AMJlG Interference Noise Study Increases Vessel Productivity by Minimising Time Share
Authors: Karwatowski J. 
Published: October 2008

It is important to minimise the time taken to evaluate licensed acreage. The acquisition of a timely quality seismic data set is a critical first step which requires maximum vessel productivity. In areas where the weather and environmental factors limit the time available to record data limited acquisition windows can result in several vessels acquiring data in the same area concurrently. This can produce interference on the recorded data. Once the noise reaches a pre-set level it is deemed unacceptable and the time available for acquisition is shared equally among all of the vessels effected, with a consequent reduction in vessel productivity. Interference was anticipated when six vessels were committed to critical programs West of the Shetland Islands (U.K.) in 1996. Minimal amount of time share was essential if these key surveys were to be completed in time. The Atlantic Margins Joint Industry Group (AMJIG) therefore initiated a study into acceptable interference levels in the area. Four key areas were investigated: the ability of brute stacks to estimate the effect of noise; the impact of the magnitude and direction of the interference; the effect of changes in geology on final data quality and; the ability of contractors proprietary processing techniques to attenuate seismic interference. The main conclusions of the study were that: brute stacks did not produce a reliable assessment of the impact of interference; time share could be minimized if the vessels were positioned end-on with respect to each other; acceptable noise levels were target-specific and varied across the area; proprietary processing techniques were effective in attenuating broadside noise. This would allow higher levels of broadside noise to be tolerated in the field. The standard interference noise measures and the methodology for establishing the need for time share which were developed in the study increased vessel productivity by reducing unnecessary time share.

Introduction

In early 1996, AMJIG's Seismic Network noted that six vessels would be working within a restricted area between the Shetland Islands (U.K.) and the Faeroe Islands (Denmark) for the majority of the upcoming season. These programs could not be delayed because they were either part of an existing license commitment or were targeted on acreage likely to be made available during upcoming rounds. Many programs were completion's of surveys on held acreage which had been suspended in bad weather at the end of the 1995 season and were required to meet 1997 drilling commitments. This need for timely data would result in several vessels attempting to acquire data in same area concurrently. Each vessel would then produce a source of interference for all the other vessels in the vicinity. When the noise reached some pre-set level it would be deemed unacceptable and timeshare requested. In early 1996, the practice within the UK and in many other areas of the world, was to grant time share on request. The time available for acquisition would then be shared equally among all the vessels affected and when more than two vessels were involved, would significantly reduce vessel productivity.