232 DEEP BOREHOLE SURVEYS AND PROBLEMS 



1. The region quite close to A, where the presence of the 

 borehole full of water and the dimensions of the electrode 

 A cause a certain disturbance. 



2. The region away from A where the equipotential 

 surfaces are affected by the earthing at B or the non- 

 homogeneity of the soil (metal casing of the hole, etc.). 



In particular, the two equipotential surfaces S and S' 

 passing through the points M and N are spheres of known 

 dimensions by reason of r and r\ These spheres intersect 

 the column of water without noteworthy distortion. The 

 measurement of potential between the electrodes M and N 

 immersed in the water is, therefore, equivalent to a measure- 

 ment made in the interior of the soil at the same distances 

 r and r' from the electrode A. 



The application of Ohm's law between the spheres S 

 and S' leads to the formula: 



i2 = 47r ^ • ~— (15) 



I r — r 



which gives the required resistance, since aV, r and r' are 

 known. ^ When the soil in the vicinity of the measuring 

 field AMN cannot be regarded as homogeneous in structure 

 the computations become more involved but nevertheless 

 furnish results that are sufficiently correct for practical 

 purposes. 



An advantage of this type of equipment is its portabihty 

 and the speed with which a well can be surveyed or logged, 

 the entire equipment weighing less than 3,000 lb. The sur- 

 veying can be done at the rate of 3,000 ft. per hour when the 

 machine is in position.^ A chart is made on special 

 paper wound on synchronized drums for lowering the cable 

 with electrodes and taking the record. As these electrodes 

 are withdrawn to the surface, readings are made at chosen 

 intervals of 5 to 40 ft., dependent on local conditions 

 and desire for information. The uncased part of the hole 



1 Lancaster-Jones, E., The Earth Resistivity Method of Electrical 

 Prospecting, Mining Mag., June and July, 1930. 



2 BiGNELL, op. cit. 



