Response Characteristic Analysis And Recognition And Correction Of Static Effect Of MT Sounding

Magnetotelluric sounding method(MT) is one of the geophysical exploration methods, generated by measuring the electric and magnetic field of the natural alternating electromagnetic field on the surface to detect the underground electric change. According to the different frequency attenuation, the information of the electromagnetic field at different depth can be detected. Because of the method does not require an artificial field source, low cost, simple and convenient field work, no shielding of the high resistance layer, high resolution of low resistivity layer, less effects of topographic, now has been widely used in oil and gas prospecting, earthquake monitoring and prediction, deep prospecting for solid mineral, engineering geology and environmental protection.Static effect is caused by the lack of the high frequency to detect shallow anomaly. Under the action of an external electric field, abnormal body boundary produced accumulated charges, and the charges course a secondary field which is proportional to the external electric field but irrelevant to the frequency, so the apparent resistivity curve in the double logarithmic coordinates moves up and down along the ordinate axis, and phase curve does not change.The static effect of two-dimensional anomaly will make YX apparent resistivity have a large longitudinal abnormal band, and the apparent resistivity and layer information obtained from inversion will be wrong.Analyzing the impact of various parameters on the response characteristics of the static effect, such as different measuring point position, transverse and longitudinal size, abnormal body thickness and depth of shallow small low and high resistance anomaly.At the same time, we design four simple models as layer, graben, faults and deep large anomalies, with shallow small low resistivity body. We use highest frequency of apparent resistivity and phase values and correlation coefficient to identify the static effect, via the phase conversion method, five point filtering method and seven point filtering method to correct the static effect. As a result, we find that under the simple and stable structure the phase conversion method is more ideal, but as the frequency increases, the resistivity curve will have a certain deformation, while five point filtering method and seven point filtering method keep the curve shape, and seven point filtering method for static effect correction is much better than five point filtering method.Finally, we recognize and correct the static effect of 10737 north-south stations on the Ordos block. After the five point filtering method and phase conversion method, the vertical anomaly contour is suppressed, and the pseudosection map shows a good layer distribution.