A study of spectral methods of estimating the depth to the bottom of magnetic sources from near-surface magnetic anomaly data

Abstract

S U M M A R YBased on a critical evaluation of several different spectral magnetic depth determination techniques on areally large synthetic layered and random magnetization models, we recommend the following considerations in the usage of the methods as necessary prerequisites to successful bottom depth determinations: (1) using windows with sufficient width to ascertain that the response of the deepest magnetic layer is captured and by verifying the spectra and computing the depth estimates with the largest possible windows (>300-500 km); (2) avoiding filtering to remove arbitrary regional fields, accomplished by compiling magnetic anomalies derived from modern spherical harmonic degree 13 Earth's main field models [e.g. recent International Geomagnetic Reference Field models (IGRF) or Comprehensive models (CM)]; (3) ascertaining the near-circularity of the autocorrelation function to avoid analysing biased spectra containing strong anomaly trends; and (4) avoid determining the slopes from the exponential, low wavenumber part of the spectra in the cases of layered magnetization. We also describe the details of the new spectral peak forward modelling method and discuss the conditions under which the method can lead to useful results. We found that, despite all these precautions, in some cases, the results can still be erroneous and, therefore, we recommend a critical evaluation of the results by modelling heat flow and taking into account seismic information on the crustal and lithospheric thicknesses and seismic velocities wherever possible. In the southcentral US, east of the Rockies, where the surface heat flow ranges between 40 and 65 mW m −2 , we obtained the magnetic bottom depth of 40 ± 10 km using the approach of the forward modelling of the spectral peak. This range is similar to the seismically derived crustal thickness of 45-50 km, suggesting, therefore, that the entire crust may be magnetic in this region. Because of the uncertainties in the various heat flow contributing parameters, such as the variations in thermal conductivity, radiogenic heat and hydraulic regime, we could not constrain the lithospheric thickness beyond an estimate ranging approximately from 100 to 200 km.

Discussion on `Spectral analysis of aeromagnetic data for geothermal energy investigation of Ikogosi Warm Spring - Ekiti State, southwestern Nigeria’

“…To obtain a reliable and accurate CPD result that is up to 21.91 km as reported in the paper, a data window not less than 138 × 138 km must be utilized. Ravat et al (2007) underscored the importance of computing the spectra and depths with large windows to ensure that the response of the deepest layer is properly captured. Nevertheless, this is not the case in the reviewed publication.…”

“…There has been an agreement among researchers in the field that the dimension of the window analyzed may need to be, in some cases, up to ten times the depth to the bottom (Chiozzi et al 2005;Ravat et al 2007). Therefore, the purpose of this review is to clarify and point out the possible errors in the recent paper authored by Abraham et al (2014) regarding data window.…”

“…In a study of different spectral methods (including the centroid method adopted by Okubo et al 1985 andTanaka et al 1999) of estimating the depth to the bottom of magnetic sources (also regarded as a proxy for an estimate of curie-point depth (CPD)), from magnetic anomaly data, Ravat et al (2007) reiterated using data windows with sufficient width to ascertain that the response of the deepest magnetic layer is captured and by verifying the spectra and computing the depth estimates with the largest possible windows. There has been an agreement among researchers in the field that the dimension of the window analyzed may need to be, in some cases, up to ten times the depth to the bottom (Chiozzi et al 2005;Ravat et al 2007).…”

Estimation of Curie point depths and heat flow from Ardebil province, Iran, using aeromagnetic data

“…Location of the spectral peak and the slope in the high wave number range are controlled by Z b and Z t , respectively. The combination of both Z t and Z b controls the slope immediately adjacent to the peak (Ravat et al 2007). The advantage of forward modelling is that it allows one to fit iteratively the position and the width of the peak and match the adjacent part of the slope more precisely and explore the model space.…”

“…The depth to the magnetic bottom is then obtained from Forward modelling of the spectral peak method Ravat (2004); Finn and Ravat (2004); Ross et al (2004) and Ravat et al (2007) proposed forward modelling of the spectral peak to better estimate the bottom depth by using Eq. 4.…”