We study aeromagnetic maps from four areas in the Western United States: Utah High Plateaus, Yellowstone National Park, Southern Great Basin, and Uinta Basin. In the first area we try to infer depth-to-bottom of magnetic source by comparison of individual anomalies with theoretical fields of vertical prisms. We conclude that this task is impossible, at least for the few anomalies considered. Assuming a bottomless prism a fit can be obtained which is not significantly worse than the best fit obtainable with any prism model. This is documented by both nonlinear and linear inverse theory.We then turn to a spectral theory like that of Spector & Grant. The advantage of spectral analysis is that the broad, low-amplitude negative flanks of positive anomalies are included even though they may be covered by adjacent positive anomalies. The disadvantage is sensitivity to finite data length: the spectra are interpreted with models derived by Fourier transformation in an infinite domain. Anomalies which are only partly included in the map can degrade the estimated spectrum. We try several techniques to deal with this problem, but none seem universally applicable. These techniques include elimination of part of the map and subtraction of a low-order polynomial. For computation of the spectral estimate our favourite procedure is: (1) average autocovariance over azimuth, (2) extrapolate to large lags as r-5, (3) Hankel transform the radial autocovariance profile to a radial power profile.Our usual model for interpretation of the low-frequency spectra is a truncated right circular cone defined by the four parameters ZT (depth to top), ZB (depth to bottom), RT (top radius) and RB (bottom radius). For the Utah High Plateaus and Uinta Basin we find a distinct preference for sloping sides, i.e. RB > RT. In analysing individual anomalies as well as spectra we find anticorrelation of ZT and ZB. We attribute this to the centroid of the average source body being better determined than its thickness.

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