Chap. 8] MAGNETIC METHOD 433 



and produces a positive anomaly of about 267. Studies of exposed 

 igneous rocks in Uvalde and Kinney Counties, Texas, were correlated 

 by Liddle^"^ with surveys of the Little Fry Pan anticline, where small 

 magnetic anomalies were found. A well located on the highest 

 point of the anticline encountered 150 feet of serpentine at 1000 feet 

 depth. Other serpentine plugs in the same area (Yoast field. Dale field 

 [serpentine at about 2000 feet]; EUstone structure [serpentine at about 700 

 feet]; Buchanan field [serpentine at 1800 feet]) were outlined by Sprara- 

 gen.^"' In the San Pedro area in Brazil, Malamphy studied the effects 

 of laccoliths and sills of basalts, found anomalies of the order of 2OO7 

 maximum corresponding to susceptibilities of about 4000 • 10~ units, and 

 compared actual depths (350 to 450 feet) with depths calculated from the 

 drop of the vertical intensity curve (which gave from 300 to 400 feet). 

 Where oil production is associated with intrusives along fracture and fault 

 zones, detailed magnetic surveys to locate the igneous sheets will be of 

 considerable help. Examples of such surveys in Mexico have been 

 published by the author.^"^ 



4. For the location of shoestring sands, only one example is known. 

 Stearn^"^ has described results obtained on several profiles through the 

 Bush City shoestring in Anderson County, whose depth is from 600 to 

 800 feet. Anomalies are predominantly negative and of the order of 20 

 to 4O7. As Stearn indicates, it would be difficult to locate shoestrings 

 magnetically because of the small magnitude and lack of definition of 

 the anomalies. 



C. Magnetic Surveys in Civil and Military Engineering 



The scarcity of magnetic surveys in civil and military engineering is due 

 not so much to adverse conditions as to lack of information on the part 

 of the civil engineer regarding the possibilities of geophysical exploration. 

 The usefulness of magnetic methods has been demonstrated in a number 

 of cases discussed below. 



1. Water supply and drainage surveys may derive much assistance from 

 magnetic exploration where water occurs in troughs underlain by crystal- 

 line or igneous rocks, whererits circulation is controlled by structure of 

 magnetic beds, where water occurs in porous magnetic igneous rocks, or 

 where its circulation is blocked by faults or igneous dikes. An applica- 

 tion of this sort is mentioned by Grohskopf and Reinoehl^°^ who found that 



2«2 A.A.P.G. Bull., 14(4), 509-516 (April, 1930). 

 203 Op. cit., 28(52) 42 (May 15, 1930). 

 ^o" A.I.M.E. Tech. Publ. No. 696 (Feb., 1936). 

 2" Colo. Sch. Mines Quart., 24(1), 61 (1929). 

 ^ofA.I.M.E. Geophysical Prospecting, 192 (1932). 

 ^"^ Miss. State Geol. Rep., App. IV (1933). 



