Magnetometry Methods

8.1.1 Magnetometry Method Introduction

Magnetometry is a passive remote sensing method that records the magnitude of Earth's local magnetic field at a sensor location. The Earth's overall magnetic field is a dipole field with the North Magnetic Pole and the South Magnetic Pole acting much like the ends of a bar magnet. There are secondary regional and local variations to the primary dipole field caused by soils and objects with different magnetic properties located above, on, or beneath the ground surface. Historically, the oersted and gamma have been the common units for measuring variations in Earth's magnetic field, and some of the older magnetometry data are displayed in these units. However, more recently, the common geophysical unit of measure for a magnetic field is called a tesla, where one tesla is equal to 104 oersteds, or 109 gammas. The Earth's magnetic field intensity varies between 0.25 and 0.65 oersteds (25,000 to 65,000 nanoteslas [nT], or 25,000 to 65,000 gammas).

Magnetometers are passive remote sensing devices that measure the magnitude of Earth's local magnetic field at the location of the sensor. Magnetometers may be placed on the ground surface, in the air (airborne magnetometer), in satellites (satellite magnetometry), or beneath the surface of Earth (borehole magnetometry). In practice, for agricultural purposes, handheld magnetometers are used with the sensor positioned within a couple meters of the ground surface. Commonly used magnetometers measure The Earth's local magnetic field with a precision between 0.1 nT and 1.0 nT. To put this sensitivity into context, a magnetometer operator cannot wear glasses, zippers, or other objects containing ferromagnetic metals (Clark, 1996; Gaffney and Gater, 2003), because these objects are likely to interfere with measurements.

8.1.2 Types of Magnetometers

Fluxgate and optically pumped magnetometers are the two most commonly used instruments. Proton precession magnetometers, a third type, are rarely used due to long data acquisition times. Fluxgate magnetometers use a nickel-iron alloy core surrounded by a primary and secondary wire wrapped around the core. The core is magnetized by the component of Earth's magnetic field parallel to the core and by an alternating current in the primary winding. The alternating current in the primary winding creates an alternating magnetic field, which induces a current in the secondary winding that is proportional to Earth's local magnetic field. Fluxgate magnetometers have a 0.1 nT precision and fast acquisition times but require careful alignment of the sensor to avoid introducing anomalies (Scollar et al., 1990).

Optically pumped magnetometers commonly use cesium vapor and the Zeeman-effect to measure the magnitude of the Earth's local magnetic field. The Zeeman-effect arises when atoms containing a magnetic moment are placed in an external magnetic field. An oversimplified model of such an atom is a bar magnet with a north-south dipole. In the absence of an external magnetic field, the bar magnet has the same energy regardless of its alignment. In the presence of an external field, a bar magnet will become reoriented to align with the external magnetic field. The aligned state of the magnet has a lower energy than the nonaligned state. The difference in energy between these two states is related to the magnitude of the external magnetic field. Optically pumped magnetometers have a 0.1 nT precision, acquisition times of ten readings per second, and do not have the fluxgate alignment issues. Because both instruments use different methods of measuring Earth's magnetic field, they have the potential to record different data when used at the same site (Scollar et al., 1990).

Both fluxgate and optically pumped magnetometers can be used in single mode or gradient mode. In single mode, a single sensor is used to record the magnetic field. Because the background magnetic field varies with time, some means of compensating for these background changes (called temporal variations) must be incorporated into the design of each magnetometry field survey. Compensation is achieved by removing the temporal magnetic field variations from the data collected during a magnetometry field survey, and the temporal magnetic field variations are determined by maintaining a separate continuously recording stationary magnetometer at a base station or by reoc-cupying a fixed location periodically (e.g., every few minutes) throughout the time of the survey. An alternative is to use two magnetometers that are horizontally or vertically separated (Figure 8.1) in a setup called a gradiometer. The difference found between the magnetic fields recorded by each sensor is divided by the sensor separation resulting in the gradient of the magnetic field. Because the gradient is a spatial measurement that can be considered to be independent of time, there is no need to compensate for temporal variations in the Earth's magnetic field. Additionally, the gradient

FIGURE 8.1 Magnetometry field survey employing the gradient mode for data collection.

is a measure of how rapidly the magnetic field is changing, resulting in an enhanced view of the shallow subsurface.

8.1.3 Magnetometry Field Principles

Data are collected discreetly or continuously while moving along transects. In discreet mode, the magnetometer is held stationary over a position along the transect while the magnitude of Earth's local magnetic field is recorded. In continuous mode, the magnetometer moves at a normal walking pace along the transect taking approximately ten readings every second. At a normal walking speed, this corresponds to a reading approximately every 10 cm. Transects are commonly separated by 25 to 50 cm when looking for human alteration of the landscape (Figure 8.1), and larger transect spacing when looking at geologic features. Data can be acquired in a unidirectional or bidirectional format. Unidirectional is where one travels along the first line from south to north, walks back to the start of the second line while not gathering data, and then gathers data along the second line while walking south to north. During a bidirectional survey, one collects data south to north along the first transect, turns around, and moves over to the second transect and gathers data while walking north to south.

8.1.4 Magnetometry Data Processing and Analysis

Magnetometer surveys produce x, y, and z data, where x and y are horizontal locations, and z is the magnitude of the vertical component (fluxgate) or the total field (optical pumping) of Earth's local magnetic field. The first step in postsurvey analysis of magnetic data is to examine the data using mapping software such as Golden Software's Surfer, Geoscan's Geoplot, or Geosoft's Oasis Montaj. These programs allow one to present the data as contour, image, shaded-relief, and surface plots. Large changes in the magnetic field caused by highly magnetic materials should be identified and "despiked." Large magnetic changes create scaling problems that obscure smaller changes of interest, and despiking removes any magnetic features above or below a selected value (Figure 8.2). Another filter can aid in removing large geologic trends.

FIGURE 8.2 Example of the "despiking" process to remove a large magnetic field anomaly shown in top plot, thereby enhancing some of the more subtle magnetic field features that are present as shown in bottom plot.

Additional defects needing attention are drift, edge matching, striping, positional errors, and periodicity due to walking gait. Drift is caused by thermal changes within the sensor and appears as a slow temporal increase or decrease of the baseline magnetic field. Edge-matching is the process of matching the magnetic baseline at the end of one survey unit to the beginning of the next. Because there is some time delay between surveying each unit, the magnetic baseline changes due to natural fluctuations caused by the Sun (Figure 8.3). Both of these defects can be removed in the field by surveying in gradient mode or eliminated postsurvey using software. Striping appears during bidirectional surveys with every other transect having a slightly higher or lower magnetic baseline when compared to its neighbor (Figure 8.3). This defect becomes large if the magnetometer operator accidentally carries ferrous metals on their person. As the operator walks in one direction, the magnetometer is located in front of the operator and sits in one end of the operator's magnetic dipole. (The operator will have a magnetic field similar to a bar magnet.) When the operator turns around, his or her dipole does not rotate with him or her due to the operator's magnetic field being induced by and aligned with the direction of Earth's magnetic field. As the operator walks in the opposite direction, the magnetometer sits in the other side of his or her magnetic dipole. Because the Earth's magnetic field is not parallel with the ground surface, the magnetometer will record a different magnetic baseline when in the operator's north pole compared to the south pole. Positional errors arise due to a range of sources such as changing walking speed, reaction time, and computer timing. These errors give the data a zig-zag look. Periodicity is caused by the natural up and down motion when walking and appears as highs and lows in the magnetic baseline on a regular interval related to the operator's stride (Gaffney and Gater, 2003; Scollar et al., 1990).

Gradiometer Gamma Homemade

FIGURE 8.3 A magnetometry data collection effort in the field is often broken up into separate survey units that together cover a study area. The data from the individual survey units are later merged together to produce a magnetic field map for the entire study area. Edge-matching is the process of matching the magnetic baseline at the end of one survey unit to the beginning of the next. Bidirectional surveys can produce striping, with every other transect having a slightly higher or lower magnetic baseline when compared to its neighbor.

FIGURE 8.3 A magnetometry data collection effort in the field is often broken up into separate survey units that together cover a study area. The data from the individual survey units are later merged together to produce a magnetic field map for the entire study area. Edge-matching is the process of matching the magnetic baseline at the end of one survey unit to the beginning of the next. Bidirectional surveys can produce striping, with every other transect having a slightly higher or lower magnetic baseline when compared to its neighbor.

8.1.5 Magnetometry Agricultural Application Example

The Oregon State University Research Dairy in Corvallis, Oregon, sits on 73 ha of working land 2.4 km west of the main campus. The dairy contains 40 registered Jersey cows and 130 registered Holstein cows and houses 20 to 140 various aged heifers. Near-surface geophysical surveys were conducted on a 1 ha portion of a pasture that is clay-tile drained with the most recent installation approximately forty years ago. This field was selected for study because excessive levels of Escherichia coli were found in adjacent Oak Creek that seemed related to the method of spraying liquid effluent as part of the nutrient and manure management strategy. Successful mapping of agricultural drainage systems was essential in understanding the relationship between effluent spraying and creek contamination.

Drainage pipe locations could not be imaged using a 500 MHz ground-penetrating radar antenna due to extreme attenuation of the signal caused by soils with high clay content (Rogers, 2003). Drainage pipe locations were successfully mapped using a Geometrics G-868 optically pumped gradiometer. A 100-meter-square study area was divided into twenty-five 20-meter-square subunits using nonmagnetic polyvinyl chloride (PVC) stakes to mark the subunit corners. Nonmagnetic (fiberglass) survey tapes marked the subunit perimeter, and nineteen blaze-orange, 0.95-gauge plastic weed trimmer line were used to mark north-south running transect lines spaced every meter. The magnetic survey was bidirectional with the sensors separated by 0.5 m in a vertical gradient mode. Under ideal conditions, establishing the survey grid and conducting the aforementioned magnetic survey could be accomplished by an experienced team of three in 4 to 5 days. Golden Software's Surfer 7 (Golden Software Inc., 1999) was used to create a shaded-relief of the plot with no postacquisition processing (Figure 8.4). An iron water pipe, the clay tile drainage system, and other magnetic signals spatially associated with subsurface objects are evident in the shaded-relief plot (Rogers et al., 2005).

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