QUANTITATIVE STUDIES CONCERNING THE VERTICAL GRADIENT AND SECOND DERIVATIVE METHODS OF GRAVITY INTERPRETATION*

In the first part of the present paper we shall investigate the possibility of localising highly situated inclined faults with the aid of the vertical gradient and the second derivative in the direction of the vertical. Since these quantities have been computed from gravity values by means of formulae of approximation we shall have to study the question as to their applicability for possible quantitative interpretation. Particular caution should be exercised when making the usual comparison with theoretical test examples. For three effects have to be taken into account which result from the application of the formulae of approximation:

• 1)   The extreme values appear more or less smoothed out
• 2)   Extremal abscissae are being displaced
• 3)   The results are influenced by the orientation of the grid which forms the basis of the calculation.

For a practical instance it was possible to locate a well under troublesome circumstances. This well is situated on the downthrown side of an inclined fault, the depth of the upthrown side being known as a result of another well.

We shall show in the second part of the present paper how small, deeply situated structures may be recognised in the diagrams of the vertical gradient and of the second derivative. In the case of two practical instances the effect of these structures is rendered unrecognisable in the isogam map in view of regional influences. The results according to the formulae of Baranov, Elkins and Rosenbach are contrasted with each other.