16 METHODS OF GEOPHYSICAL EXPLORATION [Chap. 2 



III. MAGNETIC METHODS 



General. In common with gravitational methods, magnetic prospect- 

 ing utilizes a natural and spontaneous field of force, with fields of geologic 

 bodies superimposed upon a normal terrestrial field. Coulomb's law, 

 which controls the attraction of magnetic bodies, is identical in form with 

 Newton's law; integral effects of all bodies within range are observed and 

 depth control is lacking. One important difference is that the gravita- 

 tional fields of geologic bodies do not depend on the earth's gravitational 

 field, whereas magnetic bodies frequently owe their magnetization to the 

 magnetic field of the earth. For this reason, magnetic anomalies are 

 often subject to change with latitude. Moreover, rocks may have mag- 

 netism of their own whose direction may or may not coincide with that 

 induced by the terrestrial magnetic field. An important factor in the 

 interpretation of magnetic methods is that rock magnetism, contrary to 

 rock density, is of a bipolar nature. 



In gravity methods, total field vector and the horizontal gradients of 

 the vector or of its horizontal components, are observed. In magnetic 

 prospecting, measurements of the total vector are the exception rather 

 than the rule; it is usually resolved into its horizontal and vertical com- 

 ponents. Experience has shown that the vertical component exhibits the 

 clearest relation between magnetic anomalies and disposition of geolbgic 

 bodies, at least in northern and intermediate magnetic latitudes. There- 

 fore, measurements of the magnetic vertical intensity are preferred and 

 are supplemented occasionally by horizontal intensity observations for 

 greater completeness in the evaluation of the anomalies. 



Magnetic fields are generally expressed in gauss ;^ in magnetic explora- 

 tion it is more convenient to use 1/100,000 part of this unit, called the 

 gamma (7). The accuracy requirements in magnetic prospecting are less 

 than in gravity work; hence, it is a comparatively easy matter to design 

 instruments suitable for magnetic exploration. 



The magnetic anomalies of geologic bodies are dependent on their mag- 

 netic "susceptibility" and "remanent" magnetism, properties which vary 

 much more widely than their densities. Rocks and formations fall into 

 two natural groups: igneous rocks and iron ores are strongly magnetic, 

 whereas sedimentary rocks are generally weak in magnetization. The 

 magnetic characteristics of rocks are affected by numerous factors such as: 

 magnetite content, grain size, lightning, heat, contact metamorphism, 

 mechanical stresses, disintegration and concentration, and also by struc- 

 tural forces which may alter the disposition of magnetic formations in the 

 course of geologic periods. 



* Simplest definition is lines per square centimeter (in air). See also footnote 

 on p. 295. 



