Curie depth determination from aeromagnetic spectra

Abstract

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.

Regional estimation of Curie-point depths and succeeding geothermal parameters from recently acquired high-resolution aeromagnetic data of the entire Bida Basin, north-central Nigeria

“…Each block covers a square area of 200 km × 200 km. Since magnetic source bodies having bases deeper than window length/2π may not be properly resolved by the spectral method (Shuey et al, 1977), a window length of 200 km is found to be suitable in this study. …”

“…Calculations of DBMS, which is commonly construed as the Curie-point depth, are made using diverse methods (Spector and Grant, 1970;Shuey et al, 1977;Leu, 1975, 1977;Okubo et al, 1985;Tanaka et al, 1999;Maus and Dimri, 1996;Finn and Ravat, 2004;Ravat et al, 2007;Bansal et al, 2013), namely the spectral-peak method (Spector and Grant, 1970;Shuey et al, 1977), the centroid method Leu, 1975, 1977;Okubo et al, 1985;Tanaka et al, 1999), the scaling spectral or power-law correction method (Maus and Dimri, 1996), forward modelling of the spectral-peak method (Finn and Ravat, 2004;Ravat et al, 2007), and the modified-centroid fractal method Nwankwo, 2015), among others. The conventional centroid method is used in this work for the reason that it gives better estimates with fewer depth errors compared to earlier methods (Ravat et al, 2007).…”

Distribution of the East China Sea continental shelf basins and depths of magnetic sources

“…Shuey et al (1977) demonstrated that the depth information from the power spectrum can only reach a depth of L/2π. To calculate Zo, Tanaka et al (1999) divided their area of study into several sub-regions of 2 × 2 degrees in size.…”

The Moho as a magnetic boundary

“…Curie isotherm depth, relation of Curie isotherm depth to Moho depth and source of magnetization within the crust (Shuey et al, 1973(Shuey et al, , 1977Hall, 1974Hall, , 1968Byerly and Stolt, 1977;Krutikovskaya and Pashkevich, 1977;Green, 1976;Smith et al, 1977). The Curie isotherm depth has in the past been considered the position of the Fe 304 Curie point (580o C) isotherm whose depth may vary from S 20 km (Basin and Range) to s 70 km (Sierra Nevada) depending on the geothermal gradient.…”