Computers & Geosciences 30 (2004) 591–599 A Fortran 77 computer code for damped least-squares inversion of Slingram electromagnetic anomalies over thin tabular conductors $ Derman Dondurur*, Co - skun Sarı Department of Geophysics, Engineering Faculty, Dokuz Eylu¨l University, Kaynaklar Campus, Buca, Izmir 35160, Turkey Received 22 May 2003; received in revised form 19 February 2004; accepted 27 February 2004 Abstract A FORTRAN 77 computer code is presented that permits the inversion of Slingram electromagnetic anomalies to an optimal conductor model. Damped least-squares inversion algorithm is used to estimate the anomalous body parameters, e.g. depth, dip and surface projection point of the target. Iteration progress is controlled by maximum relative error value and iteration continued until a tolerance value was satisfied, while the modification of Marquardt’s parameter is controlled by sum of the squared errors value. In order to form the Jacobian matrix, the partial derivatives of theoretical anomaly expression with respect to the parameters being optimised are calculated by numerical differentiation by using first-order forward finite differences. A theoretical and two field anomalies are inserted to test the accuracy and applicability of the present inversion program. Inversion of the field data indicated that depth and the surface projection point parameters of the conductor are estimated correctly, however, considerable discrepancies appeared on the estimated dip angles. It is therefore concluded that the most important factor resulting in the misfit between observed and calculated data is due to the fact that the theory used for computing Slingram anomalies is valid for only thin conductors and this assumption might have caused incorrect dip estimates in the case of wide conductors. r 2004 Elsevier Ltd. All rights reserved. Keywords: Slingram anomaly; Perfect tabular conductors; Least-squares inversion; Numerical differentiation; FORTRAN 77 1. Introduction Slingram (so-called Horizontal Loop Electromag- netic, HLEM) is a maximum coupled system which uses horizontal transmitter and receiver coil pairs kept at a fixed distance apart (usually 30, 60 or 90 m). The receiver system normally measures both in-phase and quadrature components of the secondary field as a percentage of the primary field intensity. The system is commonly used in exploring for conductive ore bodies and for groundwater exploration in fractured zones (Palacky et al., 1981; McNeill, 1990). In electromagnetic methods, calculation of the re- sponse of a conductive target with arbitrary shape is more complicated than that in other geophysical methods. The structures for which an analytical solution can be obtained are rather limited, e.g. a thin perfectly conductive vertical or dipping dike, sphere and disk- shaped conductors (Grant and West, 1965). Duckworth et al. (1991) suggested a method for quantitative depth estimates by transforming Slingram anomalies to a form which is free from the effect of the coil separation, however, the interpretation of Slingram data is generally achieved using type curves or Argand diagrams (Lowrie and West, 1965; Nair et al., 1968; Parasnis, 1971; ARTICLE IN PRESS $ Code available from server at http://www.iamg.org./ CGEditor/index.htm *Corresponding author. Fax: +90-232-4538366. E-mail address: derman.dondurur@deu.edu.tr (D. Dondurur). 0098-3004/$ - see front matter r 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.cageo.2004.02.003