Engineering And Environmental Applications Of The Potential Field Methods Of Geophysics.

Gravity and magnetic methods are potential field methods and used for a wide range of applications and scales in geosciences. Traditionally, they have been used for regional, large scale investigation of geological structure, such as basement relief and fault structure delineation. Despite the initial contribution of gravity methods to oil/petroleum exploration by detection and mapping of salt domes, “gravity data” is viewed primarily as a “reconnaissance tool in oil exploration” and “to provide constraints in seismic interpretation.” (quotes from Telford et al.,<br>1990) A familiar, smaller-scale (higher resolution) application of the gravity and magnetic methods is for ore body detection and mapping. The key geological factor required for<br>application of the gravity and magnetic methods is lateral variation in physical properties of subsurface materials. If the only variation in physical properties is in the vertical direction, such<br>as is the case for horizontally layered, homogeneous strata, the gravity and magnetic methods are applicable only for confirmation of the uniformity and lack of anomalous conditions.<br>Engineering and environmental (near-surface) applications of the gravity and magnetic methods were limited prior to the 1960's. Equipment developments and the emergence of new<br>classes of high priority applications have brought about a resurgence of interest in the potential field methods of geophysics and the development of high-resolution, high-accuracy field<br>procedures. The new classes of applications can be summarized as environmental (hazardous and toxic waste site assessments), archaeological, abandoned mine lands reclamation, cavity and tunnel detection and mapping, site investigations in karst areas, seismotectonic investigations, and other specialized problems. The gravity and magnetic methods are noted for two facts, one positive and the other somewhat negative: (1) the methods, which rely on measurement of variation in a naturally occurring potential fields, are fundamentally non-invasive and nondisturbing; (2) the interpretation of survey results is complicated by the fundamental, inherent nonuniqueness or ambiguity of source determination. The inherent ambiguity of gravity and <br>magnetic interpretation often requires the exercise of more geological insight and constraining direct information, such as from boreholes, for quantitative interpretation than required by many<br>of the other geophysical methods (although in fact interpretation of all geophysical survey results involves varying degrees of nonuniqueness and ambiguity). For purely qualitative, anomaly<br>mapping applications, the gravity and magnetic methods often can be used when other geotechnical methods are not effective. And, with apologies for not including other near-surface<br>geophysical methods that are legitimately potential field methods, e.g., direct current resistivity and self-potential, this paper only addresses gravity and magnetic methods and their engineering and environmental applications.