A.— MATHEMATICAL AND PHYSICAL SCIENCES 23 



with great difficulty that he was eventually persuaded to initiate what has 

 now become a common and successful practice in various parts of the 

 world. 



I may perhaps mention, too, that when I first became interested about 

 five years ago in applied geophysics, I was very doubtful of its use. Could 

 conditions underground, I asked myself, ever be so simple and free from 

 complications that physical observations on the surface would point 

 unequivocally to the solution ? The answer to this question is, generally 

 speaking, in the negative ; but here the geologist comes in again. He 

 carries out his preliminary survey by his own methods, and is often able 

 to indicate both the limited region where a geophysical survey seems 

 desirable, and in a general way the kind of formation which is to be sought, 

 thus enabling a suitable choice of method to be made. He provides, in 

 fact, the selection rules for the geophysicist, in much the same way as the 

 quantum theorist does for the spectroscopist, as regards both where to 

 look and what to expect to find. It is true that sometimes a forbidden 

 result persists in obtruding itself inconveniently upon the geological 

 interpretation, just as a forbidde7i spectral line may refuse to be extin- 

 guished. But usually the solution of a problem has to depend upon the 

 combined result of geological and physical evidence, and is only approxi- 

 mate at that. Still, the co-operative effort is undoubtedly more likely 

 to lead nearer to the truth than either singly, and physics and geology 

 must accordingly work hand in hand. 



Geophysical Prospecting. 



Before going on to consider the theoretical basis and the practical 

 methods employed, we may well inquire what justification there is for 

 carrying out geophysical surveys of the kind to be described. Why go 

 to the expense of supplementing a geological investigation by systematic 

 measurements of a physical character ? Geophysical surveying is costly, 

 either in the price of the necessary instruments, or in running expenses, 

 or in both. A high quality gravity torsion balance, for example, costs 

 ;(^i,ooo, and it may be noted that nearly two hundred of these instruments 

 were at one time in use on the oil-fields in America. The portable 

 seismometers required for an alternative method of attack are also 

 expensive, and the running costs in seismic work rapidly mount up, owing 

 to the requirement of, possibly, a hundred tons of high explosive during 

 a survey. And it is necessary to reckon, besides, the substantial salaries 

 and wages bill of the geophysical staff, often working in fields remote 

 from civilisation, and requiring all the special accessories of camp life. 



The answer to the question proposed is that everything depends on 

 relative cost. If the servant in the parable had happened to forget where 

 he had buried his talent, geophysical assistance for its recovery would not 

 have been a sound economic proposition. But if the prize is of great 

 value — millions of tons of mineral oil, for instance — extensive application 

 of suitable geophysical methods may become both justifiable and advisable. 

 Each problem has to be considered from this point of view, on the basis 

 of the experience gradually accumulating from previous surveys. But 

 it is not my purpose to discuss this interesting economic subject, except 



