Geophysical Logging is a method in which measurements are made from probes lowered into a well or boring (ASTM D5753-95). Each geophysical log responds to some physical, electrical or chemical parameter of the soil, rock, pore fluids or buried wastes.  Some logs will provide measurements from inside PVC or steel casing while others require an open hole.  Some logs will allow measurements to be made in the unsaturated zone as well as the saturated zone, while others can only be run in the saturated zone.  In contrast to driller’s logs or geologic logs (which are somewhat subjective), measurements with geophysical logs are non-subjective and virtually 100% repeatable.

While the resolution of surface geophysical measurements decreases with depth, the vertical resolution of geophysical logging in a single borehole is independent of depth.   Samples are obtained virtually continuously, providing a continuous vertical profile of geologic conditions.  Some borehole tools only image the borehole wall, while other logs sample a few inches to a few feet into the formation surrounding the borehole.   Logs that measure borehole fluids (temperature, conductivity and flow) are representative of the fluids within the borehole. Other logs (caliper, television, acoustic televiewer and borehole imaging) respond to the conditions at the borehole wall.  Nuclear tools provide 6 to 12 inches of penetration beyond the borehole wall, while electrical resistivity or induction logs sample a few feet into the formation.

Geophysical logging between borings (hole-to-hole or crosshole logging) can be used sample the properties between borings.  A common application of hole-to-hole logging is to obtain seismic P and S-wave velocities to calculate engineering properties of soils and rock for engineering design.  Other applications include locating fractures, cavities, or the continuity of soil or rock.  While seismic measurements are the most common hole-to-hole measurements.  Radar, EM and resistivity data can also be acquired in a hole-to-hole survey.  Surface to borehole measurements are also possible (e.g. vertical seismic profiling) if only one borehole is available for testing.