General Considerations for Geophysical Methods Applied to Agriculture

Barry J. Allred, M. Reza Ehsani, and Jeffrey J. Daniels contents

1.1 Introduction: Geophysics Definitions, Development Chronology, Investigation Scale 3

1.2 Geophysical Methods Applicable to Agriculture 5

1.2.1 Resistivity Methods 5

1.2.2 Electromagnetic Induction Methods 5

1.2.3 Ground-Penetrating Radar Methods 6

1.2.4 Magnetometry Methods 6

1.2.5 Self-Potential Methods 6

1.2.6 Seismic Methods 6

1.3 Aspects of Agricultural Geophysics Data Collection and Analysis 7

1.3.1 Selecting the Proper Geophysical Method 7

1.3.2 Investigation Depth and Feature Resolution Issues 7

1.3.3 Field Operations: Station Interval, Stacking, Survey Line/Grid Setup, and

Global Positioning System (GPS) Integration 8

1.3.4 Analysis of Geophysical Data 9

1.4 Potential Agricultural Uses for Geophysical Methods 9

1.5 Agricultural Geophysics Outlook 13

References 14

1.1 introduction: geophysics definitions, development chronology, investigation scale

Geophysics can be defined several ways. In the broadest sense, geophysics is the application of physical principles to studies of the Earth (Sheriff, 2002). This general definition of geophysics encompasses a wide range of disciplines, such as hydrology, meteorology, physical oceanography, seismology, tectonophysics, etc. Geophysics, as it is used in this book, has a much more focused definition. Specifically, geophysics is the application of physical quantity measurement techniques to provide information on conditions or features beneath the Earth's surface. With the exception of borehole geophysical methods and soil probes like a cone penetrometer, these techniques are generally noninvasive, with physical quantities determined from measurements made mostly at or near the ground surface. (Note: Some large-scale airborne surveys are carried out with geophysical measurements collected by airplanes and helicopters positioned well above the surface, but these types of surveys are not within the scope of this book.) The geophysical methods employed to obtain subsurface information from surface-based measurements include resistivity, electromagnetic induction, ground-penetrating radar, magnetometry, self-potential, seismic, gravity, radioactivity, nuclear magnetic resonance, induced polarization, etc.

One of the first known instruments for geophysical measurement is a seismoscope invented in a.d. 132 by the Chinese philosopher, Chang Heng (Needham, 1959). This seismoscope reportedly had the capability to not only detect earthquakes, but could also determine the direction from which the earthquake originated. Many of the geophysical methods employed today originated or were more fully developed based on the needs of the mining and petroleum industries. In fact, present levels of worldwide production for minerals, oil, and natural gas could not have been achieved without the use of geophysics as an exploration tool.

The magnetic compass was used to find iron ore as early as 1640 (Dobrin and Savit, 1988). Robert Fox devised the self-potential method using copper-plated electrodes and a galvanometer to find copper sulfide ore bodies in Cornwall, England, during 1830 (Reynolds, 1997). Robert Thalen wrote On the Examination of Iron Ore Deposits by Magnetic Methods in 1879 and contributed to the invention of some of the first magnetometers (Telford et al., 1976). These Thalen-Tiberg and Thomson-Thalen magnetometers proved very successful for mineral prospecting in Sweden during the late 1800s. Initial development of resistivity and electromagnetic induction methods for the mining industry occurred between 1910 and 1930. Airborne magnetometers refined for submarine detection during the Second World War were employed shortly afterward to quickly prospect for minerals over large areas (Dobrin and Savit, 1988). The introduction of airborne electromagnetic surveys for mineral exploration also occurred shortly after the Second World War ended.

Dobrin and Savit (1988) and Lawyer et al. (2001) detail some of the early history involving initial applications of geophysical methods for the petroleum industry. A torsion balance field device for measuring anomalies in the Earth's gravitational field was refined by Baron Roland von Eotvos of Hungary in the late 1800s. Crude seismic methods were developed by the French, British, Germans, and Americans during the First World War as a means to locate enemy artillery positions. Torsion balance gravity measurements and fan-pattern seismic refraction surveys were then used to find oil fields associated with Texas Gulf Coast salt domes in the 1920s. Conventional seismic refraction methods introduced in 1928 to the Middle East were soon found to be particularly effective within Iran for locating limestone structures containing substantial oil reserves. J. C. Karcher conducted the first seismic reflection experiments from 1919 to 1921 and then demonstrated the potential of this geophysical method for oil exploration by mapping a shallow rock unit in central Oklahoma during 1921. The first oil discovery attributed to seismic reflection occurred during 1927 with the Maud Field in Oklahoma. Seismic reflection is the predominant geophysical method used for petroleum exploration today.

Although radar technologies were introduced during the Second World War, it was not until the early 1960s that ground-penetrating radar was first employed as a geophysical tool, initially to investigate the subsurface characteristics of polar ice sheets (Bailey et al., 1964). Archeological, environmental, geotechnical engineering, and hydrological geophysical surveys became more and more common in the latter half of the past century. There was some agricultural research activity in the 1930s and 1940s related to soil moisture measurement with resistivity methods (Edlefsen and Anderson, 1941; Kirkham and Taylor, 1949; McCorkle, 1931), but for the most part, the application of geophysical methods to agriculture did not gain momentum until the 1960s, and to a greater extent in the 1970s, with the use of resistivity methods for soil salinity assessment (Halvorson and Rhodes, 1974; Rhoades and Ingvalson, 1971; Rhoades et al., 1976; Shea and Luthin, 1961). Greater historical detail on the application of geophysical methods to agriculture is provided in Chapters 2 and 3 of this book.

Geophysical surveys conducted for petroleum, mining, hydrological, environmental, geotechni-cal engineering, archeological, and agricultural applications vary dramatically in scale with respect to the investigation depth of interest. Petroleum industry oil and gas wells have been drilled to levels 8 km beneath the surface based on information obtained from seismic reflection surveys. Most geophysical surveys conducted in the mining industry have an investigation depth of interest that is less than 1 km. There are, however, some deep mining operations extending more than 3 km below ground, and therefore mining geophysical surveys can occasionally require greater investigation depths down to 3 or 4 km. A geophysical survey conducted as part of a hydrological investigation to determine groundwater resources usually has an investigation depth no greater than 300 m. Geophysical investigation depths for environmental, geotechnical engineering, and archeological applications typically do not exceed 30 m. Agricultural geophysics tends to be heavily focused on a 2 m zone directly beneath the ground surface, which includes the crop root zone and all, or at least most, of the soil profile.

With regard to the application of geophysics to agriculture, this extremely shallow 2 m depth of interest is certainly an advantage, in one sense because most geophysical methods have investigation depth capabilities that far exceed 2 m. However, there are complexities associated with agriculture geophysics not typically encountered with the application of geophysical methods to other industries or disciplines. One such complexity involves transient soil temperature and moisture conditions that can appreciably alter the values of measured geophysical quantities over a period of days or even hours. Additionally, physical quantities measured in the soil environment with geophysical methods often exhibit substantial variability over very short horizontal and vertical distances.

0 0

Post a comment