Application of borehole geophysical methods to shallow groundwater investigations

Abstract

Borehole geophysical methods have been used extensively for petroleum and mineral exploration; but due to differences in environment and survey objective, modifications of both equipment and interpretation are necessary for shallow groundwater applications. These applications require the use of complementary borehole measurements which can be related to the formation properties by comparison to core samples. Different interpretation strategies must be developed for different environments, strategies for use in unsaturated, saturated with clay present, saturated without clay present, and hydraulically anisotropic environments are developed. Parameters that can be determined are porosity, moisture content, hydraulic conductivity, groundwater velocity, cation exchange content, and pore fluid conductivity. All logging instrumentation measures a property over a short interval of the well. It is important when combining logs to insure that all logs used in the comparison are vertical averages of the same portion with respect to both depth and length of the well. It is also possible to reduce the effect of the vertical averaging by the instrumentation through the use of numerical techniques. A field example is used to demonstrate these methods. The techniques presented make it practical to determine a formation hydraulic properties on a scale of a few tens of centimeters. This high resolution distribution of the formation in turn permits the development of a class of contaminate transport models which utilize the high resolution description of the aquifer variability. These models do not require large scale dependent dispersity terms to match field data because the contaminate movement is realistically described by shear flow in the high resolution advective flow field of the model. A one dimensional model is presented as an example of this class of model.