Equipotential-line and potential-profile methods.

Equipotential lines of the current

When a source of electrical energy is grounded at two points, an electrical field is produced. Distortions of this field result from the presence of bodies of different conductivity; good conductors will attract the lines of flux, and vice versa. As it is difficult to survey these lines of flux, lines of equal potential, that is, lines along which no current flows, are mapped instead. In practice power is supplied to two grounded electrodes from an alternating current generator.

Resistivity methods.

Equipotential-line methods, while useful for the mapping of vertical or steeply dipping geologic bodies, are not suited to the investigation of horizontally stratified ground. Conversely, resistivity methods are applicable to depth determinations of horizontal strata and the mapping of dipping formations.

In resistivity procedures not only the potential difference between two points but also the current in the primary circuit is observed. The ratio of potential difference and current, multiplied by a factor depending on electrode spacing, gives the resistivity of the ground.

Potential-drop-ratio methods. The essential feature of the resistivity methods is a determination of the potential difference between two points at the surface and a measurement of the current in the external circuit. In potential-drop-ratio methods current measurements in the external circuit are not made and the potential drops in two successive ground intervals (represented by three stakes arranged in a straight line, radiating from one of the power electrodes) are compared. The potential-drop-ratio method is best suited for the location of vertical formation boundaries (faults, dikes, veins, and the like).

Electromagnetic-galvanic methods. Electromagnetic methods of electrical prospecting differ from potential methods in that the electromagnetic field of ground currents and not their surface potential (electric field) is measured. They fall into two major groups: (1) electromagnetic- “galvanic” methods in which the primary energy is supplied by contact as in the potential methods; (2) electromagnetic-“inductive” methods in which the ground is energized by inductive coupling (with insulated loops). To supply electrical energy to the ground by contact, line electrodes are laid out at right angles to the strike, point electrodes parallel with the strike.

Electromagnetic-inductive methods. In inductive procedures power is supplied to the ground by insulated loops which will cause induction currents to flow in subsurface conductive bodies. An advantage of inductive methods is the ease with which power may be transferred into the ground when the surface formations are poor conductors. Since currents induced in the subsurface conductors are dependent on frequency, interpretative advantages may be gained by regulating the frequency.

Radio methods. Since radio methods employ frequencies still higher than the high-frequency-inductive methods, they are subject to the same limitations. In one group of radio methods the effect of subsurface conductors on the emission characteristics of a transmitter is observed. In a second group a receiving arrangement is employed in addition to the transmitter, and the variation of field intensity with location is measured. In the category of radio methods belong the so-called “treasure-finders.” These are portable instruments for the location of shallow metallic objects, pipe lines, and the like.

(C.A. Heiland. Geophysical Exploration. New York, 1940)

57. Fill in the gaps with correct prepositions.

1. Electrical energy may be supplied __ the ground __ contact or __ induction.

2. Frequently the first two groups are combined ___ one group __ potential methods; the electromagnetic methods are usually subdivided ___ galvanic-electromagnetic and inductive-electromagnetic.

3. Four frequency bands may be used __ connection ___ alternating current electrical prospecting.

4. The application __ high radio frequencies is limited owing __ their lack __ depth penetration; __ greatest importance are the audio frequencies and the low frequencies.

5. One unpolarizable electrode is moved ____ the current vanishes.

6. The potential-drop-ratio method is best suited ___ the location __ vertical formation boundaries.

7. To supply electrical energy __ the ground __ contact, line electrodes are laid out __ right angles __ the strike, point electrodes parallel ___ the strike.

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