Summary

High-resolution 2D marine reflection seismic data were acquired in a narrow sound in shallow water with high acoustic-impedance contrasts. This yielded a volume of raw data with very strong, short-period multiples and weak signal below the seabed, implying extra demands for the processing. In order to avoid the loss of useful signal when attempting to enhance the S/N, a powerful f-k filter was used early in the processing, followed up by post-stack deconvolution and migration. In spite of extraordinary challenges and drawbacks in the processing the final result is promising, with stacked sections that depict intra-basaltic horizons. Some of those horizons can be identified from onshore extrapolation of their recognized positions and attitudes. After preliminary interpretation of the tight grid of seismic lines, a 3D horizon could be derived for better visualization, and this work can be used to test a recently hypothesized strike-slip fault in the study area. Consequently, as many research projects are carried out aiming at sub-basalt imaging – mostly with the application of low-frequency sources and the utilization of the bubble pulse for deeper penetration – this work exhibits the use of a high-frequency seismic source for intra-basalt imaging in the uppermost few hundred meters with quite interesting results.

Introduction

Depicting intra-basaltic horizons and other geological features within flow basalts is not a simple task, but a great challenge for geoscientists in both industry and academe. Earlier – as well as nowadays – in the NE Atlantic region much focus has been directed towards sedimentary basins predating flow basalts. However – depending on the thickness of the basalts, acquisition methods and parameters – the base basalt may be fairly hard to estimate from seismic data. This issue combined with the fact that intra-basaltic sedimentary strata in some places have been shown to contain significant hydrocarbons make it even more relevant to look for the intra-basaltic features – how and to what extent they can be distinguished in seismic data. This has resulted in a few projects – both academic and industrial as well as a combination of both – focusing on sub-basalt and intra-basalt imaging. One of these is the PUFFINS project at the University of the Faroe Islands.

This paper presents some results on intra-basalt imaging using high-resolution 2D marine reflection seismic data acquired between islands close to coasts in the Faroe Islands (Figure 1). The main challenges in the data arise because of the very high impedance contrast and the rugose surface at the water bottom. Furthermore, the data have been acquired in shallow waters – i.e. about 80 to 120 m – which implies short-period seabed multiples and, therefore, masking of useful signal at short intervals. This work demonstrates seismic data sections that have been processed with powerful f-k filtering, post-stack deconvolution and migration. It furthermore demonstrates the recognition and interpretation of intra-basaltic horizons followed up by the derivation of 3D horizons that serve to test a current hypothesis of a strike-slip fault in the study area.

Geology

The Faroe Islands is a small archipelago centred around 62º north and 7º west in the north-east Atlantic Ocean (Figure 1).

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