MODERN PROSPECTING

Hokitika Guardian, 1 November 1927, Page 1

MODERN PROSPECTING

MINERALS, OIL, WATER. METHODS OF OEOORAPI!IC\L WORK. MELROURXE. Oct, IS. An eminent, scientist. Dr R. Tvrahmann. has just, reached Australia. The purpose of his visit is to carry out a geophysical survey of an oil area in New Zealand. On his way through Australia to the Dominion he i.s to lecture upon the possibilities of geophysical work as applied to the prospecting for minerals, or in tracing out the occurrence of underground oil pools. Tn view of the coming of this scientist the question may bo asked wliat is meant by geophysical prospecting. Generally speaking surveying of the kind has beetr undertaken, more particularly in Europe. It dates back approximately, in the case of gravitometrie work, to about thirty years ago. and. with electrical methods, to a period of some five years. There are three principal methods relied on in carrying out such work—the electrical, the magnetic, and the gravitometrie. When it is desired to have an examination of a block of country with the object of ascertaining if it contains minerals, oil or water, certain delicate instruments are brought into use. Supposing the block of ground lie perfectly homogenous and does not contain any deposits of minerals. and that its physical properties arc perfectly uniform, then, it is he subjected to a current of electricity passing through it from one given point at the surface to another, there will lie no diversion in the course of such a current. If, however, there should ho a vein of ore, or a pool of nil, or water, then the current will no longer remain uniform. Tsy means of the instruments employed at the stirlace. it is possible to record these variations and determine where what may he termed the foreign hotly or formation exists. Tn other words, this nonuniformity can he measured by sensitivel instruments, and it i.s claimed that the occurrence of valuable material can therefore he detected. Differences in the specific gravity of rocks exist. That phenomenon is also availed of in the gravitometrie method to determine the presence of valuable materials lying below the surface. To do this a sensitive “ torsion balance ” i.s employed by the operator. TX OTHER LANDS.

This description is intended to give in a broad way the outlines of the tiiree principal methods of geophysical work. Already reports have heen made regarding examinations carried nut by scientists employing either of these methods in different mineral districts in various parts of the world It can he affirmed that through their investigations discoveries have unquestionably boon made. For instance, in Scandinavia a valuable deposit of iron ore was found. In another district, when a search was being carried out for deposits of pyrites, a copper lode was traced, whic has since been developed with considerable success. In the United States the claim is advnnceed that oil pools have been traced : also, that mineral deposits have heen found. As a result somewhat wide attention has been given there to geophysical work during the course of the last two or three years. The investigation carried out in the field has demonstrated that those who have to bo entrusted with the task of putting these new methods into practice have to be scientists of the highest training. It has also been empnsised that, despite the progress made, surveys of the kind have as yet to he accepted more as providing indications than ns furnishing exact data of the size or content of a hid-

den ore deposit. This was demonstrated not long ago in South Africa. Iron pyrites have high conductivity and therefore the sensitive instruments used by the operators at the surface will show u considerable variation when a vein of that mineral is traced. Tn South Africa a mining company was seeking to discover the continuance of certain zinc ore bodies. M’lien the record chart showed marked variations it was hoped that a shoot of zinc had been traced. Alining work, unlortunately, disclosed that the ore deposit consisted of a narrow vein of iron pyrites.

,Such a development shows not only that skilled operators shall be in control ol a survey, but that all work of the kind shall he regarded more or less as preliminary, not as furnishing exact data on the nature of any formation. Further, the differences to ho measured upon the instruments are exceedingly minute, and therciore necessarily call for the employment of unusually delicate instruments. The observer upon whom the responsibility of interpreting results has to he not only correspondingly qualified, hut he has also to be able to take into account any possible shortcomings of the method employed, it is not a matter of a mail holding a hazel twig in his hand and saying, “ AA r ator exists here,” hut of a deliberate mathematical survey in the carrying out of which high (lowers of observation have to he disclosed. These facts cannot be too strongly insisted upon. METHOD OF PROCEDURE.

Probably the programme to bo relied upon when field work is undertaken would be to select an area where a definite amount of geological examination lias already been undertaken. 'l'llis would mean that the inspection of blocks of country where it is almost certain that any search would be resultless would be avoided, and expense would thereby be saved because of their exclusion. Operators who have used the methods described suggest that geological and geophysical work should go hand in hand. They contend that it should not he assumed that the geophysicist is in any way the substitute for the drilling party, but that he ean certainly reduce the number of barren bores now being sunk with the object of defining ore shoots. In other words, geophysical surveying should in their opinion ho a potent agency in indicating to the prospector exactly where boring should he done, and particularly when oil is being sought. So far as the initiation of geophysical work here is concerned, a statement was recently made by the Prime Minister regarding a definite scheme. This arrangement is the result of negotiations conducted with the British Empire Marketing Board. The view of scientists and of leaders of the mining industry here is that sufficient geophysical work has been carried out in various parts of the world to show that these new methods have a definite value. Tn the view of the Empire Marketing Board and the Commonwealth Australia offers a promising field for the application oT geophysical methods. While it is realised that the work done will he of benefit to Australia. it is held that results will he of the utmost value to the Empire generally. Tf that had not been the case the Empire Marketing Board could hardly have consented to vote tlr money of Great Britain to carry om a search for minerals in Australia which would he for the benefit of the Commonwealth alone. Tf all the Empire would profit from the efforts ol the scientists in Australia, then expenditure would be justified. Tt can he seen, therefore, that the investigations of the skilled observers, to whom will be entrusted the control of geophysical survey work in Australia, will he closely followed by people interested in science and mining in every part of the British Empire.

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