Study Of CSAMT Sounding Distortion On Three-dimensional Anisotropic Bodies

The controlled-source audiofrequency magnetotelluric (CSAMT) responses ofthree-dimensional(3-D) structures due to a grounded electric bipole confirms becomesvery complex . Because of the source it will bring some "source effects" , that weonly know they exist and haven't researched their rules and emendation methods,such as near-field effect , transmitter overprint effect , shadow effect , etc . So it isvery significant to investigate integral equation modeling of 3-D CSAMT responseand the modeling software INTEM3D_v2 which is based on integral equation is veryuseful .The CSAMT method produces results similar to plane-wave responses for 3-Dtargets in the far field of a bipole transmitter . However , a large conductive orresistive body below a receiving site extends or contracts , respectively , the far-fieldfrequency range .When an inhomogeneity is located beneath the transmitter , the effects onbroadside measurements made in the modeled CSAMT survey area range from minorto considerable . When the body below the source is resistive , a slight shift of thenear-field zone to higher frequencies occurs . A smaller 3-D conductor below thesource actually appears to extend the far-field zone to lower frequencies or tolocations nearer the transmitter . Soundings from the collinear configuration when a3-D conductor lies below the transmitter do not display the large transition-zoneresponse seen in the broadside results;this contributes to a significant increase in thelow –frequency range of the far-field zone .Measurements display a shadow effect due to the primary-field decay . For thebroadside configuration and the resistive target , the results has more obvious effect incomparison with collinear configuration and the conductive target , respectively .For a 3-D conductive target , current channeling from conductive overburdeninto a 3-D conductor enhances the response of the target relative to the situationwithout the overburden . The static effect will occour due to a partial inhomogeneityabove target , that has obvious effect to apparent resistivity and slight effect to phase .