224 EXPLORATION GEOPHYSICS 



vestigation of this type across the Los Angeles Basin, t This basin is a 

 broad syncline with an axial trend northwest to southeast and is about 

 75 miles long and 25 miles wide. The geologic section shown in the figure 

 is based on geologic, drill, seismic, and gravity data. It is predicted that 

 this structural basin reaches a depth of over 40,000 feet at its deepest point. 

 The granitic basement rocks are overlain by about 15,000 feet of schists 

 and other non-crystalline metamorphic rocks, which are probably of Fran- 

 ciscan age. Overlying the metamorphosed rocks is a section of about 

 25,000 feet of soft sandstones and shales of marine origin, chiefly of 

 Tertiary age. Shallow alluvial deposits form the surface covering. High 

 angle faults of considerable displacement trend in a general northwest and 

 southeast direction. 



The magnetic investigations were conducted along a 45 mile traverse 

 and comprised: (a) vertical component measurements at intervals of ^ 

 to ^ mile and (b) horizontal component measurements at intervals of 

 from 3 to 4 miles. 



The magnetic work failed to show the positions of the large faults 

 which traverse the basin parallel to its major axis. At the shallower depths, 

 the materials on both sides of the faults have about the same permeability ; 

 hence, they show no definite anomalies at their contacts. The computations 

 for the calculated vertical and horizontal components were made by assum- 

 ing that the contact between the overlying materials and the granite is an 

 inclined smooth surface. The air, sediments, and metamorphics were given 

 a weighted susceptibility of about 40 • 10~^ c.g.s. units while the granite 

 was assigned a value of 1012 • 10~^ units. | It was assumed also that the 

 field strength was 50,000 gammas and that the direction of the field made 

 an angle of 30° with the vertical. The general agreement between the 

 theoretical calculated configuration and the assumed configuration (as 

 based on drill hole and geologic control, supplemented by seismic and 

 gravity work) is fairly typical of the results which may be expected in a 

 complex problem of this type. 



Mapping Contacts by Magnetic Methods. — The contact between 

 two materials of different magnetic permeabilities is readily disclosed by 

 magnetic studies provided their difference in permeability is sufficient to 

 produce a measurable anomaly. An application of magnetic work in con- 

 tact mapping is shown in Figure 108. The high magnetic permeability of 

 the serpentine gives rise to the large anomaly at the contact between the 

 lime schist and the serpentine at the right end of the traverse and the short 

 abrupt anomaly at the left end of the traverse. Due to the relatively small 

 difference in permeability between the quartz mica schist and the altered 

 zone of garnetized schist, and between the altered zone and the lime schist, 

 there are no appreciable magnetic anomalies or change in trend produced 



t L. F. Uhrig and S. Schafer, "Observed and Calculated Values of the Magnetic Intensity over 

 a Major Geologic Structure," Gerland's Beitrdge sur Geophysik, vol. 49, pp. 129-139, 1937. 

 X J. L. Soske, Unpublished Doctor's Thesis, Calif. Inst, of Tech., Pasadena, California. 



