SECTIONAL TRANSACTIONS.—A. 329 
Friday, September 25. 
Discussion on Geo-physical Methods of Prospecting, with special reference 
to Instruments. (Prof. A. 8. Evr, C.B.E., F.R.S.; Prof. A. O. 
RANKINE; Prof. Mrntrop, Seismic Methods; Mr. Karu SuNDBERG, 
Electrical Methods; Mr. A. Broucuton Enpcr, Spontaneous Polarisa- 
tion Method ; Dr. J. H. Jones, Portable Seismographs ; Mr. LANCASTER 
Jonzs, Gravity Methods ; Capt. Saw, Magnetic Methods.) 
Mr. A. B. Broveuton EpGEe.—Some Recent Developments in Electrical 
Prospecting Instruments. 
During the past twelve months steps have been taken in this country, under the 
auspices of the Department of Scientific and Industrial Research and with the 
co-operation of British instrument manufacturers, to produce a series of reliable 
electrical prospecting instruments and the auxiliary equipment required for their use 
in the field. The experience gained in Australia during 1928-30 by the Imperial 
Geophysical Experimental Survey has been applied to this purpose, particularly with 
regard to details of design which are essential for work carried out under rigorous field 
conditions. Amongst the instruments already completed the following possess features 
of interest :— 
Spontaneous Polarisation Method.—¥or the investigation of naturally occurring 
electrical phenomena, such as arise in the neighbourhood of certain types of sulphide 
ore-body, a portable D.C. potentiometer and non-polarising electrodes have been 
designed. The potentiometer has two ranges of 0-50 and 0-100 m. volts, is self- 
standardising, and can be used on a tripod without any special attention to levelling. 
The non-polarising electrodes, with which contact is made with the ground, are either 
of the usual copper-copper sulphate type or else are of silver immersed in a chloride 
or sulphate electrolyte—the salts of potassium or sodium being employed as a rule. 
The electrolyte is contained in a porous pot, and by preparing it in the form of a jelly 
the electrode may be kept in use for long periods without requiring attention. For 
valuable advice concerning the preparation and characteristics of silver-sodium 
chloride electrodes special acknowledgment is due to the Admiralty Research Depart- 
ment at Teddington. 
Resistivity Method.—It is commonly held that for the measurement of earth 
resistivities it is preferable to employ alternating rather than direct current, and in 
view of the natural earth potentials and the electrode polarisation e.m.f.’s that have 
to be contended with when D.C. is used, there would seem to be good grounds for this 
view. In practice, however, the use of A.C. or reversed D.C. has its disadvantages, 
one of which is that the resistivity values derived are dependent on the frequency 
employed, and in some cases upon other factors which are not always appreciated. 
D.C. measurements are free from such objections, and can be made without difficulty 
if the potential electrodes employed are of the non-polarising type and if all extraneous 
potentials are balanced before the current circuit is closed. The potentiometer 
described for the spontaneous polarisation method is provided with a special auxiliary 
os ae for this purpose, and has been found to be fully effective for resistivity survey 
work. 
Ratiometer Method.—Hitherto one of the principal difficulties experienced in surface 
potential electrical surveys, in which A.C. is used, has been to take proper account of 
the phase displacements which arise in the neighbourhood of concealed conductive 
deposits, and which are of prime importance when making a search for the actual 
position and extent of the latter. In order to overcome this difficulty a simple form 
of A.C. bridge or ratiometer has been devised which serves a variety of useful purposes. 
Potential and phase anomalies may be investigated along selected lines of traverse 
or a complete picture of the surface potential distribution may be obtained over a 
large area. In the latter case two distinct series of lines are followed over the ground 
surface and are known as equipotential (E.P.) and equiyuadralure (E.Q.) lines respec- 
tively. These lines may either be traced directly over the ground with the ratiometer 
or they may be derived from observations made with the same instrument along a 
closed series of traverses. Since such a survey is so conducted that the potential 
spacing of the lines in each series is known, it follows that the surface ellipse at any 
