Research And Application Of CSAMT Two-dimensional Constraint Inversion

Controlled Source Audio Magneto Telluric(CSAMT) method is a frequency domain geophysical exploration method which developed rapidly in recent years. This method has a high exploration efficiency, strong anti-interference ability, sensitive to low resistivity layers, so it is widely used in geothermal research, oil and gas exploration, engineering geophysical prospecting and many other aspects of the research work. Although CSAMT method has the strong anti-interference ability, in ore concentration areas, economic developed areas, high voltage line laying areas or other strong interference areas, electromagnetic fields are highly susceptible to humanities interference, the quality of collected data is not very high, and even there is a big difference with the real situation of underground electrical structures. In order to improve the effect of the CSAMT application, and make the measured data in disturbed areas reflect the actual underground electrical structure as true as possible, improve the inversion accuracy, this paper presents electrical constraint inversion method, providing further measured data on local target structure on the basis of conventional image inversion, by using log data obtained from actual drilling for electrical constraint inversion, we can improve the inversion interpretation accuracy, thus contributing to obtain a richer geoelectric information, provide technical support for the detailed study of geological structural feature.This paper first introduces the basic theory of CSAMT method, and deduces the wave equation of electromagnetic field which derived from maxwell’s equations, From this basis, different electric and magnetic formulas in different diatance was deduced by electromagnetic wave from the horizontal electric dipole source, and finally got the apparent resistivity and apparent phase formula of CSAMT. On the basis of conventional OCCAM inversion algorithm, we improve the model roughness of the objective function, and deduce the iterative formula of electrical constraint inversion algorithm according to the modified objective function. By cleverly adding a priori geoelectric model constraint items in inversion objective function and adding the known geophysical data and other priori information in the priori geoelectric model what electromagnetic inversion required, then in the process of inversion iteration, correction model will gradually move closer to the priori information in the area of priori information is already available, and finally achieve the inversion effect of electrical constraints, improve the fitting accuracy of the model response with the measured data.This paper presents a few simple model which simultaneously under the condition of conventional OCCAM inversion and improved the electrical constraint inversion, then compare the two results. The results show electrical constrained inversion can make the boundary contour position of target body inwardly convergent and more compact, resistivity values within the target body region is more close to the initial value of original model, It was to some extent, improve the inversion accuracy both in the aspect of target boundary position and resistivity value, and more accurately reflect the actual form of the model.Geothermal exploration in changbai mountain area as an example, using the CSAMT exploration data as resistivity inversion model, and use the log data derived from survey profile as constraint data, we can realize that electrical constraint inversion method can improve the inversion accuracy, thereby increasing resolution of exploration, thus better for people to understand underground electrical structure in the study area, reduce the economic loss caused by wrong explanation to guide the engineering construction, and as a method to guide the development of different types of constraint inversion.