The Delineation of Deep Crustal Magnetic Bodies from Total Field Aeromagnetic Anomalies

Abstract

Studies were made of accurate, total field aeromagnetic data of excellent uniformity over an area in NW. Ontario extending from 4845'N to 53N latitude and from 86W to 94W longitude. The rocks in this area are of early Precambrian age (Archaean era). The regional effects from the data were removed. The residual values were then analytically filtered to obtain the total field stripped of its near-surface, high amplitude components. Eighty-five individual anomalies were selected and analyzed to determine the horizontal dimensions and the depths to the tops and bottoms of the causative bodies and the magnitude and direction of total polarization vectors associated with the bodies. The bodies were assumed to be vertical rectangular prisms of arbitrary polarization. A brief summary of the method used is presented.The horizontal dimensions of the bodies vary from 0.8 km (.5 milez to 5.1 kms (3.2 miles). The depths to the top of the bodies in the filtered aeromagnetic map are found to be in the range 4.8 kms (3 miles) to 8 kms (5 miles) from the surface of the ground. The depths to the bottom of the bodies are between 17.7 kms (11 miles) and 24 kms (15 miles) for most of the anomalies, with a mean depth of 20 kms (12.4 miles), which is interpreted as the mean depth to the Curie point geotherm. The horizontal vectors of the polarizations are subparallel to the magnetic lineation of the filtered aeromagnetic map. An interesting and surprising result of this analysis is the detection of the presence of alternatingly normal and reverse polarization roughly in the East-West direction. The mean inclination and declination for normal polarizations are 67.6 and 258.7 respectively whereas those for reverse polarizations are -67.9 and 81.3 respectively. The consistency in the directions of magnetization in the two clusters for normal and reverse polarization indicates strongly the absence of any dominant component of magnetization parallel to the present field. On the basis that the magnetizations are permanent and the reversals are due to field or selfreversal, the calculated pole position for the rocks of Archaean era is found to be at a latitude of 31.5 N and a longitude of 136.9 W. The intensity of magnetization of the bodies lies within a wide range from. 0006 to. 349 cgs emu with the average of. 055 cgs emu.

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

“…Thus the Curie temperature isotherm corresponds to the basal surface of magnetic crust and can be calculated from the lowest wavenumbers of magnetic anomalies, after removing the approximate regional field from the aeromagnetic data (e.g. Bhattacharyya and Morley 1965;Spector and Grant 1970;Mishra and Naidu 1974;Byerly and Stolt 1977;Connard et al 1983;Hamdy et al 1984;Blakely 1988;Tanaka et al 1999;Salem et al 2000;Ross et al 2006). …”

“…More recently, investigations have been made for parts of the territory of Japan (by Okubo 1985Okubo , 1989Okubo , 1994, USA (by Mayhew 1985;Blakely 1988), Greece (Tsokas et al 1998;Stampolidis and Tsokas 2002), Portugal (Okubo et al 2003), Bulgaria (Trifonova et al 2006(Trifonova et al , 2009 and Turkey (Dolmaz et al 2005;Maden 2009). Authors consider the power spectrum of the total geomagnetic field intensity anomaly over a single rectangular block using the expression, which was first given by Bhattacharyya (1965). The equation was transformed into polar wavenumber coordinates (S,ψ), and the average depths to the top of magnetized bodies from the slope of the log power spectrum were calculated.…”

Curie Point Depths of Albania Inferred from Ground Total Field Magnetic Data

“…Magnetic data have been used in various parts of the world in order to estimate Curie point depths (e.g., Vacquier and Affleck, 1941;Serson and Hannaford, 1957;Bhattacharyya and Morley, 1965;Shellinger, 1972;Smith et al, 1974;Leu, 1975, 1977;Byerly and Stolt, 1977;Shuey et a1., 1977;Wasilewski et a1., 1979;Blakely and Hassanzadeh, 1981;Mayhew, 1982;Connard et al, 1983;Huppunen, 1983;Okubo et al, 1985Okubo et al, , 1989Blakely, 1988;Smith and Braille, 1994;Tsokas et al, 1998;Tanaka et a1., 1999;Salem et al, 2000;Stampolidis and Tsokas, 2002;Ates et al, 2005).…”

Curie Point Depths Based on Spectrum Analysis of Aeromagnetic Data, West Anatolian Extensional Province, Turkey

“…Curie point depth estimation from magnetic data has been used extensively to estimate the thermal structure of crust in various tectonic settings (e.g., VACQUIER and AFFLECK, 1941;SERSON and HANNAFORD, 1957;BHATTACHARYYA and MORLEY, 1965;SMITH et al, 1974SMITH et al, , 1977BHATTACHARYYA andLEU, 1975, 1977;BYERLY and STOLT, 1977;SHUEY et al, 1977;BLAKELY and HASSANZADEH, 1981;CONNARD et al, 1983;MAYHEW, 1982MAYHEW, , 1985OKUBO et al, 1985OKUBO et al, , 1989OKUBO and MATSUNAGA, 1994;BLAKELY, 1988;HISARLI, 1996;TSO¨KAS et al, 1998;TANAKA et al, 1999;RAVAT, 2000;STAMPOLIDIS and TSO¨KAS, 2002). The Curie point depth is known as the depth at which the dominant magnetic mineral in the crust passes from a ferromagnetic state to a paramagnetic state under the effect of increasing temperature.…”