2014 刘云鹤 3D anisotropic modeling for airborne EM systems using finite-difference method

3D anisotropic modeling for airborne EM systems using finite-difference method

Yunhe Liu,Changchun Yin ⁎

College of Geo-Exploration Sciences and Technology,Jilin University,Changchun 130021,China

a b s t r a c t

a r t i c l e i n f o Article history:

Received 18January 2014Accepted 2July 2014

Available online 11July 2014Keywords:EM

Airborne EM 3D modeling

Electrical anisotropy Finite difference method

Most current airborne EM data interpretations assume an isotropic model,which is sometimes inappropriate,especially in regions with distinct dipping anisotropy due to strong layering and strati fications.In this paper,we investigate airborne EM modeling and interpretation for a 3D earth with arbitrarily electrical anisotropy.We implement the staggered finite-difference algorithm to solve the coupled partial differential equations for the scattered electrical fields.Whereas the current density that is connected to the diagonal elements of the anisotropic conductivity tensor is discretized by using the volume weighted average,the current density that is connected to the non-diagonal elements is discretized by using the volume current density average.Further,we apply a divergence correction technique designed speci fically for 3D anisotropic models to speed up the modeling process.

For numerical experiments,we take both VMD and HMD transmitting dipoles for two typical anisotropic cases:1)anisotropic anomalous inhomogeneities embedded in an isotropic half-space;and 2)isotropic anomalous inhomogeneities embedded in an anisotropic host rock.Model experiments show that our algorithm has high calculation accuracy,the divergence correction technique used in the modeling can greatly improve the conver-gence of the solutions,accelerating the calculation speed up to 2times for the model presented in the paper.The characteristics inside the anisotropic earth,like the location of the anomalous body and the principal axis orien-tations,can also be clearly identi fied from AEM area surveys.

©2014Elsevier B.V.All rights reserved.

1.Introduction

Airborne electromagnetic method (AEM)is an effective and ef ficient survey tool for geological mapping,mineral exploration,ground water,environmental and engineering exploration (Smith,2010).Due to its moving platform for EM transmitter and receiver,no human access to the survey area is required;and renders it possible for this technology to be widely used in areas like deserts,high mountains,swamps and those covered by heavy greens or forests.

AEM data are generally processed and inverted (or transformed)based on 1D layered earth model (Auken et al.,2005;Brodie and Sambridge,2006;Chen and Raiche,1998;Christensen et al.,2010;Farquharson et al.,2003;Huang and Fraser,2002;Sattel,2005;Tartaras et al.,2000;Viezzoli et al.,2009;Yin and Hodges,2007).All models used so far for AEM assumed an isotropic layered earth,which is,in many cases,a good approximation to the geophysical reality.How-ever,in regions with distinct dipping strati fications,where the earth cannot be considered as isotropic,these kinds of isotropic models are no longer adequate.

Electrical anisotropy considered in geophysical prospecting general-ly means the macroanisotropy or structural anisotropy that occurs for thin layers of different materials that are isotropic themselves,or for faults saturated with water that have a preferred direction,or for a parallel alignment of conductive rods channeling currents (Everett and Constable,1999).The series and parallel connections resulted from these small structures lead to the change of resistivities with the direction of the current flow (i.e.electrical anisotropy).

In the past,the effect of electrical anisotropy on EM survey systems mainly focused on ground and marine EM.Weidelt (1999)put forward a 3D anisotropic modeling algorithm for MT,Chen et al.(2009a,b)studied the integral equation (IE)algorithm for CSAMT and marine CSEM,while Li and Dai (2011)used finite-element (FE)method for 2.5D marine CSEM anisotropic modeling.In contrast,airborne EM data have been processed and inverted (or transformed)mainly by an isotro-pic earth.Avdeev et al.(1998)and Newman and Alumbaugh (1995),respectively studied the AEM system responses over a 3D isotropic earth by staggered finite-differences (FD)and the integral equation algorithm,while Cox and Zhdanov (2008),Cox et al.(2010),Sasaki (2001),and Zhdanov and Chernyavskiy (2004)developed 3D algo-rithms for airborne EM inversion.Among these researches little atten-tion has been paid to the effect of the electrical anisotropy of the earth on the AEM system responses,except for Yin and Fraser (2004)who

Journal of Applied Geophysics 109(2014)186–194

⁎Corresponding author.

E-mail addresses:liuyunhe@ (Y.Liu),yinchangchun@ (C.

Yin).

/10.1016/j.jappgeo.2014.07.0030926-9851/©2014Elsevier B.V.All rights

reserved.

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