Resistivity Methods
Resistivity methods are numerous, almost too many to count. We will discuss the most common here and present example surveys of some of these methods in appendices of this manual. Most methods can be divided into either “soundings” or “profiles”. Soundings are sometimes referred to as VES; Vertical Electrical Sounding. Profile methods can be simple or more complex, such as the dipole-dipole technique, which can provide both profiling and sounding information.
Azimuthal surveys can be used to locate faults, fractures and dikes. Square Array surveys have been used for fracture studies and especially in archeological surveys. The MiniRes can be used for all of these methods.
In vertical soundings, a series of field observations are taken, each successive observation has the electrodes at greater separation. The center of each observation remains the same. With the separation small, the apparent earth resistivity of the shallow earth is determined. With the separation greater, the apparent earth resistivity of the shallow and intermediate depth is determined. The resistivity of the intermediate earth can be determined by correcting for the already determined shallow earth resistivity. The electrode separation continues to be increased and the apparent earth resistivity of greater and greater depth is determined.
The two most common methods of electrical soundings are the Wenner method and the Schlumberger method. They will be discussed in the next few pages.
The qualitative interpretation of soundings may be made by examining a plot of apparent resistivity versus electrode separation. Traditionally LOG-LOG graphing is used. Quantitative interpretation has gone through an evolution culminating in computer forward and automated reverse modeling and interpretation. Several commercial programs are available. L and R Instruments has Windows and DOS versions of RESPAC, a computer program for forward modeling, automated reverse modeling and constrained reverse modeling.