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Attenuation of potential field anomalies with increasing distance from the source is, of course, independent of sampling, so the Nyquist problem may always be avoided with increased cost, as Davies pointed out. However, attenuation does produce a complete loss of some signal with increasing distance because instruments operate in a discrete or space domain. So all instruments are filters in this sense, and no amount of sampling overcomes this unfortunate characteristic.

The material in this paper was presented at the Annual Meeting in Houston in November 1966 and was enthusiastically received. Certainly many have looked forward to its publication. However, your attention is directed to some errors and subjective material which detract from the application of this new tool.

Six examples of relationships between aeromagnetic anomalies and producing areas are presented by maps of subsurface structure, observed aeromagnetics, second vertical derivatives of the aeromagnetic fields, and maps of the magnetic basement computed from the aeromagnetic data whenever all these are available. In addition, a copy of an original aeromagnetic traverse is included. The examples embrace Puckett in the Delaware Basin of West Texas, Grieve in the Wind River Basin of Wyoming, Tioga and Onstad in the Williston Basin of North Dakota, Swanson River in the Cook Inlet Basin of Alaska, and Zelten in the Sirte Basin of Libya. These include both “before and after” examples with basement maps prepared in some cases before discoveries, and in other cases afterwards. The paper purports to demonstrate that correlative magnetic anomalies exist in either event. Moreover, these anomalies would be disclosed by any competent technique, so techniques of interpretation are not a part of the argument. It is well known that the larger observed aeromagnetic anomalies result from intrabasement contrasts of magnetization. Anomalies resulting from structure alone ordinarily are smaller by a factor of the magnitude of ten. Of the six cases, Puckett, Tioga, and Onstad represent structures with associated intrabasement effects, whereas anomalies at Grieve, Swanson River, and Zelten are considered to be purely structural effects. There are difficulties involved in using either type of anomaly quantitatively for determination of depth to basement and the relief of possible basement structure. But these examples are part of a much larger amount of evidence which seems to justify improving the definition of the observed aeromagnetic field so that the small residual structural effects can be better defined in order to find more of them and to interpret them more accurately. New instrumentation with higher sensitivities is now available for this second venture in aeromagnetic prospecting.

Aeromagnetic data are analyzed to study the configuration of the Precambrian rocks in the eastern Paradox basin, principally in the vicinity of the Gypsum and Dolores structures. The analysis shows that the Precambrian surface is uplifted several thousand feet on the southwestern flanks of the surface structures and that the uplifted Precambrian blocks are strongly mineralized ferromagnetically. It is postulated that this mineralization is contemporaneous with deformation that, in turn, resulted in the formation of the “salt anticlines” in the adjacent deep basement areas to the northeast. Gravity anomalies in the area are found to be entirely minimal and attributable to the masses of the salt “bulges” within the surface anticlines. These masses are quite well defined in the instances of these structures from the width of outcrop at Gypsum and from the depth to the Precambrian computed from the aeromagnetic data, the pre‐salt Paleozoic interval being quite uniform.

The magnetic basement map computed from the 1950 Peace River aeromagnetic survey of 27,000 sq mi in western Canada is compared with the subsurface Precambrian surface determined by 169 deep tests subsequently drilled. The computed map has a standard deviation of 6.5 percent and an average error of 7.4 percent. The errors of the computed depth estimates are biased toward the shallow side; that is to say, the depth estimates are generally not deep enough. Moreover, excessive errors, those over 20 percent, are conspicuously shallow. This asymmetrical distribution is attributed to the failure to utilize plate models in the interpretation, as both intrabasement and suprabasement anomalies may be identified in the observed field. Consequently, the comparison confirms the existence of the intrabasement magnetization units within the Precambrian by establishing the validity of the coefficients derived from them and used in computing the depth estimates.