| Time-domain electromagnetic method(TDEM)also refereed as transient electromagnetic method(TEM),is an important branch of geophysical electromagnetic exploration.The source types of TDEM typically include loops and grounded dipole,where the secondary electric fields are usually recorded to interpret the distribution of conductivity of the underground.TDEM has been extensively used in mineral and geothermal resource exploration,deep crustal studies,nvironmental and partly engineering investigations,as its advantages of higher detection efficiency,lower cost,higher resolution and the flexible survey configuration.Electrical anisotropy of strata has been long recognized by field and laboratory observations from 1960 s,where the large effects of electrical anisotropy on magnetotelluric method(MT)and frequency-domain electromagnetic method(CSEM)have been well studied these years.To clarify the influence of electrical anisotropy on TEM responses,we developed a three-dimensional(3D)robust finite-volume(FV)algorithm for simulating the time-domain electromagnetic diffusion fields in an arbitrarily anisotropic medium efficiently.The source type can be set as loop-source and grounded dipole source.The influences of anisotropy on TEM response are well studied through a series of numerical experiments for 3D models with different types of electrical anisotropy.Lastly,long-offset transient electromagnetic method(LOTEM)data will be processed and interpreted using the 3D anisotropic modeling algorithm.Inversion modeling is a vital procedural in the interpretation of TEM data,and forward modeling plays a central role of inversion modeling.The anisotropy is an important parameter of conductivity,which can affect the distribution of the electromagnetic diffusion field.In this paper,the time-domain Maxwell's equations are discretized using the mimetic finite-volume method(MFV)on a conventional staggered grid in the space domain and discretized in the time domain using the backward Euler method.Adaptive time-step doubling method and direct solver are selected to solving the system matrix efficiency.The initial fields are calculated using two different methods which include analytic and numerical method.The initial fields of loop-source only include the static magnetic field stimulated by the current in the loop,while the initial fields of grounded dipole source are consist of the static magnetic filed and the direct current electric field injected from the electrodes.As the initial fields and source configurations are definitely different,the accuracy of our algorithm is validated separately for these two types of method through comparing against quasi-analytic solutions of 1D layered anisotropic models and published numerical solutions of 3D isotropic models.Then,various series of anisotropic models are designed and simulated to analyze the effects of anisotropic parameters which include the anisotropic coefficient,principle resistivities,anisotropic dip angle and strike angle.The modeling results and conclusions provide important insights into survey design and data interpretation for TEM surveys in different type anisotropy geological environments.Based on the anisotropic analysis of LOTEM response from synthetic model above,a profile of LOTEM data from an old mining area in eastern Thuringia,Germany are analyzed.The general existence of black shale had been demonstrated in this area,producing a bulk anisotropy.Based on the 1D and 2D analysis of LOTEM data,using trial-and-error modeling method,a 2D anisotropic model is designed which can explain the LOTEM data and results in a reasonable quantitative data fit.The derived 2D anisotropic model is in good agreement with the geological information.The Schur complement method is utilized to reduce the modeling time consumption.The case study can be referred to processing the TEM data in the area with distinct anisotropy to avoid misinterpretation. |
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