Earth Magnetometer

The AlphaLab Earth Magnetometer is used to map the earth's magnetic field. It shows the variation from place to place down to a resolution of .01 milligauss (= 1 nanotesla, or 1 gamma), which is about 1/500 percent of the earth's field strength. Because the earth's field fluctuates by typically 3-30 nanoteslas per hour, a resolution better than 1 nanotesla will not be of much additional benefit. Until now, resolution of 1 nanotesla or .01 milligauss could be achieved only with very expensive, heavy, and high-power-consuming magnetometers such as proton-, overhauser-, and cesium- types.

Earth Magnetometer (complete with battery & probe rod): Price $700 (US dollars). BUY IT NOW

Price includes shipping in the US. For shipping to Canada by USPS add $20 (US dollars). Outside the US and Canada add $40 (US dollars). 

 

The AlphaLab Earth Magnetometer weighs only 18 ounces (= 510 grams), and runs about 8 hours on the included standard 9-volt alkaline battery. It measures difference in field strength to accuracy of +/-0.5% of the difference, either for point-to-point or time-to-time measurements. In other words, if one location is 2.00 milligauss (200 nanotesla) stronger than another location, the Earth Magnetometer will indicate that difference to accuracy of +/- .01 milligauss or 1 nanotesla.

This magnetometer has a magneto-resistive sensor, which uses the spins of electrons flowing through a circuit, rather than their charge, to measure magnetic field. This type of device most closely resembles a “flux gate” magnetometer, but the magneto-resistive technology is newer, smaller, less expensive and intrinsically more stable over temperature.

Measuring the Earth's Magnetic Field

Geomagnetometers, such as this meter, are usually used to measure the strength of earth's field at several points on the land surface. These points are sometimes arranged in a square grid (with spacings such as 10 x 10 feet). In this case, a measurement is recorded at every point on the grid, and a map can be made from these numbers. The map might show for example, that the field is stronger in a circular area about 20 feet in diameter in the center of the grid than it is at the edges of the grid. This would suggest that a magnetized object is buried roughly 20 feet underground.

Instead of measuring on a square grid, measurements are sometimes made at several points along a straight line. The relative strength of field is measured at each point (relative to the field strength at the starting point). If there is an area with a stronger (or weaker) field than average, then the place along that line where the field is strongest (or weakest) can be found with the meter. Then measurements are made along a straight line that is perpendicular to the original line, starting at that strongest (or weakest) point on the original line. If you're looking for the location of the strongest (or weakest) field, this method is faster than measuring every point of a grid.

There is one complication involved with measuring the field strength: magnetic field is a vector, which means it has both a strength (“magnitude”) and a direction. At each point (or position) that a measurement is taken, both the strength of field and the direction may be different from that of the previous measured point. Most geomagnetic mapping is concerned with the strength only, and not with the direction. When using the more expensive meters such as proton magnetometers, the direction cannot be determined, because these types of meters measure the strength only. (In comparison, our Earth Magnetometer is capable of showing the field direction, but this additional information is difficult to quantify, because it would require that you measure the angle between the sensor rod and the vertical direction. A measurement like this could be done using a plumb-bob.)

All types of geomagnetic meters require that you point them, at least approximately, in a certain direction (with respect to the earth's field direction). Otherwise the readings will be inaccurate. With a proton magnetometer, the sensor can be pointed as much as 30° off from the preferred direction before the readings start to become noisy. It's easy to keep the sensor pointed with +/- 30° of a certain direction when measuring in the field, so this is not much of a problem. However, the AlphaLab Earth Magnetometer is a vector magnetometer, and it must have its sensor pointed (at least momentarily) within one-third of a degree of the correct field direction in order to measure the actual field to .01 milligauss precision. This kind of precision required in pointing the sensor had been a problem when using vector magnetometers to measure the exact earth field strength. A “fluxgate” magnetometer (which is also a vector magnetometer) could have been available a long time ago as a less expensive alternative to the proton magnetometer, had it not been for the accuracy of pointing needed.

The expensive geomagnetic field meters now in use measure the magnitude of the field and are accurate if pointed roughly within +/- 30° of a preferred direction (the “preferred direction” is actually a certain amount of angle off-axis from the earth's field direction, but that detail is not important here). These meters will read the same number if pointed anywhere within that +/-30°. In contrast, vector magnetometers, like fluxgate or the AlphaLab Earth Magnetometer, use sensors that read a field strength that is correct only when the sensor is pointed in the same direction as the field; if pointed in a different direction, they will read a lower number. The number they read is the actual field strength multiplied by the cosine of the angle between the sensor direction and the actual field direction. For example, if the actual field is 500.00 milligauss, and a vector sensor is pointed in exactly the direction of the field, it will read “500.00” (subject, of course, to the sensor's accuracy). If the sensor is 1/3 of a degree off from the correct direction of the earth's field, it will read 500.00 milligauss x cos(1/3°) = 499.99 milligauss (about 1/500 percent low). If the sensor is 3 degrees off, it will read 500 x cos (3°) = 499.31 milligauss, or about 1/7 percent low. The sensor will always read low unless pointed in exactly the direction of the field. In theory, this fact could be used to find the actual field strength. You could just look at a digital display of the field strength that the sensor measures, and then “tweak” the direction of the sensor until you see the highest possible strength. If the sensor is correctly pointed in the direction of highest field, then any direction you tip the sensor away from this correct direction will make the digital display read a lower number. That “highest” number is in fact the true strength of the field. The problem with this method is that it is slow—it may take literally a minute to determine the field strength this way.

The Earth Magnetometer solves this problem by using an analog meter (needle-type) to tell you if you're tipping the sensor closer to or farther from the correct position. This analog meter does a continuous auto-zero to keep changes as small as .01 milligauss clearly visible while still being able to handle a several hundred milligauss dynamic range. The digital display shows the highest number that occurred (since the last reading) and holds that number. Therefore, if at any time, the sensor was briefly pointed within 1/3° of the correct direction, the digital display will show the correct strength of the field and will hold that number until it's time for the next reading. The correct reading will continue to be held even if you point the sensor in a completely wrong direction. To take the next reading, just press the “reset” button. A reading can be done every 5 seconds this way.

This is not as fast as the more expensive geomagnetic meters; they generally require one second to make each reading. (Each measurement is a timed internal process, but some of the more expensive versions can read faster than once a second). However, one reading every 5 seconds is not extremely slow, because it usually requires 5 seconds or more to walk to the next measurement position.

The Earth Magnetometer also has a rapid scan function. This is only sensitive effectively down to 1 milligauss (but theoretically down to 0.1 milligauss, or 1/50 percent of the earth's field). It allows you to walk along, continuously reading the field. You can see “blips” as fast a 1/10 second.

The Earth Magnetometer also has an “offset” function that adjusts the display to a reading of zero +/- .01 milligauss at whatever time you want to. Then you can measure how many nanoteslas stronger or weaker the field is, compared to your starting point. (This offset function only works on the more sensitive range, not the rapid scan range).

Accuracy

This meter has maximum error of .5% of the reading +/- .5 milligauss. Because .5% of 500 milligauss is 2.5 milligauss (=250 nanoteslas), this may seem to be a problem when reading changes in the earth's field from one point to the next with precision of .01 milligauss (= 1 nanotesla). However, if you're measuring how much the field varies from point to point, this amount of variance is accurate to +/-0.5% of the variance. For example, if you use the offset controls to set your first point at “0.00” milligauss, and you measure the next point as “-1.53” milligauss, then that next point really is 1.53 +/- .01 milligauss weaker (note “minus” sign) than the field at the first point. A change of up to 2.00 milligauss is actually correct +/- .01 milligauss, which is just the rounding error of the number. A change of 4.00 milligauss is accurate to +/- .02 milligauss. When doing these variance or differential measurements, the .5 milligauss maximum offset error will cancel itself and so does not introduce any inaccuracy. The other source of error is due to temperature change of the sensor; that error is +/- 1 milligauss/°C (.0056 milligauss/°F) at maximum. The biggest problem with measuring the earth's field is that some magnetism is created in the ionosphere, which is a layer of charged particles near the top of the atmosphere. There is no way to distinguish between the small contribution from the ionosphere and the big contribution from the earth; all types of meters simply read the sum of both fields. The earth's field stays fairly constant, but the ionosphere's field varies randomly from minute to minute. The variation is faster during the day when the sun is contributing solar wind to the ionosphere. A typical variation between day and night is +/- .30 milligauss. Typical random fluctuation during the day is .10 milligauss in one hour, and during the night it is .03 milligauss in an hour. However, these numbers can be much higher during strong solar activity. To see plots of these variations at specific locations, look up “magnetogram” on the internet. These are usually plotted in nanoteslas.

Fortunately, the variation contributed by the ionosphere is fairly uniform over distance on the surface of the land. That is, if the ionosphere is currently adding .13 milligauss at a certain location, it will probably be adding .13 +/- .01 milligauss everywhere else within a square kilometer, because the ionosphere is roughly 100 kilometers up. This is sufficiently distant that the ionosphere cannot create "high-resolution" magnetic patterns on the land (with significant changes between points separated by only 1 km). This means that if you need an accurate grid map of how the field varies from point-to-point, go back to the reference point and re-measure it periodically, no matter which type of geomagnetic meter you're using. The actual map of variance of the field from point-to-point remains the same from day to night, even though the field strength at every point may rise or fall. This is because the field strength at all locations rises or falls by the same amount from hour to hour. If you periodically return to re-measure the same point, you can make a graph of how much variation the ionosphere is actually contributing. This will clarify how reliable the measurements are.

Earth Magnetometer (complete with battery & probe rod): Price $700 (US dollars). BUY IT NOW

Price includes shipping in the US. For shipping to Canada by USPS add $20 (US dollars). Outside the US and Canada add $40 (US dollars). 

Warranty for this meter is one year. Made by AlphaLab, Inc. (USA).

Earth Magnetometer (DC Milligauss Meter) Instructions

  1. Assemble the 2 half-rods into a single straight rod (but assembly into an "L" shaped part is also possible and may be more convenient to use if the local field direction is almost horizontal). "Capture" the multi-colored ribbon cable near the center and top of the rod with the clear plastic spirals. The white sensor block should be near the white pointed end of the rod (the "bottom" of the rod).
  1. Set all three offset knobs to zero. The upper knob is in 12 "clicks"; the lower two are continuous.
  1. Turn the MILLIGAUSS RANGE to 1999.9, and put the white pointed end of the rod on the ground. You'll find that there is a certain direction to point the rod so that the digital display reads a maximum positive number. This maximum number is the true magnitude of the Earth field (+/- .5% of reading +/- .5 milligauss) at that location. If you are in the southern hemisphere, you may need to slide the sensor block off the rod and slide it back on in the reverse direction in order to make the number read positive when the rod is vertical.
  1. If you press (and release) the black button, the analog display "holds" the field strength at that instant; whenever the field at a later time is higher than the "hold" number, the needle will go to the right of center. Whenever you rotate the rod so it "sees" lower field than at the time you pressed the button, the needle will go left. The digital display, however, is not affected by the button (in this 1999.9 range).
  1. Use this needle to locate the direction of maximum field. Press (and then release, of course) the button when you're pointing the rod near the maximum field direction, and then change the direction of the rod while looking at the needle to find where the needle points most to the right. The needle is most sensitive to changes in field when the needle is close to center position. To find the direction of maximum field, press the button a few times as you get closer and closer to the maximal direction. This will keep the needle near the center. Watching the needle like this can help you find the azimuth and elevation directions of the rod to maximize the field easily. You are pointing the rod in the maximal-field direction if when you increase or decrease elevation angle (like latitude on a globe), or if you increase or decrease azimuth (like longitude on a globe), the needle will go more toward the left. That is, the needle goes more left with any change in direction of the rod. Imagine the white top of the rod being capable of sweeping out the shape of a half-sphere (centered at where the bottom of the rod touches the ground). Imagine this half sphere as the northern hemisphere of the Earth (even though it's not really oriented in same direction as the Earth). The "latitude" on this half sphere is the elevation angle (0 - 90°) and the "longitude" is the azimuth. With the top of the rod touching this imaginary half-sphere at an arbitrary latitude and longitude, imagine the two ways that the top point of the rod is capable of moving on that half-sphere: A tipping motion (elevation) which moves it up and down in latitude, and a motion which looks like an arc of a circle, as seen from above (azimuth), which moves it back and forth in longitude, but at fixed latitude. If the rod is pointed to the exact "latitude" and "longitude" (elevation angle and azimuth angle) for maximum field, you can push the button that resets the analog display's zero, and any motion away from the correct elevation and azimuth will make the needle travel to the left.
  1. While still in the 1999.9 range, click the COARSE OFFSET knob so that at the maximum field, the display is made to read a lower number. It must read a positive number that is between 000.0 and 199.9 (and preferably between 000.0 and 100.0). Usually, "-4" is selected on the knob. This represents only approximately a 400 milligauss subtraction. The actual subtraction that occurs when you set the knob at "-4" may be something like "-413.86", but it will always be that number, even if you measure again days later.
  1. Switch the range to 199.99. This causes the digital and analog displays to act slightly differently from the 1999.9 range.
  1. Press the black button. This resets the digital display now, and this display now shows and holds the highest (in the positive sense) number that the sensor was exposed to since the most recent button push. The analog display is also slightly different; it can only go as high as the center line now. The analog display is showing the current field strength minus the highest field strength since the last button push. Therefore, the analog display can never read higher than the center line. This is because, by definition, the "maximum" strength is always at least as high as the current strength.
  1. Press the button, and then start at 90° elevation (rod straight up) and tip approximately toward the direction of maximal field. You'll see the analog display stay near the center line as you get closer. The digital display will continue to read a higher and higher number. When you finally overshoot the maximal field direction, the needle will then go to the left, and the digital display will stop increasing and hold its number. If you reverse and go back up in elevation, you'll find an elevation where the needle is highest (but this will be just below the center line). Then hold the rod at that elevation angle and vary the azimuth angle. This will look as though the top of the rod is moved through a circular arc, as seen from above. Stop at the azimuth angle that leaves the needle at the closest to the center line, and try varying the elevation again while holding at constant azimuth. After some practice, you'll be able to find this point in about 5 seconds per reading. This "point" is a circle about one inch (2.5 cm) in diameter on the imaginary half-sphere (or half-globe). If the top of the rod passes through any part of this one inch imaginary circle, the digital display will lock onto the correct strength of the Earth field within .01 milligauss, and will hold that reading, even if the rod is re-pointed to a different direction.
  1. The bottom two knobs will subtract an adjustable amount from the digital display. The MEDIUM OFFSET will subtract up to slightly more than 100 milligauss, and the FINE OFFSET will subtract up to about 5 milligauss. These two lower offsets only work on the 199.99 range, and not on the 1999.9 range. You can adjust the held digital number down to zero by using these two knobs. Then you can compare the field at any other location to this field. You can also use the knobs to display absolute field, except that the top digit will be missing. For example, if in the 1999.9 range, you made a measurement of the absolute field at 513.6 milligauss, then you went to the 199.99 range and measured the (relative) field at 19.26, you can adjust the bottom two knobs to make the held number read "13.60", which is the actual field, but without the "5" (500) digit. This allows the display, which can read only up to 20,000 counts, to show a high number in high resolution, like "(5)13.60". In order for this to work correctly, the COARSE OFFSET when using the 199.99 range, would have to be set at -5 (or perhaps -4). This is what would approximately subtract the "500" milligauss from the reading. However, use COARSE OFFSET at "0" when on the 1999.9 range. Remember that whenever you use the 199.99 range, the COARSE OFFSET must be adjusted so that the display is not over-range while the button is being pressed and while the rod is pointing in the approximate direction of maximum field. Therefore, the COARSE OFFSET knob will generally be set to -3, -4, or -5 when on the 199.99 range.
  1. There is a fast-scan function. Set the range on 1999.9 and the COARSE OFFSET to 0. (The other two offset knobs don't affect the reading on the 1999.9 range). Walk in the east-west direction to do this fast scan. Face east or west and hold the rod near the center, where it is balanced. Tip it to the direction which reads maximum field, and then press (and release) the button. The analog display will help you find the direction. You might want to press the button a few times as you get closer to the correct direction. Once you've found the correct direction for maximum field, it's fairly easy to keep holding the rod pointing in that direction (+/- about 1 milligauss) as you walk east or west. As you walk along, both the analog and digital displays will show the magnetic deviations from average. You will need to find the maximum field direction again every once in a while. You would have to tip the rod (accidentally) about 4 degrees off maximum direction to get the reading to drop 1 milligauss. Holding the rod to +/- 4 degrees is relatively easy.
  1. Interpretation: Buried magnetized (or at least magnetizable) objects may either increase or decrease the field strength in the vicinity. Generally, directly over a buried object, the deviation of field strength is at a maximum. For example, the field may be “1.62” milligauss directly over an object, compared to an average around “0.00” in the field surrounding the object. As you approach this object, the deviation becomes larger and larger, peaking at -1.62, which is directly over it. A certain distance away from this peak is a zone on the ground where the deviation is only about half as much, or -0.81 milligauss. (This “zone” will actually be approximately a circle surrounding the object). The depth that the object is buried is generally between 1 1/3 and 2 times the radius of this circle, or 1 1/3 to 2 times the distance between the point where the deviation is maximum, and any point where the deviation is half that much.

Contrary to popular belief, underground water generally does not produce any magnetic field. This is true whether or not the water is flowing. The only way that water can produce a (DC) magnetic field is if (DC) electric current is flowing through it. Without any DC current, the presence of water can only occasionally be inferred in instances where the underground stream of water displaces magnetic minerals; in other words, if the “absence” of magnetic minerals is atypical of the area.

Certain objects, such as cell phones, GPS devices, boot zippers, and key chains, may be magnetized. If these are too close to the sensor block, the readings may be affected. You can check to see if anything you're wearing will affect the sensor (and how close you have to get to affect the sensor). Set the rod against a wall pointing in approximately the direction of maximum field. Set the RANGE on 1999.9 and look for any changes in the display as you walk closer or move the questionable objects near the sensor block. Remember that the orientation of the objects in space (if held up-down, or east-west, etc) will affect the amount of the reading. In the field, keep these objects at least twice as far away as the distance required to make the display change by 0.1 milligauss. The battery in the meter may also affect the sensor block, but it will not if kept at least 2 ½ feet away from the sensor.

You can make high-resolution instantaneous readings to resolution .01 milligauss, by disabling the peak-hold function. To do this, just press and continue to hold the button.

Although this meter has a maximum inaccuracy of +/- .5% of reading +/- .5 milligauss, actual shift with temperature is under .01 milligauss (= 1 gamma = 1 nanotesla) per °C. Therefore when reading the difference between points in the earth field, this difference is accurate to +/- .5% and the offset due to temperature will generally change slower than natural variations in the earth field, which is typically about .30 milligauss from day to night. When LOW BATTERY is displayed, slide off the back door and change the alkaline battery. Battery life is about 8 hours with an alkaline.

For information or to place an order :

AlphaLab, Inc. - 1280 South 300 West - Salt Lake City, UT 84101


HOME