iron in a geological structure like this, so the survey should be designed to cross the dike perpendicularly every 5 meters or so, making sure to cross over the dike several times. Each pass over the dike may exhibit an anomaly of similar amplitude, and the feature will show up as a clear linear feature in the final processed map. For general mapping of geology, you have a lot of options. Most commonly mineral exploration surveys are done over very large areas, so the line spacing is wider to save time as the lower resolution model that results still accomplishes the task of finding large mineral deposits. If more detail is required, then a more fine-grained survey can be done later. 20m-50m line spacing is typical for mineral exploration surveys. Design a survey grid to completely encompass the area of interest (i.e. make sure you get some data outside of the areas of interest, in case an interesting anomaly lies right along the edge). The founder of Geometrics the late Sheldon Breiner called this the Law of Search, as he often found his targets of interest along the edges of his archaeological magnetic surveys. It is important to make sure the operator of the magnetometer is magnetically clean before surveying with the magnetometer. This means no Steel toe boots, glasses or hats with metal fittings, cellphone, belt buckle, etc. Magnetometer data acquisition is fairly simple, but data interpretation can be complex. You may need a base-station too. Please refer to the Base-Station information.