Exploration Geophysics - Volume 12, Issue 4, 1981

Two functions, H(x) and V(x) involving the combinations of both the horizontal Tx(x) and vertical Th(x) gradients of the total magnetic intensity anomaly due to a buried sloping step are defined as

H(x) = Tx(x) cos 2/β + Th(x) sin 2/β and

V(x) = Th (x) cos 2/β - Tx (x) sin 2/β

where β is the effective inclination of the magnetization vector. By combining the two gradients Tx(x) and Th(x) in this fashion the effect of inclination of the inducing field is removed in the two functions H(x) and V(x). Characteristic points on H(x) and V(x) curves such as zero crossings and equi-gradients are used to evaluate the parameters of the step. This technique can be extended easily to magnetic anomalies in vertical and horizontal components.

Fictitious time shifts with consequent time and velocity errors are introduced when conventional replacement static corrections are used on marine seismic data, recorded over irregular or steeply dipping water-bottom environments. Correct common depth point (CDP) stacking of seismic reflection signals involves the determination of appropriate time-variant static corrections for different receiver geo-phone positions. Static corrections are shown to depart from surface consistency over irregular water-bottom environments.

To study these problems, raypath modelling has been carried out for two irregular water-bottom situations which are typically complex. A technique is illustrated for determining static corrections in such areas.

A gravity survey of the Woy Woy district, south of Gosford, New South Wales, comprising some 500 gravity stations is conducted to an accuracy of 1 GU in Bouguer anomalies. The resulting map shows predominance of a regional gradient mainly associated with the continental margin. It is suggested that a first degree surface for the regional effect is preferable to a second or higher degree surface. It is shown that the orientation of the linear gradient can be explained almost completely by the attitude of the continental slope and the regional southward dip of the underlying Permian rocks. The residual anomalies show a main gravity low over the townships of Woy Woy and Ettalong. The low reaches a minimum of 18 GU and trends north-south, a major regional drainage direction. It is interpreted to reveal an ancient channel of Brisbane Water, now filled with unconsolidated young sediments replacing the rocks of the Gosford Formation and reaching a maximum thickness of about 90 m.