A Survey of Engineering Geophysics Capability and Practice in the Corps of Engineers

The Repair, Evaluation, Maintenance, and Rehabilitation REMR Research Program Work Unit Geophysical Techniques for Assessment of Existing Structures and Structural Foundations was conceived to address the problems posed to standard geophysical methods by the presence of an existing structure at a site. The problems posed by an existing structure are geometrical and physical. A few examples to illustrate these problems will suffice. The presence of concrete structures or paved access roads defeat or greatly complicate the application of standard geophysical survey methods. Standard interpretation methods used for interpreting electrical resistivity surveys, for example, assume that the surveys are conducted on the surface of a halfspace. How then is a resistivity sound conducted on the crest of a dam to be interpreted Changes in direction of the center line of a dam, levee, or highway or railroad embankment complicate the conduct and interpretation of standard engineering geophysics methods such as electrical resistivity and seismic refraction, which require long, colinear electrode or geophone arrays. The presence of a structure such as a dam moves the points of application of surface geophysical methods further from the foundation materials and hence geometrically attenuates the signatures of normal or anomalous conditions in the foundation. Also, the presence of the structure physically attenuates and distorts signatures and complicates their recognition because of vertical and horizontal variation within the structure. These factors are just examples of problems which must be dealt with or overcome when applying engineering geophysics to existing structure sites.