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There is a small volume of published examples of magnetic surveys and basement depth maps. Some are in areas where the interpretations have been followed by drilling, which permits a reasonably objective basis for evaluating the precision and usefulness of the results. The following examples include instances of broad-scale mapping of the basement surface to determine general basin features and also those where surveys and interpretations have been made over local basement features closely related to oil field structures.


An interesting example of the application of the basement depth mapping procedures outlined above is available from a magnetic survey made for the French government in Senegal. Figure 64 (Nettleton, 1962) is a generalized geologic map of the area. From this map alone we can determine very general features only. It can be inferred that much of the map is over a basin area, because there are outcrops of older rocks to the east and there are some outcrops of igneous rocks (not indicated on the figure) in the vicinity of Dakar. This surface geologic information alone would give very little basis for evaluation of the area for its petroleum possibilities.

A rather loose reconnaissance magnetic survey was carried out over the area. This was not sufficiently closely controlled to give a detailed map of the basement surface but approximate basement depths could be determined throughout the area. Figure 65 shows generalized contours, at an interval of 1000 m, on the basement surface. While this reconnaissance has provided little detail there is now a much more specific

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