that all such techniques are merely objective methods of drawing smooth regional 

 curves and have no physical superiority to the subjective curves drawn by eye. 

 One popular variation of the mathematical technique which they sometimes used 

 was the derivative method, which combines the steps of computing a smooth 

 curve and subtracting it from the observed values. 



Amprex's geophysicists realized that all techniques employed to depict areas 

 of anomalous rates of change of gravity, thus emphasizing local anomalies, can 

 be useful tools if interpreted by experienced men. They were aware of the 

 difficulties of interpreting gravity data in areas such as the western margin of 

 the Arenoso Basin, where the basement rocks are shallow, and in other parts 

 of the basin where the basement was considered to be of a heterogeneous com- 

 position. They knew from experience that when the composition of the base- 

 ment varies, it is difficult to determine which anomalies are due to intra-basement 

 density contrasts and which are due to structure in the sediments overlying the 

 basement surface. 



From studies of the gravity and magnetic surveys of the Arenoso Basin, 

 Amprex's geophysicists defined many anomalous areas and trends. Perhaps 

 most important was that the total effect of the gravity data provided them with 

 a general outline of a basin of maximum sedimentary fill. Steep gradients in 

 gravity data and irregularities in magnetic data indicated two anomalous trends 

 in the west-central part of the basin that might be indicative of strong fault 

 zones having large displacements downthrown to the east. The most westerly of 

 these two trends could be interpreted as a major hinge belt within the basin. Just 

 west of this feature was a trend of gravity maximum anomalies that indicated 

 possible anticlinal structures. Other gravity anomalies were outlined and added 

 to the prospect files for further investigation. 



Most of the Paleozoic rocks of the basin were covered by a relatively thick 

 section of alluvium, and Amprex found that its usual methods of surface 

 geological exploration were in many places ineffective. Broad alluvium-covered 

 areas within the basin, such as flood plains and alluvial fans, were both difficult 

 and expensive to explore with conventional methods. 



However, the photogeologists found such areas to be ideally adapted for 

 certain types of their studies, for they knew from experience in other areas that 

 in alluvial regions of low relief, even very small structural uplifts and down- 

 warps may disturb stream adjustment and cause local abnormalities to develop 

 in the normal regional drainage pattern. From their studies of the airphotos 

 of the area, they discovered many anomalies, and each of these leads was 

 checked at a later stage of exploration by either the core drill or the seismo- 

 graph, or both. 



Utilizing all of the available information, Amprex's geologists and geo- 

 physicists combined their talents and constructed a series of regional geologic 

 cross sections, both dip and strike, which depicted the regional stratigraphy and 



833 



