Surface Geophysical Methods
Surface geophysical surveys investigate specific areas of land (i.e., sites) using geophysical equipment deemed most appropriate for the objective(s) that are defined during survey planning. A common objective is locating a geophysical target (e.g., a utility line or contamination). The method(s) most able to detect a target depend upon its hypothesized physical properties. Though a survey objective may be more comprehensive (e.g., understanding the hydrogeology for conceptual site model development) than mapping utilities, the method selection process is determined by the hypothesized properties of the target, which may be a discrete object or an earth process, such as groundwater - surface water interactions, and the survey objectives or goal of the investigation.
Generally, the geophysical equipment is moved about the ground surface within the confines of the site and measures certain physical properties at depth within the earth. For each survey, the surface location of the equipment is typically georeferenced, which can allow for the determination of the spatial distribution of subsurface properties. Surface geophysical methods can collect data and, thus, image the subsurface to a depth of investigation (DOI), which depends upon the method, instrumentation, and site properties.
Typically, at least two methods are selected for a single objective, and, ideally, the separate lines of geophysical evidence converge to support a single interpretation. Such approaches are analogous to medical procedures that use both an x-ray and MRI, which respectively image bone and soft tissue to more comprehensively understand an injury. Thus, the results produced by each geophysical method, which is chosen for its detection capability, are used in tandem to support interpretations.
Surface geophysical methods require little to no penetration into the ground. At most, an electrode, geophone, or similar sensor may be driven or placed within 12-24 inches (~ 30-60 cm) below the surface. Hence, surface geophysical methods are considered non (or minimally) invasive, which is often reflected in the associated time and financial costs.
Surface geophysical methods can be subdivided into five broad categories, which are based on physical principles. These, along with each category’s most commonly applied methods to shallow subsurface investigations for environmental studies, are as follows: