Controlled Source Audio-frequency Magnetotelluric Method Source Effect Research

Controlled source audio magnetotelluric method in mining, geothermal, water, oil and gas exploration has played a significant role especially in the areas of industrial interference, due to the use of artificial source, you can get a larger signal,this method become one of the powerful tools in the shallow geophysical exploration. In the controlled source audio magnetotelluric method (CSAMT) field exploration, often, there are source effects,for example, near-field effects, overprint effects, shadow effects, such as the effects of the field source is the larger problem has long plagued the CSAMT field work and data processing and interpretation, it is a serious impact on the processing and interpretation of results, currently only know of their existence, the impact of the law and the correction method is still in the research stage, therefore, identify the source effect, the characteristics and laws of the analysis of field source effects,it is very importan to the CSAMT’s practical application effectt.In this article, for the problems brought about by these field sources effect, we designed a different geological models, the use of the3D forward modeling software Emigma to simulate, and select a few field test areas that are similar with the previous design theoretical geological model, test data acquisition. Comparative study with the theoretical model of the response data and the measured data of the field test area, we have a new understanding of the characteristics of the field source effects.First, different distance of receiver and transmitter,variety of unusual body or construct under the field sources (bedrock depth, fracture, lithology contact zone, etc.) and abnormal body (resistivity high or low, size) between the field source and the receiving point.In the cases, the simulation results show the influence the characteristics and laws of the source effect on the apparent resistivity and impedance phase:the greater transceiver distance, the deeper the probing depth; the geological conditions below to change the source field source and the source of the original field different, have a greater impact on the observed apparent resistivity and impedance phase; high resistance bedrock deeper, underground resistivity lower than expected; source is located in the bedrock shallow, on the contrary; the greater the fracture width, the greater affection the apparent resistivity and impedance phase; low resistivity body on the layout of the field source can cause false anomalies; There are abnormal body between the field source and the receiving point, the affection of low resistivity body is greater; the anomalous body reaches a certain size, to the observed curve impact.Secondly, choose a few test areas which are similar with geological models designed previously (from the size of the transceiver, change source, bedrock depth, there is a lithological contacts, etc.), test data collection, the same conclusion with the forward modeling.