Research On The Response Law Of Controlled Source Audio-frequency Magnetotelluric Method With Topography

With the in-depth development and wide application of the controlled source audio-frequency magnetotelluric method in the actual exploration field,many influencing factors in the exploration also appear,especially the complex topography conditions in mountainous areas will lead to serious distortion of CSAMT exploration results and have a great impact on the detection accuracy of geological abnormal bodies.Starting from the basic theory of the controlled source audio-frequency magnetotelluric method,this paper establishes different models according to the actual geological conditions,and studies the electromagnetic response characteristics of CSAMT with different topography and geoelectric conditions.The main conclusions are as follows:1.One dimensional model:(1)With the Type H model,only when the resistivity of the overlay is much higher than the intermediate lower resistance layer or is much greater than the cover layer thickness,the response to the low resistance layer can be clearer,and the120/140-point filter coefficient is better than the 47/61-point filter coefficient;(2)With the Type A model,when reducing the resistivity of the coversage,the response of the three-layer section can be fully reflected;when the thickness difference between the top layer and the middle layer is increased,it more accurately reflects the model;(3)With the Type K model,when increasing the resistivity of the cover layer,the reflection of the visual resistivity curve on the high blocking surface becomes apparent,the longer the receiving and receiving distance,the more obvious the reflection result is,and the 47/61 point filter coefficient is better than the 120/140 point filter coefficient;when the thickness of the high resistance layer is increased and the thickness of the covered low resistance layer is reduced,the visual resistivity simulation results of both filtering coefficients reflect the high resistance more clearly;(4)With the Type Q model,changing the layer thickness more fully reflects the model,it shows that the layer thickness size plays an important role in the positive response results.2.Two dimensional model:(1)Both apparent resistivity(ρ_s)and apparent phase are affected by topography:false high resistivity anomaly and low phase anomaly at valley and false low resistivity anomaly and high phase anomaly at ridge.(2)Compared with the pure valley topography,the valley topography contains low resistivity bodies,which makes the apparent resistivity decrease and the apparent phase increase;The inclusion of high resistivity increases the apparent resistivity and decreases the apparent phase.Compared with the pure ridge topography,the ridge topography contains low resistivity bodies,which decrease the apparent resistivity and increases the apparent phase;The inclusion of high resistivity increases the apparent resistivity and decreases the apparent phase.(3)When the horizontal width of topography is reduced from 480 m to 400 m,the amplitude difference of apparent resistivity in valley topography(including low resistivity body)decreases,and the apparent resistivity value at the abnormal center changes from 834Ω·m to 789Ω·m;The response range and amplitude difference of apparent resistivity in valley topography(including high resistivity body)are reduced,and the value of apparent resistivity at the center of anomaly changes from 11Ω·m to 10.36Ω·m;The response range and amplitude difference of apparent resistivity in ridge topography(including low resistivity body)increase,and the value of apparent resistivity at the center of anomaly changes from 173Ω·m to 115Ω·m;The response range and amplitude difference of apparent resistivity in ridge topography(including high resistivity body)increase,and the apparent resistivity value at the abnormal center changes from 8.96Ω·m to 8.47Ω·m.(4)When the elevation difference of topography increases from40 m to 120 m,the response range and amplitude difference of apparent resistivity in valley topography(including high resistance body or low resistance body)increase,the apparent resistivity value at the abnormal center point in valley topography(including low resistance body)changes from 644Ω·m to 829Ω·m,and the apparent resistivity value at the abnormal center point in valley topography(including high resistance body)changes from 10.2Ω·m to10.4Ω·m;The response range and amplitude difference of apparent resistivity in ridge topography(including high resistance body or low resistance body)increase,the apparent resistivity value at the abnormal center point in ridge topography(including low resistance body)changes from 353Ω·m to 144Ω·m,and the apparent resistivity value at the abnormal center point in ridge topography(including high resistance body)changes from 9.55Ω·m to8.76Ω·m.(5)When the resistivity of surrounding rock in topography increases from 250Ω·m to 1000Ω·m,the amplitude difference of apparent resistivity in valley and ridge topography increases,the apparent resistivity value at the abnormal center point in valley topography changes from 358Ω·m to 1588Ω·m,and the apparent resistivity value at the abnormal center point in ridge topography changes from 128Ω·m to 428Ω·m.(6)When the buried depth of abnormal body in topography increases from 120 m to 320 m,the response range and amplitude difference of apparent resistivity in valley and ridge topography decrease.The apparent resistivity value at the abnormal center point in valley topography changes from 106Ω·m to 51Ω·m,and the apparent resistivity value at the abnormal center point in ridge topography changes from 98Ω·m to 45Ω·m.(7)With the increase of the size of the anomalous body in the topography,the response range and amplitude difference of the apparent resistivity in the valley topography decrease,and the apparent resistivity value at the center of the anomaly changes from large to small to large;The response range and amplitude difference of apparent resistivity in ridge topography increase,and the apparent resistivity value at the anomaly center changes from large to small to large.Finally,the topography correction method is studied,and the CSAMT measured data of a coal mine in Shanxi Province are corrected,and the results are verified by drilling.