Gravity and Magnetic Interpretation in Exploration Geophysics

Gravity and magnetic methods are among the oldest and cheapest geophysical methods, that are routinely used to solve quickly a wide variety of geological problems. However, gravity and magnetic anomalies are contributed simultaneously by density and magnetization contrasts at different depths, and their interpretation in terms of the causative sources at several depths is many times ambiguous. Thus, very little effort is generally put in for a serious interpretation of these anomalies, which in many cases ends with identification of trends and their correlation with known geology. When properly interpreted, gravity and magnetic anomalies can provide geological information quite disproportionate to the money and time spent on their collection. However, practicing geophysicists and researchers are seriously handicapped by the typical absence of published books with catalogues of curves and listings of programs that can be routinely used for a better interpretation of gravity and magnetic data. The present book is an attempt to present a series of computer programs and ready-to-use charts to serve the interests of reserachers and practicing geophysicists. Its major contribution is to provide computer software in the form of 33 Function Subprograms/Subroutines and 22 main programs, all coded in FORTRAN 77, to be used at different stages of gravity and magnetic interpretation in the space domain and for a wide variety of models. They can be executed on personal computers. The listed programs are generally adequate to deal with the cases of isolated anomalies attributed to any geophysical model. The subroutines are also useful to develop easily any new program, to be applied to anomalies of multiple, sources. The book also presents several interpretation techniques, rules of thumb, and charts for ready reference by field geophysicists. Anomaly equations of simple geometric models are briefly derived, their properties are listed and the theory leading to development of methods of interpretation is outlined for the benefit of students. The programs are edited in such a way that the Function Subprograms and Subroutines can be used elsewhere on other programs by the users. Each main program lists, in its comment statements, its purpose, the data to be supplied to the computer and the parameters printed as the output. It also shows the set of subprograms to be collected and compiled along with it. A few subroutines, solving a system of simultaneous linear equations, achieving numerical integration of data, or calculating some mathematical functions are included separately in the appendix. All programs and subroutines are made available on a diskette, along with instructions to create data files and sample outputs for each program. Barring one or two exceptions, programs are so chosen that their objectives are not duplicated. Details of FORTRAN coding are deliberately omitted; but a serious reader will find from the listed programs several novel ideas used to avoid singularities, to achieve generalization or to increase efficiency.