MODERN PROSPECTING

Evening Star, Issue 21515, 13 September 1933, Page 1

MODERN PROSPECTING

DR FDCKEN SPEAKS OF GEOPHYSICAL METHODS Taking as his subject ‘Modern Methods of Prospecting—Geophysical,’ Dr C, M. Focken, 8.M.E., addressed the meeting of the Otago Institute in the University physics lecture room last night. Dr F. J. Turner presided. By way of introduction; the speaker said that prospecting for minerals by geophysical methods was a comparatively receiit advance in applied science, and now played an important part in the domain of economic geology. Although, nearly all the important developments in applied geophysics had taken place recently, yet one of the seeds from which the science grew was the Chinese seismoscope, invented by Choko in 136 a.d. x Geophysical prospecting was based on established scientific principles, and therefore was clearly dissociated from a group of ''inexplicable prospecting methods to which properly belonged the divining rod. Geophysical methods for determining what lay beneath the surface at relatively shallow depths without digging or boring had been developed primarily in the laboratory, and subsequently in the field, where they had been subjected to many rigorous tests. As they all depended upon the physical properties of the minerals knd surrounding rocks, the essential condition for success was that there should be a detectable Contrast in at least one of these properties. Most mineral deposits possessed one or more characteristic properties, such as high density or electrical conductivity. Hence it was possible by a careful application of suitable methods under favourable conditions to establish reliable information regarding the position, form, and size of a deposit, even when it lay at a considerable depth. Tectonic features, also, which were likely to be .associated with mineral concentrations, such as faults, anticlines, and, salt domes, could frequently he investigated by the characteristic anomalies—i.e., changes in physical properties, to which they gave rise. Many precious minerals did not occur in sufficient quantities or concentrations to be detectable directly by geophysical methods. Gold was the most conspicuous example. Even in such cases the mineral could'sometimes bo tracked by its association with other minerals present in bulk or with special horizons or structures. The physical properties upon which the principal methods depend were those of magnetic susceptibility, electrical conductivity, density, and elasticity. These methods were, therefore, appropriately termed the magnetic, theelectrical, the gravitational, and the seismic methods respectively. Methods depending on radio-activity and temperature, gradient in the surface layers had been used to a limited pxtent. Three successive steps were common to the general procedure in all methods. TU variation at the surface of some selected physical effect was first carefully measured and mapped, and then the sub-surface distribution of the corresponding physical property was determined. The third step, which had proved, the greatest barrier-to success, was the interpretation of the geological structure which most probably corresponded with the pre-deterrained distribution. The advisability of operating several independent geophysical methods over the same area had been demonstrated many times. Under favourable conditions most geophysical methods could be used to depths of several thousand feet, but they were more usually employed for depths of less than I,oooft. By means of numerous slides and diagrams the lecturer explained in a comparatively simple way the physical principles underlying each of the important geophysical methods of prospecting. In doing so, he drew attention to tho limitations and characteristic advantages of ekeh of the methods, and mentioned certain! successes they had achieved. Four chief electrical methods were dealt with —viz., the spontaneous polarisation, the resistivity, the equipotential, and the electromagnetic method. A complete seismic recording apparatus which was being constructed in the physics department was exhibited. By means of it and a hot-wire microphone the detection of small shocks was clearly demonstrated.

Geophysics, said the lecturer, was not a magic method for revealing all that was valued within the earth’s crust. Like a new method of diagnosis, it must be used by experts with discrimination) and, even under such favourable conditions, the results were often negative. The role of geophysics as a corrective measure against the abusive practices often common in the initial stages of a mining venture was one which .should be of increasing importance. Even in its present state of development it filled a definite need in the preliminary exploration stage of a mining venture, ami its practical value was being recognised more and move by mining engineers and geologists. In some cases its proper use would enable a logical plan of development to be formulated and carried through. Dissatisfaction with the achievements of geophysical prospecting had been due to a misconception of its capabilities and limitations, or to a poor application of its technique, or to a faulty perspective as to its proper place in geological work.

Much remained to be done in this subject in order to put it on a more secure basis and to determine more certainly the scope of its applications. They had in the past been far too dependent on the brains of German scientists, and the enterprise, initiative, and financial resources of American mining engineers, who had sought to buy and apply the newest ideas and instruments available in Germany. Considerable interest had now been awakened in British countries, and a chair of geophysics had recently been established at the Imperial College of Science in London. It was to bo expected that British workers would take a more active part in future progress, and that more use would be made of the methods in those parts of the ■ Empire where economic gains were likely to follow. The lecturer surveyed the geophysical investigations which had been carried out in the dominions, and by means of a set of slides, lent by Dr Marsden, he gave some idea of the nature and

scope of these activities in New Zealand. A vote of thanks to the lecturer was carried by acclamation.