266 



EXPLORATION GEOPHYSICS 



Bulk Dry 



Sedimentary Rocks and Near- Density 

 surface materials^ (Bd) 



Anhydrite 2.9 



Cap Rock 2.6 



Siltstone 2.3 -2.47 



Gypsum 2.2 - 2.6 



Rock Salt 2.1 - 2.4 



Dolomite 2.04-2.54 



Sands and Clays 2.5 



Sands and Clays 2.0 - 2.2 



Sands and Clays 1.7 - 2.2 



Mudstone 1.90 - 2.28 



Sylvite 1.9 -2.0 



1.8-2.6 



Bulk Density Porosity Range 

 Water Per Cent 



Saturated 

 2A7-2M 



2.28'-"2.65 



(pr 



3 

 4 



7.4"-" 15.4' 

 6.2* *-' '8.6 



2.19-2.42 



13.9 -28.6 



1. 



Chalk 



Glacial Drift 

 Limestones 

 Sandstones 

 Carnallite . . 



Shales 



Alluvium . . 



Sands 



Gravels .... 



Clays 



Silts 



Soils 



Marl 2.25-2.60 



Marl 0.98 



Diatomaceous Shale 0.9 - 1.1 



Loess 0.75-1.60 



1.74 



1.60 



1.6 



1.56 



1.5 



1.4 



1.4 



1.3 



1.2 



1.0 



2.76 

 -2.68 

 -1.7 

 -3.17 

 -1.6 

 -1.8 

 -2.2 

 -2.4 

 -1.8 

 -2.0 



2.12-2.72 

 1.99 - 2.77 



0.0 -37.6« 

 0.0 -51.3 



1.92 - 3.21 

 2.0 



1.9 

 1.7 

 1.63 

 1.8 

 1.5 



-2.1 

 2.4 

 2.47 

 2.2 

 2.4 



2.5 

 42.3 

 30.2 

 20.2 

 36.6 

 31.4 

 40 



36.3 

 43.3 

 48.4 

 i7.7 

 62.9 

 53.6 

 50 



1.59 

 1.4' "-"i. 93 



60.6 



10 



20.6" '-'69.4' 



1 Values listed in table represent averages. 



2 Relationships: P (%) = 100 (1 -Bd/Sg), wherein P = total porosity; Bd = bulk density or 

 apparent density of the dry sample; Sg = the specific gravity of the mineral grains in the sample. 



3 No data available. 



* From salt domes. 



5 At 7,000 feet, Texas-Louisiana Gulf Coast. 



8 More complete data under tabular headings on earth materials is given in Handbook of Phys- 

 ical Constants, G. S. A. Special Paper No. 36, January, 1942, p. 14. 



' At shallow depth, Texas-Louisiana Gulf Coast. 



* Items listed in the plural represent density determinations averaged from many samples. 



9 Pleistocene, Texas-Louisiana Gulf Coast. 

 1" San Joaquin Valley, California. 



" Loess, plural. 



Quantities Measured in Gravity Prospecting. — An analysis of the 

 variations of the residual Ag''o values allows predictions to be made of 

 subsurface conditions. Detailed discussions of various type of gravity 

 surveys are given later in this chapter. The following brief analysis of 

 over-simplified type conditions is intended merely to illustrate the general 

 application of gravity theory to the prediction of subsurface conditions. 



(a) Ag"o Profiles. 



The value of the force of gravity at a station or series of stations can 

 be measured with an accuracy of better than 0.001 dyne. As has also been 

 discussed, when the theoretical value of the force of gravity for the latitude 

 of a station has been subtracted from the observed gravity and the necessary 

 corrections made for local conditions, the residual quantity is the gravity 

 anomaly or Ag'^o value. 



A profile of Ag'\ gravity values for a series of closely-spaced stations 



