Bulletin 54. Geophysical Methods of Exploration and their ...

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NEW JERSEY GEOLOGICAL SURVEY
34 GEOPHYSICAL WORK IN NEW JEllSEV frequently embrace areas of diverse surface geology with marked changes in density whose effects are ahnost completely masked by the large anomaly and only show up in the small residuals left after renloving the effect of the regional anomaly. Gravitational wdues therefore, like magnetic ones, reflect the combination of size of the disturbing body, its depth below the surface, and its departure in physical property from that of the surrounding material. The latter factor in this case is _hc density of the rocks involved rather thau their magnetism. Some of the densities of the more common rocks and minerals that might give rise to anmualies arc listed in the following table. TABLE 2.--DENSITIES Minerals Rocks Gold ........................ 19.4 Perldotite .............. 3.55 Galena ...................... 7.6 Gabbro ................ 3.09 Chalcocite ................... 5.8 Norite ................. 3.2 Magnetite ................... 5.2 Diorite ................ 2.99 Franklinit e .................. 5.l Dolomite .............. 2.8 Barite ....................... 4.7 Diabasc ................ 2.7 -- 31 Chromite .................... 4.5 Granite ................ 2.5 -- 2.7 Sphalerite ................... 4. Serpentine ............. 2.5 Limonite .................... 4. Shale .................. 2.3 Salt ......................... 2.2 Sandstone ............. 22--2.5 Sulphur ..................... 2. Clay ................... 1.6 Coal .................. 1.2 Just as with magnetic data, certain corrections have to be made to gravitational data before an interpretation of readings can be made, These include a correction for elevation, since the gravitational value'varies as the square of the distance and it is thus necessary to reduce all data to a common datum ; a correction for the latitude due to the fact that the earth is shaped like an orange and hence the gravitational force increases both north and south of the equator; a drift correction in gravimeter reading for errors introduced by slight changes in the elastic behavior of _he springs; a terrane correction to take into consideration the effect of the mass of the material lying beneath the station and down to the reference datmn (i.e. sea level); and in areas adjacent to mountains, an isostatie correction to compensate for the deficiency in mass a_sociated with mountain ranges. The need for exact elevations is a drawback to gravitational methods since this frequently necessitates special survey parties to obtain this data and this adds to the cost of the measurements. Likewise, the terrane correction requires a knowledge of the topography surrounding the station so that the mass effects of hills rising above the station and valleys lying betleatb it may be eliminated. Though this NEW JERSEY GEOLOGICAL SURVEY
Page 1 and 2: BULLETIN 54 Geologic Series Geophys
Page 3 and 4: • BOARD OF CONSERVATION AND DEVEL
Page 5 and 6: PREFACE At the request of Meredith
Page 7 and 8: ILLUSTRATIONS PAGE Frontispiece, Ou
Page 9 and 10: Outline map of New Jersey showing'
Page 11 and 12: 14 GEOPHYSICAL WORK IN NEW JERSEY m
Page 13 and 14: 16 GEOPHYSICAL WORK IN NEW JERSEY k
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Page 19 and 20: 22 GEOFHYSICAL WORI( IN NEW JERSEY
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Page 33 and 34: 36 6EOPHYSICAI_ WORK 1N NEW JERSEY
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Page 47 and 48: 50 (IEOPHYSICAL WORI_ IN NEW JERSEY
Page 49 and 50: 52 (_EOPHVSICAL WORK IN NEW JERSEY
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Page 65 and 66: 68 GEOPHYSICAL WORK IN NE'W JERSEY
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Page 69 and 70: NEW JERSEY GEOLOGICAL SURVEY
Page 71 and 72: 7 72 • r_ X GEOPHYSICAL IN\ ES_HG
Page 73 and 74: GEOPHYS1CA [J INVESTIGATIONS 73 TAB
Page 75 and 76: \ DIAGRLPI TO ILLVJT_ATE _A_EF/£NT
Page 77 and 78: 76 G'EOPHYS1CAI_ WORK IN NEW JERSEY
Page 79 and 80: GEOPHYSICAL INVESTIGATIONS 77 bro i
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GEOPHYSICAL INVESTIGATIONS 79 nitud
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NEW JERSEY GEOLOGICAL SURVEY
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STRUCTURAL ANALYSIS ON GEOLOGIC - S
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84 GEOPHY _.[CAL \_ ORK IN NEW JERS
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86 GEOPHYSICAL WORK IN NEW JERSEY t
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88 GEOPHYSICAL WORK IN NEW JERSEY l
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