Fundamental Theory And Instrumentation Of Electrical Logging

The standard electric log of today consists of two basic types of recordings. On the left side of the log appears the spontaneous potential of the bore hole. Usually this is one curve although under special conditions the same curve can be presented in different scales. The right hand side of the log is devoted to recordings of the electrical resistivities of the formations as encountering in the bore hole. There are various systems of measuring resistivities but those most commonly employed are multi-electrode "normal" and "lateral" resistivity recordings. Generally the logs consist of two normal resistivity curves of different electrode spacings and one lateral curve. Any of these three resistivity curves can be presented in several scales to cover the full range of resistivities encountered in any particular bore hole. Of course, the spontaneous potential and the various resistivity curves are simultaneously recorded against a depth scale. All modern logging equipment can also simultaneously record the curves on at least two depth scales. The most commonly accepted depth scales are 2 in. = 100 ft and 5 in. = 100 ft. Occasionally a depth scale of 1 in. = 100 ft is recorded. On some specialized services such as the Microlog or Dipmeter an expanded depth scale of 25 in. =100 ft or 60 in. = 100 ft is frequently used. In very rare cases the expanded scale of 120 in. = 100 ft can be recorded. The Spontaneous Potential Curve The SP log (spontaneous or self-potential log), is a record of the naturally-occuring potentials measured in the mud at different depths in a drill hole. Usually, the SP log consists of a base line, more or less straight, having excursions or peaks to the left (Fig. 1). The base line corresponds in most cases to shales, whereas the peaks are generally opposite permeable strata. The shape and the amplitude of the peaks may be different, according to the formations, but there is no definite correspondence between the magnitude of the peaks and the values of permeability.