Examples of Basement Mapping
Published:January 01, 1971
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
Figures & Tables
Elementary Gravity and Magnetics for Geologists and Seismologists
The purpose of this work is a general review of the gravity and magnetic nlethodsods of geophysicael xplorationa s applied in the search for petroleum. This material is not designed for the gravity and magnetic specialistb ut rather lo)r the geologistsa nd seismologistwsh o may not have a thorough appreciation of the applications of these metht)ds in the overall expl()ration picture. A subtitlc for this monograph might well be "-l'hc Other Five Percerot." This is because the seismic method and its associated data processing account for sornc 95 percent of the total expenditures Ik)r petroleum exploration geophysicss o that whatever application is made of the gravity and magnetic noethods comes out of the other 5 percent. This does not mean that these methods make a proportionately small contribution to the overall exploration effort. Because of the relatively rapid rate of progress in the field, particularly by airborne magnetics. the total area covered by gravity and magnetic surveys may bc greater than that covered by the much greater seismic expendituresA. s a very rough rule-ofthumb, the relative cost per unit area of magneticg, ravity and seismicf ield work with data processings tand in the ratio of I to 10 to 100. It is the hope and purposeo f this monographth at a better appreciatioonf the valueo f the potential methods and understanding of their applicationsm ay be broughta bouts t) that they can be applied with proper perspective in the overall exploration picture. From the beginning of geophysical exploration in the petroleum industry in the 192()'s, three basic physical principles were used: i.e., the measurement of small variations in the magnetic field, the measurement of small variations in the gravitational field, and the propagation of elastic waves through the earth. These three and only these three physical principles are the basis for practically all of the geophysical work up to the present time. Many other methods have been conceived and tried in the field in a limited way, but none has persisted to the extent that field operations are carried out n a scale at all comparable with that of the three primary methods listed above. The seismic method, of course, usually is much more direct in its relation to the geologyt han the potentialm ethodsR. etlection zones or horizons frequently are directly correlative with geologic strata and give relativelya ccuratem easureosf their depth and form.