REVEALING SECRETS

Evening Post, Volume CXII, Issue 8, 9 July 1931, Page 18

REVEALING SECRETS

BELOW THE SURFACE

HOW GEOPHYSICS WORKS

A NEW SCIENCE

Possibly there are some people who think of geophysics, the newest of the sciences, as being something akin to the methods used by the'diviner, who, with forked hazel twig or bent wire, locates or professes to locate, metals, water, or oil, lying beneath the earth's surface. At first sight there, is some similarity between the two, but a closer examination of geophysical methods discloses great differences. . There is no trace of charlatanism'about geophysics. It is a pure science, in its infancy certainly, but rapidly coming into its own. Its early failures militated against its acceptance as a science at first, and caused criticisms, many of which were not justified. But that is a thing of the past now, and in a surprisingly short time geophysical methods have been developed and perfected to such an ex-' tent that the necessity of their application is to-day fully realised in many quarters. A description of geophysical methods was given last night to an interested audience in the Dominion Museum, when Mr. Norbert Modriniak addressed members of the Technological Section of the Wellington Philosophical Society on '' Geophysical Prospecting.'' Mr. Modriniak is an Austrian scientist who has been in the Dominion during the last four years and who intends, if possible, to make'his, home here. His lecture was followed with keen attention, and was provocative of discussion afterwards. The ft speaker had little need to apologise, as he did, for his lack of acquaintance with the English j language,, for he made himself quite clear on most points.

. Hitherto, when investigating what lies beneath the surface of the ground, mining engineers and others have had to obtain information from bores, which cost a lot of money, or from the speculations of geologists. Tho element of chance has entered largely into mining operations, for a bore may miss a lo ( de by only a few feet. In the history of many mines, even in New Zealand, the loss of the lode has meant the end of the mine, and in only comparatively few cases has a lost lode been found again. It has been tho same with boring for oil. In the sinking.of boros speculation as to the best spot has figured largely, but now the science of geophysics, which allies itself closely to those .of geology and physics, steps in, and with uncanny exactitude says where bores must be sunk to achieve the desired results. The proportion of rock exposures in most districts, Mr. Modriniak pointed out, is only about 2, per cent. What the prospector wants to know is where beneath the surface is the remaining 98 per cent. Expensive tunnelling and boring may locate some of the remainder, or.it may not: the science of geophysics, by one or other of its several methods, will locate it all. ■ Geophysics is valuable, also to the civil engineer, for it can tell him about the formation 'underlying tho top soil, and whether it is suitable for carrying the proposed weight. ■ It can tell him further whether the underlying strata are uniform or disturbed, whether fault lines or other distortions are presents and whether there is under it all a final solid foundation. ft GEOPHYSICAL METHODS. It is the funetiou of the geophysical prospector to ascertain tho gravity, density, elasticity, magnetic susceptibility, radio-activity, electrical conductivity, and temperature of tho strata lying underground. These are detected and measured, by many ingenious and delicate instruments. Then, in accordance with the facts laid out by geology, the data are converted into disclosures of practical value. Different kinds of rock and ore give differing results in these geophysical tests, and Mr. Modriniak explained how the expert can tell from the data what kind of rock lies below the surface and exactly at what depth and over what area it occurs. The torsion balance is widely used by the geophysicist when; he is using gravimetric methods. One of tho oldest methods of geophysical prospecting is the magnetic method,' first used in Sweden to locate magnetite. , This method depends on the fact that the magnetic- field varies,according to the j nature of the "rock. ~ - v | HUMOTJSS OF GEOPHYSICS. The fundamental physical property on which the': seismic method of geophysical prospecting, is based is the variation in theft speed of transmission of the elastic' earth .waves in the .different geological formations. Artificial explosions sot up a series of waves in tho -earth, and these aro' carefully measured. In , this cohnbetion Mr. Modriniak told .how he was'billed for £15 in Taranaki because b neofhis explosions was in the vicinity of a herd of Jersey cows., '■■ The animals'were frightened, and the milk supply for days showed a distinct decline. He told another good story of how some delicate instruments and recording apparatus had been set up.on a lawn covered with long grass. During tho temporary absenco of tho' geophysicist tho handyman of the place came along with the mowing machine, oausing records to be obtaiuedftof a kind never seen before or since. RADIO-ACTIVITY. Tlie electrical methods of geophysical prospecting, tho speaker said, have accounted for over a hundred' patents, some of which are of no practical value. After describing those electrical methods which were' of use, Mr. Modriniak said that great things had been expected from tho radio-activo method, especially in tho determination ,of oil accumulations, but up to • the< present no practical results had been obtained. Some experiments in the direction had proved the possibility of realising tho ago of geological formation. "In conclusion," said Mr. Modriniak, "I would like to point out that 'this science is young, especially so in Now I Zealand, and needs all possible assistance. ' It. cduldbe greatly helpod if laboratory/work could bo done by Universities and other scientific institutes. Students of geology, and mining engineers especially, should know tho essentials of tho science, "not to specialise themselves in this lino, but to know enough to call in its assistanco when necessary. "I am pleased to add that I have had great assistance and interest shown regarding tho science by members of the various Government Departments in Now Zealand."

After questions and discussion, a hearty vote of thanks to Mr. Modriniak was proposed by Dr. E. Kidson, chairman of the meeting, and was carriod by acclamation. ■