Analysis of the magnetic data on the basalts in Polour area
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Article Type:
Research/Original Article (دارای رتبه معتبر)
Abstract:
Summary: Polour basalts are located 75 km away from Tehran. It is situated in northeast of Tehran and south of Damavand volcano. The basalt emission point has not been distinguished yet, whereas its occurrence is a debatable topic among geologists. The procedure of formation, geological setting and the way of ascending have been always as controversial issues. Petrologically, Polour basalts are basalt–basalt trachyte rocks. Similar units are found at the east of Damavand volcano and as expected, the composition, age and feeder source are different from Damavand volcano. Polour basalts occurred as arc-shaped bodies at shallow depth in central part of study region, where they mostly consist of old terraces and alluvial fans. Because of having considerable amount of magnetic susceptibility, magnetometric survey might delineate the location of such geological units. Various depth estimation techniques have been employed to determine the depth of causative sources associated with the basaltic units in the region of interest. Introduction: Several well-known techniques have been developed to estimate the depth of causative sources in potential field studies. Among them, AN-EUL, Euler deconvolution and analysis of power spectrum methods are the most efficient ones, which provide valuable pieces of information about the geometry of the prospect geological sources. Methodology and Approaches: AN-EUL technique, as a combination of the analytic signal and the Euler deconvolution methods, is an automatic algorithm for simultaneous estimation of depth, location and geometry of the subsurface sources in the potential field studies. The derivation of the main equations of this technique is based on the substitution of the derivatives of the Euler homogeneous equation into the analytic signal of the potential field data. Location of sources can be approximately estimated from the position of the maximum value of the analytic signal amplitude, and subsequently, the formulae of depth and structural index (SI) estimation are calculated at this point. An important advantage of the AN-EUL method is that it is not restricted only to idealized sources (i.e. having integer structural index). Its wider applicability means that the SI can be a fractional number that indeed describes sources with various arbitrary shapes. Analysis of power spectrum is also one of the methods used widely to estimate the depth of geological structures. The sources of magnetic anomalies within a region are assumed to average out so that spectral properties of an ensemble of sources are equal to the average of all causative sources responsible for potential field anomalies. This approach is advantageous since it is (1) statistically oriented, (2) averaging source depths over a region containing complex patterns of anomalies, (3) less affected by interference effects due to overlapping anomalies and high-wavenumber noise than other methods because it is based entirely on analyzing the wavelengths of the anomalies, (4) independent of the directional attributes of the magnetization of the sources and the geomagnetic field, and finally (5) it can be used to study a wide range of depths by varying the window involved in the data analysis. Results and Conclusions: Employing the above-mentioned methods for estimation of the depth and geometry of subsurface anomaly to the collected ground-based magnetometric data leads to obtain valuable information about the subsurface anomaly. Thus, in this study, the depths and structural indices of the subsurface anomalies have accurately been estimated that have been strongly in good agreement with the geological information of the study area. The maximum depth on both arcs is about 95 m below the surface topography. In addition, the 3D model from the area reveals the basalt root in both bodies amazingly is from separated sources. Due to the short length of profiles, that causes to have only shallow depths of identified bodies, the trend of creeping magma from the chamber is unknown. Subsurface anomalies and their depths indicate that the Polour basalts have deep roots and have not flown on the ground surface. By proving the existence of the anomalies as signatures of basalts, the depths and structural indices of the anomalies have accurately been estimated.
Language:
Persian
Published:
Journal Of Research on Applied Geophysics, Volume:4 Issue: 2, 2018
Pages:
323 to 337
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