Geological applications

7.1 INTRODUCTION A major use of geophysics in engineering investigations is as a tool in unravelling the subsurface geology. In this fundamental and early application, it is the geology that is the target, and engineering considerations and material properties are secondary. The simplest geological structure, which can be investigated, is a horizontal interface such as that between bedrock and overburden. Geological structures are rarely simple, and surveys have to be designed to tackle complex situations. Geological structure can be considered as lateral variation in the properties of the subsurface rocks. In its simplest form it might be represented by a single dipping or irregular interface such as a bedrock surface. In more complex situations it might include thickness changes, faults, folds, or igneous intrusions. The first section examines the measurement of depth to bedrock, as this is probably the most common boundary problem. Other geological structures are briefly considered under different types of geological hazard. 7.2 GEOLOGICAL BOUNDARIES 7.2.1 Depth to bedrock The question of the depth to bedrock and its measurement is a frequent problem for engineering site investigations and groundwater studies (Fig 7.1). However, the definition of what constitutes bedrock depends very much on the field of application. A geologist might define bedrock as the older consolidated rock formations lying below unconsolidated deposits (generally Pleistocene and Recent), but an engineer might define bedrock or engineering rockhead as the level at which the rock has adequate bearing capacity for large structures. Sometimes bedrock will be defined in a contractual