The Limitations of Geophysical Methods and the New Possibilities Opened Up by an Electrochemical Method for Determining Geological Formations at Great Depths

THE paper deals generally with the standard methods of geophysical survey, drawing attention to their several advantages and disadvantages. An outline is then given of a newly developed electrical method, based on the simultaneous measurement of an input current and the resultant polarisation current set up at the junction of two beds of different composition. These currents are separately recorded by galvanometers on a moving film against time marks accurate to 10-4 seconds. From an analysis of these records in terms of the time factor for different electrode spacings, determinations can be made as to the depth of the various formation junctions. The important features of the new methods are (a) An electrical method dependent, not on conductivity, but on the chemical composition of the strata. (b) Records can be obtained up to depths of 6000 feet with an input current of only 1-2 watts. (c) The screening effect which limited the older methods in many cases is almost entirely eliminated. The ultimate aim of geophysical prospecting methods. should be the direct locating of minerals. The development of such direct methods would mark a real advance of this science; great difficulties, however, have to be overcome before such a result could be achieved. Up to the present time, the different methods can only give additional information to the geologist, and although the value of this is now duly appreciated especially where geological evidence is sparse and uncertain, as where the strata to be investigated are covered with recent alluvium, drift sand or lava flows, this by no means represents finality from the geophysical standpoint. Each of the different geophysical methods depends for its success on one physical property of the strata and each has its own special possibilities and limitations which must be clearly understood. Disappointments are due not to any fault in principles but to the misapplication of methods through failure to recognise their limitations. The underlying principles of the methods are based on the classic laws of physics, but their success depends entirely on the interpretation of the results into correct geological meaning. In prospecting for oil, geophysical methods are used solely for elucidating hidden geological structures suitable for the accumulation of oil, and at the present time the direct locating of oil has not been possible. The methods are based on the differences existing between the density, magnetic susceptibility, elasticity and electric conductivity of the different strata. Some of the methods have already reached the limit of their scope as far as both the direct and indirect location of the oil are concerned. The Gravimetric Method reliably locates hidden structures, but does not give exact depth determinations. The Magnetic M