Standard resistivity tools, such as the deep laterolog and the deep induction log, generate an electromagnetic wave which operates in a frequency range of 35 to 20,000 hertz (where hertz = cycles/second). At these frequencies, the predominant influence on the wave is the conductivity of the substance it is traveling through. In a reservoir, conductivity is strongly influenced (of course) by the salinity of formation water.

At higher frequencies in the 30 megahertz (million hertz) to 1.1 gigahertz (billion hertz) range, the dielectric properties of a substance become very important to wave propagation (Hilchie, 1982). High dielectric constant values are associated with polar compounds like water. Since water is a polar compound, it requires energy to orient all its dipoles. Thus, an electromagnetic field moving through water is weakened. Rock matrix and hydrocarbons are both non-polar compounds with very low dielectric constants, and weaken an electromagnetic field less than water does.

The Gearhart and Dresser Atlas logs measure dielectric constant and operate at frequencies of 30 MHZ and 47 MHZ, respectively. Schlumberger's log is called an electromagnetic propagation tool (EPT)† and operates at 1.1 gigahertz. It measures the propagation time of the electromagnetic wave by reduction in wave amplitude and by shifts in the phase of the wave (Serra, 1984).

Table 7 lists dielectric constants and propagation times for various materials. It is apparent that water has a much greater wave travel time and dielectric constant than any other material on the list. Because of this, the dielectric log can be used to detect water-versus hydrocarbon-bearing zones (for a detailed discussion of the dielectric and EPT† logs, see Serra, 1984; Dewan, 1983; and Hilchie, 1982).