GOLD EXPLORATION

Hokitika Guardian, 23 May 1933, Page 8

GOLD EXPLORATION

THE GEOPHYSICAL METHOD

IMPORTANT EXPERIMENT'S,

CHRISTCHURCH, May 20.

Valuable results in the location of gold’" Efeiaring deposits in Otago and on the West Cbast have been obtained Within recent months by experts working under the Department of Scientific, and Industrial 'Research, according to Dr. E. Marsdeh, .secretary to the ’department, who discussed the importance of this work in ihn interview yesterday. The methods employed are considered to be of nr ini ary importance in enabling n-old bearing areas to be located without pros pee tors ' having to undertake the expensive ar.d lengthy work ol exploratory boring.

“DiU'ing recjnt; months the department lias been undertaking research into the applicability of various moJhnds of detecting underground strutA.irfes. These methods were in the fiist instance evolved by the large ail' 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 Iras so far been published on : the question as the secrets of geophysical research have been jealously guarded.

“The methods used hear directly upon th.o physical properties of strata, and materials. For instance, certain recks are different'’ magnetically, or have a different susceptibility to magnetism, and they can be traced through the surface by their influence oil the; earth’s magnetism. Since the war very delicate instruments have been evolved to perform this work of detection. Mr If. F. Baird, of the Magnetic Observatory, is at present using this method on Cromwell Flat and north Cromwell for tracing old river beds by their ironsand deposits.

(TESTING 'SEISMIC WAVES

“Another way of deteting 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 piepeiud of rooks, beto.v the surface. Jiff'drent l'-ocks transmit the waves •alb ditF'C'idaf' velocities ’Schist, for instance, transmits these waves about L8,(1.,0ft. a second. . Quartz conglomerate transmits the waves about XDJIt. a second, and shingle about ■soooft. 'a second. ‘‘This method has been used to trace schist bottom in the Mamiherikia Valley,- the Ida Valley, and 'the Vlaiiitot-o -Plain. Most «f 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 tlx e , edges of cliese' 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 liver, beds Und rebifs, and it '.has been found that definitely valuable results can be obtained. These have been particularly marked in Central Otago and oil 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 main advantages of tho methods being employed is that the best places for boring can be indicated- Baling 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 off "the results to be obtained from the application of scientific methods was given in ap area worked by the department’s experts. About 40 bores were. .sunk without success, hut the geophysical work indicated the pvesencei of a channel not readily revealed by even the large number of bores pnfe down. Tho work of geophysical survey is always done in conjunction with detailed geological survey, ‘and the methods are merely an adjunct to the geologist to test his findings and to lead toward more accurate determination and 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 th. 3 surface. There is also another important means of testing, known as the gravitational method. This depends upon the. attraction of heavy bodies beneath the surface as compared with those of lighter structure. This method is made much use of in detecting oil. but the instruments are extremely costly.”