team to recognize critical sedimentary units and to prepare preliminary sub- 

 surface maps showing their concepts of thicknesses and facies. 



Gradually, the basic information accumulated. Surface geologists complet- 

 ed their areal geologic maps of the basin and the surrounding hills and moun- 

 tains. The airborne magnetic survey was completed and different types of resid- 

 ual and derivative maps were prepared from the basic magnetic profiles. The 

 two gravity crews working in different areas, respectively, finally met near the 

 central part of the basin and "tied" their gravity values to create a composite 

 survey of the basin. From these original gravity data, residual maps were pre- 

 pared to assist in the interpretation of the data. Working with the seismologists, 

 the geologists prepared regional cross sections from the seismic profiles. 



From a study of the airborne magnetometer data (fig. 41-6) and the sub- 

 surface control of the four stratigraphic tests, the geophysicists and geologists 

 were able to construct a topographic map that depicted the general configuration 

 and depth of the basement surface. From detailed study of residual maps of the 

 magnetic data, several anomalous areas and trends were noted that suggested 

 structural anomalies in the basement rocks. Several probable fault zones and 

 two possible anticlinal trends were rather poorly denned by the magnetic data. 



Amprex's geophysicists considered the gravity meter to be one of the most 

 useful tools in reconnaissance exploration. They had found it more definitive 

 than the magnetometer in locating structures that result from deformation of the 

 sediments because gravity anomalies are not necessarily dependent on relief of 

 the basement surface. Also, the density of control is usually greater for gravity 

 data, and corrections which are applied to the raw data are, as a rule, more 

 precise than those applied to the magnetic data. Figure 41-7 is a generalized 

 copy of Amprex's Bouguer gravity map of the Arenoso Basin showing results 

 of measurements at almost 15,000 stations, an average density of three stations 

 per square mile. Its construction involved corrections for elevation, latitude, 

 and irregularities of terrain. It shows the resultant effects of both regional and 

 local structures. While the major anomalies are readily apparent, some local 

 structural effects are obscured by large regional effects, particularly where the 

 local structural strike parallels the strike of the regional gravity contours. In 

 such places the only effect is a slight widening or narrowing of the contours. 

 In an effort to make these anomalies more apparent and easily localized, Am- 

 prex's gravity interpreters prepared a residual map by estimating the regional 

 effect and subtracting it from the total effect. To the extent that the regional 

 effect is correctly estimated, the resulting residual map is a representation of 

 the local effect (fig. 41-8). Various techniques of estimating the regional effect 

 were applied. A common one utilized profiles showing the corrected, or Bouguer, 

 gravity values, with the regional profile sketched in by eye to give a smooth curve 

 approximating the Bouguer curve. They also used mathematical techniques for 

 computing a regional effect from the observed values, although they realized 



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