Interpretation of magnetic anomalies using some simple characteristic positions over tabular bodies

The authors are thankful to the reviewers for their critical analysis of the work particularly to Dr Amanda Buckingham for patiently correcting and suggesting some points.

Åm, K., 1972, The arbitrarily magnetized dike; Interpretation by characteristics: Geoexploration 10, 63–90.
| Crossref | GoogleScholarGoogle Scholar | Bott M. H. P. , 1973, Inverse method in interpretation of magnetic and gravity anomalies. In: Methods in computational physics. B. A. Bott (Ed.), Vol. 13, pp. 133–162. Academic Press.

Breiner, S., 1973, Application manual for portable magnetometers. Geometrics.

Cook, K. L., 1959, Quantitative interpretation of vertical magnetic anomalies over veins: Geophysics 15, 667–686.
| Crossref | GoogleScholarGoogle Scholar | Grant, F. S. and West, G. F., 1965, Interpretation theory in applied geophysics: McGraw Hill Book.

Green, R., 1979, The harmonic method of inverting a magnetic profile over a contact: Geoexploration 17, 261–268.
| Crossref | GoogleScholarGoogle Scholar | Heiland C. A. , 1940, Geophysical Exploration: Prentice-Hall, Inc.

Hood, P., 1965, Gradient measurements in aeromagnetic surveying: Geophysics 30, 891–902.
| Crossref | GoogleScholarGoogle Scholar | Jakosky, J. J, 1940, Exploration Geophysics: Times-Mirror Press.

Koulomzine, Th., Lamontagne, Y., and Nadeau, A., 1970, New methods for the direct interpretation of magnetic anomalies caused by inclined dikes of infinite length: Geophysics 35, 812–830.
| Crossref | GoogleScholarGoogle Scholar | Nettleton, L. L., 1940, Geophysical prospecting for oil: McGraw Hill Books.

Nettleton, L. L., 1976, Gravity and magnetics in oil prospecting: McGraw Hill Books.

Rao P. T. K. S. , 1987, Dike model in magnetic known properties and new ideas for quantitative interpretation. Published in A. E. G. Souvenir of 13th Annual Convention, 12–14 November 1987.

Rao, P. T. K. S., 1988, Interpretation of magnetic gradient anomalies using characteristic positions of equi-angular separation. Geological contact and vein: Geoexploration 25, 199–209.
| Crossref | GoogleScholarGoogle Scholar | Reeves C. V. , 1981, Optimizing the interpretation of magnetic and gravity data by computer inversion of numerous collected anomalies – paper presented at the 51st Annual International Meeting and Expo. Society Exploration of Geophysics, Los Angles.

Reford, M. S., 1964, Magnetic anomalies over thin sheets: Geophysics 29, 532–536.
| Crossref | GoogleScholarGoogle Scholar | Telford, W. M., Geldart, L. P., Sheriff, R. A., and Keys, D. A., 1976, Applied Geophysics: Cambridge University Press.

ΔF — magnetic anomaly. (ΔT — total component and ΔV — vertical component anomalies)

C_F — Amplitude coefficient.

θ _F — Index parameter.

T — Total intensity of the earth’s main magnetic field.

K — Susceptibility contrast.

B₀′ = B₀ (1 – cos² I_T sin² A_T)^1/2. It is the component of the Earth’s normal flux density, B_0, in the plane of the profile.

I_T — inclination of the Earth’s magnetic field.

A_T — declination of the Earth’s magnetic field from x-axis.

i — inclination of the resultant magnetization.

a — declination of the resultant magnetization from x-axis.

I — resolved direction of the induced component of magnetization in the xz plane.

Y — resolved direction of the resultant (remanence and induction) component of magnetization in the xz plane.

tan I = tan I_T/cos A_T.

tan Y = tan i/cos a.

α — strike of the body.

X — distance of the point of observation from the origin.

h — depth to the top of dyke, sheet, fault or contact.

H — depth to the bottom of the fault.

2w — width of the dyke.

t — thickness of sheet/thin plate.

δ — geological dip.

Magnetic expression for the dyke is given by

where

and

and

Magnetic expression for the sheet is given by

where

and

Magnetic expression for the vertical step is given by

where

and

Table A1. Characteristic properties of the anomaly curve to fix the correct, quadrant of θ _F (θ _F is as obtained from the method).

The method discussed in the present paper has been developed for interpreting the magnetic anomalies over single and simple tabular geometries. When the interpretation method is applied over a magnetic profile of overlapping anomalies (anomalies over two or more than two bodies), there may be a small amount of error in the determination of origin and datum. To show the effect of the interpretation technique, Synthetic anomalies over two (2.5 dimensional) bodies separated by 200 m were considered. On applying the geometric procedure for determining the origin and datum over the two bodies, the error percentage was found to be small. For computing the error percentage in datum, the peak to peak amplitude was used. The synthetic profile over two bodies and the interpretation is shown in Figure 10. The interpreted results along with actual values and per cent errors are given in the Table A2.

Table A2. Geomagnetic Field Parameters: Intensity = 42 000, Inclination = 60°, Declination = 0°, The profile bearing = 0°, reference height = 0. For both the bodies strike length = 100 m.