Engineers use ground-penetrating radar to study soil.
Several geophysical methods, and shallow drilling, are possible alternatives for ground-penetrating radar (GPR). GPR's uses include determining the depth of soil, soil strength, ice thickness and detection of voids in the subsurface, including covered pits. How appropriate a particular alternative is would depend on the physical properties of the target being investigated. Any geophysical method is able to detect a subsurface target only when at least one of its physical properties is different than that of the surrounding medium.
Ground-Penetrating Radar
A GPR survey unit consists of a transmitter-receiver antenna pair that emits high-frequency electromagnetic energy into the subsurface and then detects the reflections. The reflections arise when the transmitted rays hit an object with permittivity -- an electrical characteristic -- that is different than in its surroundings. Reflections recorded by the receiver are computer-processed to produce three-dimensional maps of the subsurface, accurate to a few feet. The depth of investigation is usually restricted to less than a hundred feet, except in ice, where it could be much deeper.
Geophysical Seismic
Geophysical seismic technology -- based on elastic property contrasts between the exploration target and the surrounding medium -- can be used as an alternate to GPR. The three-dimensional seismic method can produce results comparable to GPR in accuracy. The older and somewhat more primitive hammer-seismic method can only map the depth of the soil. Energy to probe the subsurface is generated usually by explosives but sometimes by hammer impacts. The reflected signals from the target are detected by an array of geophones and are then computer-processed to produce images of the targets.
Geophysical Electrical
Subsurface structure can be mapped by geophysical electrical methods because of the contrasts in the electrical resistivity, and other electrical properties, between geologic media. The resistivity method can probe to about the same depth as GPR but is far less accurate. However, it is quite efficient at detecting voids. Field equipment consists of a pair of electrodes that feed current into the ground and another pair that senses the voltage. Electromagnetic methods can also be substitutes for GPR for certain targets.
Drilling
Shallow drilling is an alternate to GPR.
GPR and other geophysical methods map large geological volumes, but the results can be ambiguous and imprecise. On the other hand, drill holes provide precise, accurate engineering, geological or hydrological information, but that data is restricted to the very small volume of the borehole. However, data from a properly planned array of shallow drill holes can be interpreted to cover large areas. Drilling is not only a substitute for GPR, but it is also an excellent follow-up to confirm the GPR interpretation.
Tags: computer-processed produce, depth soil, different than, drill holes, geophysical methods, pair that, physical properties
