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.

58. Fill in the gaps with the terms from the text.

1. In _______ procedures power is supplied to the ground by insulated loops which will cause induction currents to flow in subsurface conductive bodies.

2. __________________ are portable instruments for the location of shallow metallic objects, pipe lines, and the like.

3. The essential feature of the _____________ methods is a determination of the potential difference between two points at the surface and a measurement of the current in the external circuit.

4. _____________ methods are applicable to depth determinations of horizontal strata and the mapping of dipping formations.

5. Distortions of __________ field result from the presence of bodies of different conductivity; good conductors will attract the lines of flux, and vice versa.

6. The ______________ method is the only electrical method in which a natural field is observed.

7. _______ methods employ frequencies still higher than the high-frequency-inductive methods.

8. An advantage of ____________ methods is the ease with which power may be transferred into the ground when the surface formations are poor conductors.

59. State whether the following sentences are True or False; correct the false ones.

1. Two main groups of electrical methods can be distinguished.

2. The application of high-frequencies are limited owing to the lack of their resolving power.

3. Comparable to seismic refraction methods are self-potential and electromagnetic galvanic ones.

4. Comparable to seismic reflection methods are inductive ones.

5. Equipotential-line methods are suited to the investigation of horizontally stratified ground.

6. Resistivity methods are applicable to the mapping of vertical or steeply dipping geologic bodies.

7. 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.

8. To supply electrical energy to the ground by contact, line electrodes are laid out at right angles to the strike, point electrodes vertical to the strike.