EXPLORING FOR GOLD.

Press, Volume LXIX, Issue 20860, 20 May 1933, Page 10

EXPLORING FOR GOLD.

GEOPHYSICAL METHOD. IMPORTANT EXPERIMENTS, Valuable results in the location of gold bearing deposits in Otago and on the .West Coast have been obtained within recent months by experts working under the Department of Scientific and Industrial Research, according to Dr. E. Marsden, secretary to the department, who discussed the importance of this work in an interview yesterday. The methods employed are considered to be of primary importance in enabling gold bearing areas to be located without prospectors having to undertake the expensive and lengthy work of exploratory boring. "During recent months the department has been undertaking research into the applicability of various methods of detecting underground structures. These methods were in the first instance evolved by the large oil companies in locating structures associated with oil. They were extremely successful, and a fairly large percentage of oil finds was made as the result of geophysical work. Little information has so far been published on the question I as the secrets of geophysical re- j search have been jealously guarded. "The methods used bear directly upon the physical properties of strata and materials. For instance, certain rocks are different magnetically, or have a different susceptibility to magnetism, and they can be traced through the, surface by their influence on the earth's magnetism. Since the war very delicate instruments have been evolved to perform this work of detection. Mr H. F. Baird, of the Magnetic Observatory, is at present using this method on Cromwell Flat and north of Cromwell for tracing old river beds by their ironsand deposits. Testing Seismic Waves, "Another way of detecting strata is by the so-called seismic method. A charge of explosive is fired in the ground, and the time of arrival of the explosive wave through the ground at microphones placed at various distances indicates the presence of rocks below the surface. .Different rocks transmit the waves at different velocities. Schist, for instance, transmits these waves about 18,000 ft a second. Quartz conglomerate transmits the waves about 9000 ft a second, and shingle about 5000 ft a second.

"This method has been used to I trace schist bottom in the Manu- > herikia Valley, the Ida Valley, and the Manitoto Plain. Most of this rock immediately overlies the \ quartz conglomerate containing fine gold on the sides of depressions at such places as St. Bathans and the numerous mining valleys on the edges of these areas. "The main object of the work is to trace the contours of the bottom so as to indicate the best places for exploratory boring and mining from the viewpoint of drainage troubles and depth from the surface. So far the tests have been performed with the object of tracing old river beds and reefs, and it has been found that definitely valuable results can be obtained. These have been particularly marked in Central Otago and on the West Coast. The results to date 'are now being tested out by the sinking of shafts and bores. Reduction of Cost. "One of the maiji advantages of the methods being employed is that the best places for boring can be indicated. Boring on the basis of 15s a foot as an operating cost, a 100 ft bore would cost £75, and it is held that if the geophysical methods can result in the most likely places being set aside the cost and time of putting down blind bores would be done away with, thus giving a distinct commercial advantage to those working the areas. A striking example of the results to be obtained from the application of scientific methods was given in an area worked by the department's experts. About 40 bores were sunk without success, but the geophysical work indicated the presence of a channel not readily revealed by even the large number of bores put down. The work of geophysical survey is always done in conjuncition with detailed geological survey, and the methods _ are merely an adjunct to thg geologist to test his findings and to lead toward more accurate determination of depths. "In addition to the magnetic and seismic methods, recent experiments have been made with the electrical method, which depends upon the conductivity of certain layers, such as old stream beds, below the surface. There is also another important means of testing, known as the gravitational method. This depends upon the attraction of heavy bodies I beneath the surface as compared | with those of lighter structure. This method is made much use of in deI tecting oil, but the instruments are I extremely costly."