Research On1-D Forward Modeling And Investigation Depth Of Borehole-surface Electromagnetic Method By Vertical Line Source

With the development of national economy, the growing demand of petroleum, naturalgas and kinds of mineral resources is presented, and research on the deep resource explorationmethod theory and detection instrument system has attracted much attention.Borehole-surface electromagnetic method receives electromagnetic response on the groundfrom launching alternating current in the formation. Emission source is put directly intounderground, which overcomes the interference of surface and shortens the distance of thesource and object detection. Because of the advantages of wide exploration depth and highresolution, the method has broad application prospects in the area of deep resource detection.Combining with the needs of the deep resource exploration, this text provides theory ofborehole-surface electromagnetic method and gives design idea of the instrument system. Thepaper studies the1-D forward calculation of electromagnetic method, and numericalsimulation results of several typical models are given under different parameters such asemission current and transmitting frequency. The instrument design is provided theoreticalsupport through calculating exploration depth and effective detection range. The main detailsand results are as follows:Through the introduction of vector, Maxwell’s equations is converted to solve theproblem on Helmholtz equation of vector from Maxwell’s equations, and Helmholtz equationis solved through variable separation method. The numerical solution of borehole-surfaceelectromagnetic for method vertical line source is received with using the formula of linearfilter. Comparing respectively electromagnetic forward modeling results and uniform pointsource and vertical electric dipole electromagnetic field in full space, Reliability of thealgorithm is verified.Based on one dimensional forward calculation formula of borehole-surfaceelectromagnetic method, electromagnetic response characteristics were analyzed in theuniform whole space, uniform half space model and three layers space. The paper gives thenumerical simulation of borehole electromagnetic response in three layers of the earth modelwith respectively changing the launch electric current, frequency, source characteristics andelectrical resistivity, which provide theoretical basis for parameter of electromagnetic launchsystem settings.According to the requirements of the deep exploration, the computing of prospectingdepth using borehole-surface electromagnetic method by vertical line source is given.Comparing electromagnetic response in the two layers of the earth model and uniform halfspace model, studies the abnormal electromagnetic anomaly response from the abnormal layerdepth, which determine the detecting precision of the receiving system requirements. At the same time, the text calculates electromagnetic prospecting depth data in the case of differentlaunch electric current, frequency, line source characteristics and distribution of earthresistivity when instrument resolution of electric field is10-8V/m. and analyzes influence lawof electromagnetic prospecting depth, which provides theoretical support for improvingexploration depth of the electromagnetic method.Due to requirements of resources exploration depth and high resolution, receiversmeasurement are put into the wells cooperating ground measurement way, which will improvethe ability of exploring deep resources. The electromagnetic response calculation formula inthe borehole is got when detection point in the theory of one dimensional forward is set in thewell. Electromagnetic field simulation of uniform the earth model and typical three layermodel is done. At the same time, the paper contrasts the surface and borehole electromagneticresponse in the equivalent space between receiver and transmitter. the results are received:surface electric field value is bigger, the borehole magnetic field component is bigger. Theeffect results of resistivity and magnetic conductivity for effective exploration area in theborehole are given. Finally, based on the second prospecting space detection depth, instrumentparameters are given by the forward calculation, and propose the instrument system designand data processing flow, which will provide theoretical foundation for the electromagneticfield test.Based on forward modeling theory of the borehole-surface electromagnetic method,electromagnetic field response by vertical line source is calculated, and gives characteristicsanalysis of electromagnetic responses and exploration depth calculation through the numericalsimulation. At the same time, effective exploration is solved when receiving device is set inthe borehole. These studies will provide theoretical basis for different detection instrumentsand indicators, and have practical significance and reference value for electromagneticmethod applied in the deep resource exploration and improving measurement accuracy.