Geophysical studies provide an inverse solution to the resolution of environmental problems, and are analogous to a murder mystery investigation. In a murder investigation, we know a murder occurred, but don’t know the cause (i.e., the murderer or weapon). Environmental geophysics then is used to image a site (i.e., the murder scene) and to identify the target (i.e., the murderer and weapon). In this way, geophysical representations of the subsurface provide a fast and inexpensive method for understanding the environmental problems prior to complex and expensive intrusive methods.
Environmental geophysical tools exploit field conditions (passive) or measure induced conditions (active) and collect data in either a profiling or sounding method, with computers producing real-time images. Passive methods include measurements of acoustic, electrical potential, electromagnetic, thermal, radioactivity, gravimetric, and magnetic fields. The last three methods are the most commonly employed environmental methods.
Active or induced methods include acoustic, seismic disturbance, electric induction, and electromagnetic imaging. Seismic reflection is used on a routine basis to image deep in the earth; however, seismic methods can also be used to image the shallow subsurface. Resistivity is a method for profiling the subsurface using electric current. Electromagnetic methods (e.g., time-domain, frequency-domain) use a specific electromagnetic frequency to induce a secondary eddy field and are the most common tools used in environmental geophysical investigations. Ground penetrating radar, a pulsed electromagnetic tool, can provide detailed subsurface profiles.
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