Discussion on `Spectral analysis of aeromagnetic data for geothermal energy investigation of Ikogosi Warm Spring  Ekiti State, southwestern Nigeria’
 Levi Ikechukwu Nwankwo^{1}Email author
DOI: 10.1186/s4051701400113
© Nwankwo; licensee Springer. 2014
Received: 9 July 2014
Accepted: 27 August 2014
Published: 17 October 2014
Abstract
Background
The recent paper by Abraham et al. (2014) `Spectral analysis of aeromagnetic data for geothermal energy investigation of Ikogosi Warm Spring  Ekiti State, southwestern Nigeria’ applied spectral analysis in the interpretation of aeromagnetic data for estimation of curiepoint depths using a data window of 55 × 55 km; however, the employment of such small window may not be consistent with derived curiepoint depth results.
Discussion
Here, I would like to clarify and point out the possible errors in the paper.
Summary
It is suggested that the curiepoint depth results be recomputed with appropriate window width. This would tremendously assist researchers in appropriate spectral calculation and curiepoint depth investigations.
Keywords
Spectral analysis Aeromagnetic Geothermal Data window Curiepoint depthBackground
Spectral analysis has been widely acknowledged as a robust method for automated depth estimations from magnetic anomaly data (Nabighian et al. [2005]). The approach considers the anomaly to be caused by an ensemble of magnetic sources in order to determine their average depth. Spector and Grant ([1970]) showed that logarithmic radialpower spectra of gridded magnetic data contain constantslope segments that can be interpreted as arising from statistical ensembles of sources, or equivalent source layers, at different depths. However, the areal extent of the subset of data analyzed (the window) limits the maximum depth being investigated. For a given horizon depth, there is an optimal window size that detects the horizon with the least noise and greatest stability. Window sizes are chosen to focus on different depths of investigation; small windows target shallow depths, as the shallow sources produce highfrequency anomalies, and larger windows target greater depths with deepseated bodies (Archimedes Consulting [2013]).
In a study of different spectral methods (including the centroid method adopted by Okubo et al. [1985] and Tanaka et al. [1999]) of estimating the depth to the bottom of magnetic sources (also regarded as a proxy for an estimate of curiepoint 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]). 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. This would tremendously assist researchers in appropriate spectral calculation and CPD investigations.
Discussion
In the recent paper by Abraham et al. ([2014]) `Spectral analysis of aeromagnetic data for geothermal energy investigation of Ikogosi Warm Spring  Ekiti State, southwestern Nigeria’, 22 randomly selected blocks of dimension 55 × 55 km (data window) were utilized for CPD evaluation using spectral centroid method. Consequently, they obtained CPD results ranging between 11.48 and 21.91 km. However, bearing in mind the extent of deepseated magnetic investigations such as CPD, the utilization of such a small window width may be a fundamental error in the application of spectral methods for aeromagnetic interpretation for geothermal explorations. The authors themselves admitted that the spectrum of a magnetic map only contains depth information to a depth of length (L)/2π, and if the source bodies have bases deeper than L/2π, they cannot be resolved by spectral analysis (Shuey et al. [1977]; Salem et al. [2000]).
Considering a data window of 55 × 55 km used by Abraham et al. ([2014]), only depth information to a depth of 8.75 km could be satisfactorily resolved by spectral analysis. Consequently, the CPD values reported in the paper could likely be erroneous. The 55km dimension windows will neither be adequate to analyze 11.48 nor 21.91km base depths as reported by Abraham et al. ([2014]). 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.
Summary
Accordingly, the estimated CPD values and ensuing geothermal gradients and heat flows by Abraham et al. ([2014]) could be erroneous. Therefore, it is recommended that the CPD results be recomputed with appropriate window width.
Abbreviation
 CPD:

curiepoint depth
Declarations
Acknowledgements
The critical and constructive comments of all the anonymous reviewers and editors are highly acknowledged.
Authors’ Affiliations
References
 Abraham EM, Lawal KM, Ekwe AC, Alile O, Murana KA, Lawal AA: Spectral analysis of aeromagnetic data for geothermal energy investigation of Ikogosi Warm Spring  Ekiti State, southwestern Nigeria. Geothermal Energy 2014, 2: 6. 10.1186/s4051701400060View ArticleGoogle Scholar
 Archimedes Consulting (2013) Mapping subsalt and subbasalt structures from magnetic and gravity data. Accessed 2 July 2014, [http://www.geoexpro.com/articles/2013/02/mappingsubsaltandsubbasaltstructuresfrommagneticandgravitydata]
 Chiozzi P, Matsushima J, Okubo Y, Pasquale V, Verdoya M: Curiepoint depth from spectral analysis of magnetic data in central southern Europe. Phys Earth Planet Int 2005, 152: 267–276. 10.1016/j.pepi.2005.04.005View ArticleGoogle Scholar
 Nabighian MN, Grauch VJS, Hansen RO, LaFehr TR, Li Y, Peirce JW, Phillips JD, Ruder ME: The historical development of the magnetic method in exploration. Geophysics 2005, 70(6):33–61.View ArticleGoogle Scholar
 Okubo Y, Graf RJ, Hansent RO, Ogawa K, Tsu H: Curie point depths of the island of Kyushu and surrounding areas, Japan. Geophysics 1985, 53: 481–494. 10.1190/1.1441926View ArticleGoogle Scholar
 Ravat D, Pignatelli A, Nicolosi I, Chiappini M: A study of spectral methods of estimating the depth to the bottom of magnetic sources from nearsurface magnetic anomaly data. Geophys J Int 2007, 169: 421–434. 10.1111/j.1365246X.2007.03305.xView ArticleGoogle Scholar
 Salem A, Ushijima K, Elsiraft A, Mizunaga H (2000) Spectral analysis of aeromagnetic data for geothermal reconnaissance of Quseir area, northern Red Sea, Egypt. In: Proceedings of the world geothermal congress, KyushuTohoku, 28 May10 June 2000Google Scholar
 Shuey RT, Schellinger DK, Tripp AC, Alley LB: Curie depth determination from aeromagnetic spectra. Geophys J Roy Astr Soc 1977, 50: 75–101. 10.1111/j.1365246X.1977.tb01325.xView ArticleGoogle Scholar
 Spector A, Grant FS: Statistical models for interpreting aeromagnetic data. Geophysics 1970, 35: 293–302. 10.1190/1.1440092View ArticleGoogle Scholar
 Tanaka A, Okubo Y, Matsubayashi O: Curie point depth based on spectrum analysis of the magnetic anomaly data in East and Southeast Asia. Tectonophysics 1999, 306: 461–470. 10.1016/S00401951(99)000724View ArticleGoogle Scholar
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