THE USE OF THE SEISMIC REFRACTION METHOD IN PAPUA

Abstract

Among the advances in geophysical techniques during the past ten years, particularly those related to multiple coverage using the seismic reflection method, the seismic refraction method has become somewhat neglected. The neglect has often been necessary since the seismologist is increasingly required to detail more complex structures and locate hydrocarbon bearing features that have been formed by subtle variations in stratigraphy or lithology, refinements that cannot be produced by the refraction method.

However, the refraction method has been used with success in areas where the primary drilling target is an interface at which there is a large increase in seismic velocity, such as in Iran where the Asmari limestone provides a good refraction horizon. Similar conditions exist in Papua where the Miocene limestone was an early target and it was with expectation of similar success that the Australasian Petroleum Company, with the Anglo Iranian Oil Company (which later became BP) as a partner, embarked on a programme of refraction exploration in Papua. Early surveys, planned before the nature of the limestone structures was fully understood, failed to provide satisfactory structural information, but as the programme progressed and expanded a pattern of refractors became recognised.

In the Eastern Delta, basement is recorded at depths of over 20,000 feet and an unreliable refractor from within the Miocene is evident. As we move towards the west, the top of the Miocene limestone becomes apparent as a refractor, increasing in intensity as we continue westwards until multiple limestone refractors become evident in the Western Delta area.