Geophysical Borehole Logging:

Geophysical borehole logging involves gradually lowering a probe down a borehole, while the probe measures a physical property of the surrounding rock or soil. Probes can be designed to measure any one of a variety of physical properties. Since the measured physical property is related to the composition of the surrounding rocks and soils, borehole logs can be used to map the subsurface.

The array of available probes, designed to measure different properties, enables borehole logging to be used in a wide variety of applications, where boreholes are present.

NGA most commonly utilizes induction (electrical conductivity) logs. Natural gamma logs, temperature logs, vertical flow meters, and caliper (borehole diameter) logs are also utilized. Those logs can be run in a 2 inch diameter PVC cased hole.

Conductivity logs measure the electrical conductivity of the soil or rock surrounding the borehole. They provide a detailed measure of changes in conductivity with depth. The electrical conductivity of soil or rock (and its reciprocal, electrical resistively) depends on the porosity, groundwater conductivity, degree of saturation, clay content, and other bulk soil properties. Hence it is a useful tool in determining the changes with depth of any of these properties. When integrated with other information, such as natural gamma logs, lithologic logs, or other geophysical logs, the geologic (or hydrogeologic) cause of the conductivity log response can often be deduced.

The principles of operation of borehole induction logging are illustrated in the figure above. Faraday's law of electromagnetic induction states that an oscillating magnetic field has an associated electric field. A small transmitter coil in the borehole probe creates a primary magnetic field. That magnetic field creates a toroidal electric field in the material surrounding the borehole (soil or rock), which in turn creates electrical "eddy current" flow within the soils. The strength of those eddy currents depends on the conductivity of the material. The eddy currents, in turn, create a magnetic field, which is detected and measured with a receiver coil within the probe. Within the normal range of operation of the borehole equipment, the quadrature signal (i.e., the signal 90° out of phase with the primary field) is proportional to the conductivity of the material surrounding the borehole (soil or rock). Most induction loggers, including the Geonics EM-39, have additional "focusing" coils to sharpen the vertical resolution of the probe. The EM-39 probe is most sensitive to materials in an annulus 20 cm to 100 cm from the borehole. Hence the effects of drilling mud and grouting surrounding the casing are minimized. The EM-39 has an intercoil spacing of 0.5 m. This provides a vertical resolution of somewhat less than 0.5 m. Layers as thin as 0.1 m can be detected, if they have sufficient conductivity contrast with the adjacent layers. Increased definition of vertical layering can be attained by modeling of the geologic section to obtain a "best-fit" to the data.