1D and 2D interpretation of the Magnetotelluric (MT) data of northeast Gorgan plain



A detailed standard Magnetotelluric (MT) study was conducted to recognize brine bearing layers in depths of less than 2000 m in the northeast of Iran close to the southeastern shore of the Caspian Sea. Long and medium period natural-field MT methods have proved very useful for subsurface mapping purpose by determining the resistivity of the near surface structure.
MT data were analyzed and modeled using a 1D inversion scheme. Then corresponding data on eight profiles were inverted using 2D inversion schemes.
Down to 2 km, the resistivity model obtained from the MT data is consistent with the geological information from a 1200 m borehole in the area. Analysis of the MT data-set suggests signatures of salt water reservoirs in the area which are distinguished potentially positive to contain iodine. Due to the very conductive nature of the sediments regardless of all difficulties in the interpretation stage because of the lack of a considerable resistivity contrast we could recognize the more conductive zones in the less conductive host as layers of saline water.
Conductive structures are ideal targets for the Magnetotelluric method when located in a considerably resistive host. They produce strong variations in underground electrical resistivity. In cases where the electrical resistivity of the target is not substantially different from that of it would be quite difficult to reach a promising result. Despite this limitation, we could get some useful results in our study.
Dashli-Boroon area is located in Golestan Province in the northeastern part of Iran right at the border with Turkmenistan. Geologically it is a part of the Kopeh-Dagh sedimentary basin. Kopeh-Dagh has was formed by the last orogeny phase of Alpine and the erosion that followed. Topography relief is very smooth and basically it is a flat plain consisting of loesses occurring naturally between the Elburz mountain range and the desert of Turkmenistan. Quaternary sediments including clay and evaporates and particularly salt are impenetrable.
An MT survey was carried out using GMS05 (Metronix, Germany) and MTU2000 (Uppsala University, Sweden) systems in February 2007. MT data were collected at 60 sites in a network of 2 by 2 km meshes along eight EW profiles.
For data processing a code developed by Smirnov (2003) was used. 1D and 2D inversions are conducted to resolve the conductive structures. 1D inversion of the determinant (DET) data using the code of Pedersen (2004) as well as the 2D inversion of TE, TM, TE+TM and DET mode data using a code from Siripunvaraporn and Egbert (2000) were performed.
A supplementary goal of this work is to evaluate the possibility of using surface MT measurements on the very conductive sediments to monitor the underground salt water bearing layers or bodies. Our concern which is followed in the current paper, only in the frame of one- and two- dimensional (1D and 2D) interpretation, is to place emphasis on the characteristics of the extremely conductive structures which are supposed to bear iodine in economic meanings. Based upon the MT results some sites were proposed for detail exploration by excavating deep exploration boreholes. As results the resistivity sections show a clear picture of the resistivity changes both laterally and with depth.