Geophysical Methods Applicable To Agriculture

Geophysical methods can be classified as passive or active. There is no artificial application of energy with passive geophysical methods. On the other hand, active geophysical methods do require the artificial application of some form of energy. The three geophysical methods predominantly used for agricultural purposes are resistivity, electromagnetic induction, and ground-penetrating radar. All three of these predominantly employed methods are active, and each is summarized within this book; resistivity in Chapter 5, electromagnetic induction in Chapter 6, and ground-penetrating radar in Chapter 7. Chapter 8 provides shorter descriptions of three additional geophysical methods: magnetrometry (passive), self-potential (passive), and seismic (active), all of which have the potential for substantial future use in agriculture, but at present are being employed sparingly or not at all for agricultural purposes. To provide an introduction, the six geophysical methods— resistivity, electromagnetic induction, ground-penetrating radar, magnetometry, self-potential, and seismic—are all concisely defined as follows.

1.2.1 Resistivity Methods

Resistivity methods measure the electrical resistivity, or its inverse, electrical conductivity, for a bulk volume of soil directly beneath the surface. Resistivity methods basically gather data on the subsurface electric field produced by the artificial application of electric current into the ground. With the conventional resistivity method, an electrical current is supplied between two metal electrode stakes partially inserted at the ground surface, while voltage is concurrently measured between a separate pair of metal electrode stakes also inserted at the surface. The current, voltage, electrode spacing, and electrode configuration are then used to calculate a bulk soil electrical resistivity (or conductivity) value.

1.2.2 Electromagnetic Induction Methods

Electromagnetic induction methods also measure the electrical conductivity (or resistivity) for a bulk volume of soil directly beneath the surface. An instrument called a ground conductivity meter is commonly employed for relatively shallow electromagnetic induction investigations. In operation, an alternating electrical current is passed through one of two small electric wire coils spaced a set distance apart and housed within the ground conductivity meter that is positioned at, or a short distance above, the ground surface. The applied current produces an electromagnetic field around the "transmitting" coil, with a portion of the electromagnetic field extending into the subsurface. This electromagnetic field, called the primary field, induces an alternating electrical current within the ground, in turn producing a secondary electromagnetic field. Part of the secondary field spreads back to the surface and the air above. The second wire coil acts as a receiver measuring the resultant amplitude and phase components of both the primary and secondary fields. The amplitude and phase differences between the primary and resultant fields are then used, along with the intercoil spacing, to calculate an "apparent" value for soil electrical conductivity (or resistivity).

1.2.3 Ground-Penetrating Radar Methods

With the ground-penetrating radar (GPR) method, an electromagnetic radio energy (radar) pulse is directed into the subsurface, followed by measurement of the elapsed time taken by the radar signal as it travels downward from the transmitting antenna, partially reflects off a buried feature, and eventually returns to the surface, where it is picked up by a receiving antenna. Reflections from different depths produce a signal trace, which is a function of radar wave amplitude versus time. Radar waves that travel along direct and refracted paths through both air and ground from the transmitting antenna to the receiving antenna are also included as part of the signal trace. Antenna frequency, soil moisture conditions, clay content, salinity, and the amount of iron oxide present have a substantial influence on the distance beneath the surface to which the radar signal penetrates. The dielectric constant of a material governs the velocity for the radar signal traveling through that material. Differences in the dielectric constant across a subsurface discontinuity feature control the amount of reflected radar energy, and hence radar wave amplitude, returning to the surface. As an end product, radar signal amplitude data are plotted on depth sections or areal maps to gain insight on below-ground conditions or to provide information on the position and character of a subsurface feature.

1.2.4 Magnetometry Methods

This geophysical method employs a sensor, called a magnetometer, to measure the strength of the Earth's magnetic field. Anomalies in the Earth's magnetic field indicate the presence of subsurface features. An anomaly is produced when a subsurface feature has a remanent magnetism or magnetic susceptibility that is different from its surroundings. A gradiometer is an instrument setup composed of two magnetometer sensors mounted a set distance apart. Gradiometers are typically used to measure the vertical gradient of the magnetic field, which is not affected by transient magnetic field changes. In comparison to a single magnetometer sensor, the gradiometer has the additional advantage of being better adapted for emphasizing magnetic field anomalies from shallow sources.

1.2.5 Self-Potential Methods

Self-potential methods collect information on a naturally occurring electric field associated with nonartificial electric currents moving through the ground. Unlike resistivity methods, no electric power source is required. Naturally occurring electric potential gradients can arise a number of different ways, including the subsurface flow of water containing dissolved ions, spatial concentration differences of dissolved ions present in subsurface waters, and electrochemical interactions between mineral ore bodies and dissolved ions in subsurface waters. Self-potential methods are fairly simple operationally. All that is required to obtain information on a natural electric field below ground is the voltage measurement between two nonpolarizing electrodes placed or inserted at the ground surface.

1.2.6 Seismic Methods

Seismic methods employ explosive, impact, vibratory, and acoustic energy sources to introduce elastic (or seismic) waves into the ground. These seismic waves are essentially elastic vibrations that propagate through soil and rock materials. The seismic waves are timed as they travel through the subsurface from the source to the sensors, called "geophones." The energy source is positioned at the surface or at a shallow depth. Geophones are typically inserted at the ground surface. Seismic waves move through the subsurface from source to geophone along a variety of direct, refracted, and reflected travel paths. The velocity of a seismic wave as it travels through a material is determined by the density and elastic properties for that particular material. Differences in the density and elastic properties across a subsurface discontinuity feature control the amount of reflected or refracted seismic energy, and hence the seismic wave amplitudes returning to the surface. Information on the timed arrivals and amplitudes of the direct, refracted, and reflected seismic waves measured by the geophones are then used to gain insight on below-ground conditions or to locate and characterize subsurface features.


1.3.1 Selecting the Proper Geophysical Method

A clear goal must be defined in the initial planning stage of a geophysical survey regarding the soil condition or subsurface feature information that needs to be acquired. In order to choose the proper geophysical method for monitoring changing soil conditions, consideration must first be given to the different physical properties responded to by the various geophysical methods and then whether any of these physical properties are influenced by the soil condition of interest. Delineating a subsurface feature with geophysics requires there to be a contrast between the feature and its surroundings with respect to some physical property responded to by a geophysical method. To summarize, the geophysical method selected must respond to a physical property that is in turn affected by temporal changes in soil conditions or the spatial patterns of subsurface features; otherwise, useful information cannot be obtained on these soil conditions or subsurface features of interest. For example, soil cation exchange capacity (CEC) will often have a substantial impact on soil electrical conductivity (or resistivity); therefore, resistivity or electromagnetic induction methods that measure soil electrical conductivity may be useful for delineating spatial patterns in CEC. On the other hand, magnetometry methods respond to anomalies in remanent magnetism or magnetic susceptibility, properties that are not likely to be affected by CEC, and consequently, magnetometry methods would not be a good choice for delineating spatial patterns in CEC.

1.3.2 Investigation Depth and Feature Resolution Issues

Once a geophysical method is chosen, there are usually options with respect to the equipment and its setup. The investigation depth required and the size of the feature to be detected are two important issues that should be taken into account when deciding on the equipment to use and its setup. There is normally a trade-off between the investigation depth and the minimum size a feature must have to be detected. Finding a large, deeply buried object or a small, shallow object with geophysical methods is much easier than locating a small, deeply buried object. One potential example is the use of GPR to locate buried plastic or clay tile agricultural drainage pipe. The radar signal penetration depth and minimum size at which an object can be detected are both inversely related to GPR antenna frequency. Low-frequency GPR antennas are better for locating larger deeply buried objects, and high-frequency GPR antennas are more applicable for small, shallow objects. Therefore, a GPR unit with 100 MHz transmitting and receiving antennas might work well at finding a 30 cm diameter drainage pipe 2 m beneath the surface in a clay soil, and a GPR unit with 250 MHz transmitting and receiving antennas is likely capable of finding a 10 cm diameter drainage pipe 0.5 m beneath the surface in a clay soil. But, finding a 10 cm diameter drainage pipe 2 m beneath the surface in a clay soil is probably an extremely difficult undertaking regardless of the GPR antenna frequency employed.

An important implication with respect to the issues of investigation depth and feature resolution (detection) is to use equipment with the proper setup that provides an investigation depth similar to the investigation depth of interest. Using an equipment setup with an investigation depth substantially greater than the investigation depth of interest results in the minimal size for feature resolution being increased over what would be the case if the equipment investigation depth coincided with the investigation depth of interest. Additionally, by using an equipment setup with an investigation depth substantially greater than the investigation depth of interest, a problem could arise of not being able to determine whether a detected feature is located within the investigation depth of interest or at a deeper level. An equipment setup investigation depth substantially less than the investigation depth of interest means that features positioned between the equipment setup investigation depth and the depth of interest will not be detected. For example, when a resistivity survey is employed to map lateral changes within a well-developed soil profile, a specific electrode array length might be chosen to provide an approximate 2 m investigation depth. Significantly shorter or longer electrode array lengths than that selected for a 2 m investigation depth would respectively produce investigation depths much less or much greater than 2 m, thereby producing information that does not include the entire soil profile (short electrode array length problem) or information where it is difficult to determine whether resistivity changes occurred within the soil profile or at a greater depth (long electrode array problem). Finally, there are instances where small, deeply buried features are unlikely to be detected, and therefore, time and expense should not be wasted conducting a geophysical survey.

1.3.3 Field Operations: Station Interval, Stacking, Survey Line/Grid Setup, and Global Positioning System (gps) Integration

The distance is usually fixed or at least fairly consistent from one geophysical measurement location to the next along a transect, and this distance between measurement locations is referred to as the station interval. A short station interval provides a better chance for finding the smaller features that are capable of being resolved with the geophysical equipment used. Reducing the station interval has the downside of increasing the time needed to conduct a geophysical survey. Consequently, it makes sense to use the shortest station interval possible that still allows the geophysical survey to be carried out in the time allotted.

Often, several measurements are collected at each measurement location and then are added or averaged. This overall process is called stacking. Unwanted signals referred to as noise tend to be random and can thus be cancelled out by adding or averaging multiple geophysical measurements obtained at the same location. Although data quality is improved, increased stacking can slow the geophysical survey. Data collection procedures should be optimized to provide the greatest amount of stacking possible within the time frame during which the geophysical survey needs to be conducted.

For a larger subsurface feature where the general directional trend is known, a sufficient number of geophysical measurement transects should be oriented perpendicular to the feature's trend so as to better delineate the feature. A measurement transect parallel to a linear subsurface feature, but offset from it by sufficient distance, will in all probability not detect the feature. A geophysical survey grid covering a study area is commonly composed of either one set of parallel measurement transects or two sets of parallel measurement transects oriented perpendicular to one another. Setting up a geophysical survey grid composed of two sets of parallel measurement transects oriented perpendicular to one another reduces the risk of not finding long, narrow subsurface features, such as agricultural drainage pipes, whose trend prior to the survey is unknown. The spacing distance between adjacent transects is usually fixed at some constant or fairly consistent value for a particular set of parallel transects. This spacing distance should be set small enough, within reasonable limits, to avoid missing important features.

The integration of Global Positioning System (GPS) receivers with geophysical equipment is becoming more and more common, particularly with regard to agricultural applications. GPSs can provide accurate determinations of measurement locations while the geophysical survey is in progress. As a result of GPS integration, marking off a well-defined grid in the field is no longer required, thereby allowing rapid geophysical data collection over large areas, especially in regard to horizontal soil electrical conductivity mapping with resistivity or electromagnetic induction methods. The importance of GPS to agricultural geophysics will undoubtedly continue to experience growth in the near future; therefore, a detailed discussion on aspects related to GPS is certainly warranted and can be found in Chapter 9.

1.3.4 Analysis of Geophysical Data

Depth sections and contour maps are two of the most common geophysical data analysis end products. Two-dimensional depth sections characterize the distribution of some geophysically measured property beneath a measurement transect along the surface. Different geophysical methods employ different computer processing steps to produce these depth sections. Contour maps are typically used to show the horizontal spatial pattern of some geophysically measured property. Various spatial interpolation algorithms are employed by the computer software used to generate these contour maps. Where there is a choice, careful consideration is needed in selecting the interpolation algorithm so as not to introduce features on the contour map that do not truly exist or to remove features that are actually present.

Rather than focusing just on a single geophysical data set at a time, the integration of several geophysical data sets along with other spatial information is an approach that can potentially improve agricultural data interpretation for a particular farm site. Integration of multiple geophysical and nongeophysical spatial data sets is accomplished using a geographic information system (GIS). A GIS is a powerful data analysis tool that is just beginning to find widespread use in agricultural geophysics. Because GIS is expected to become essential to agricultural geophysics in the future, a detailed discussion on some important GIS elements is definitely relevant and is presented in Chapter 10.

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