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KIRTLAND AIR FORCE BASE ALBUQUERQUE, NEW MEXICO ...

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APPENDIX B<br />

SOP B1.8<br />

Surficial Geophysical Surveys<br />

Many types of geophysical surveys are available to assist in the overall analysis of a site, including<br />

resistivity, magnetometry, electromagnetic induction (EMI), seismic refraction, and ground penetrating<br />

radar (GPR). The measurement of subsurface resistivity is applicable to determine and provide indirect<br />

information on the porosity and permeability of subsurface materials, degree of saturation, subsurface<br />

materials, subsurface lithology, dissolved electrolyte plumes, and presence of buried wastes. The<br />

application of magnetic surveys centers on confirming the presence or absence of buried ferromagnetic<br />

objects. EMI can be useful for locating and mapping subsurface utilities, trenches, buried objects, and<br />

contaminant plumes. Seismic refraction is an effective method for the determination of the depth and<br />

thickness of subsurface geologic layers, including the depth to bedrock or water table. GPR can be a<br />

powerful tool for locating and mapping metallic and non-metallic items such as buried drums,<br />

foundations, non-containerized wastes, underground utilities, and many other artifacts.<br />

At Kirtland AFB, it is anticipated that the geophysical methods to be employed will be limited to the<br />

investigation of buried wastes and plume delineation through magnetometry surveys, EMI, and GPR.<br />

These methods are described in this section. Should it be advantageous to use resistivity or refraction<br />

techniques at one or more of the sites, these will be discussed in detail in project-specific addenda.<br />

In the EMI method, the electrical conductivity of a geohydrologic section is measured by transmitting a<br />

relatively high-frequency electromagnetic field into the earth that produces eddy currents. These currents<br />

generate secondary electromagnetic fields that are detected by a receiver. The eddy currents are induced<br />

in the earth by an above-ground transmitter coil, and the resulting secondary electromagnetic fields are<br />

coupled to an above-ground receiver coil. Thus, EMI measurements do not require direct ground contact,<br />

and surveys across a line or area may be performed quite rapidly.<br />

Magnetometry consists of measuring local variations in the earth‘s magnetic field along a traverse or<br />

across an area on the surface. Because the intensity of the earth‘s magnetic field depends in part on the<br />

magnetic susceptibility of subsurface materials, knowledge of variations in field intensity provides an<br />

indication of variations in the distribution of materials with different magnetic susceptibilities.<br />

GPR operates on the principle of time-domain reflectometry, in which the difference in strength and the<br />

time delay between a transmitted electromagnetic pulse and its reflection from an object are measured.<br />

The time delay (t) is directly related to the propagation velocity of the electromagnetic waves (v) and to<br />

the distance between the transmitter and the object (D) as follows:<br />

t<br />

2D<br />

v<br />

Because GPRs are normally used at or near the surface of the ground, the distance D corresponds to the<br />

depth of buried targets that scatter the radar signals.<br />

Borehole geophysical surveys utilize acoustic, electrical, magnetic, electromagnetic, and passive and<br />

natural radiation source methods to determine the geohydrologic characteristics surrounding a borehole.<br />

Data acquired with appropriate QA/QC procedures are repeatable and comparable over different time<br />

periods, which provides the opportunity to evaluate changes over time. Often times the log suite selected<br />

investigates a larger borehole volume than can be evaluated using only the drill cuttings or samples,<br />

therefore, characteristics of the medium outside of the borehole diameter can be inferred.<br />

Kirtland AFB<br />

SOPs for Field Investigations B-65 April 2004

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