Imaging Below Shallow Volcanics Using Long Offset Data.
Duncan Emsley, Phillips Petroleum Company United Kingdom Ltd.
Abstract:The presence of shallow volcanic rocks can severely limit the success
of standard seismic methods in imaging the deeper section. This problem is
prevalent in the Rockall Trough and the Faroes-Shetland Basin where Tertiary
volcanics, related to the opening of the Atlantic, mask potentially prospective
sediments below them. This paper is a review of Phillips’ attempts to develop a
method to overcome this.
Standard seismic methods are thought to fail for several reasons: Poor
penetration of the basalt owing to the strong reflection coefficient at the top
basalt; The rugose and heterogeneous nature of basaltic rocks can lead to a
scattering of energy and potential travel time issues in the stacking process; The
effort required to remove strong multiple energy generated at the basalt
interfaces can lead to the inadvertent removal of the relatively weak Pwave
energy that does penetrate the basalt.
In 1996, Phillips conducted an experiment to attempt to image below the basalts
in the Northern Rockall Trough. Modelling suggested that at offsets beyond the
basalt critical angle, it might be possible to record energy that could be used to
image below the basalt. This energy was modelled as being mode converted to
Swave at post-critical distances. Invoking a similar conversion back to P on the
upward leg of the journey, the energy could be recorded by standard, towed
acquisition methods.
Based on the modelling, an acquisition was performed, employing offsets up to
12.5km. Events were identified that were similar in appearance to those modelled
and these were processed and stacked by standard processing techniques, by
virtue of the symmetric travel path, to give a credible image beneath the basaltic
horizon. Depth Imaging work further enhanced the image and a long offset
Pwave depth image gave some encouragement that the events being imaged
were real.
Since this work was performed, the use of long offsets has become
commonplace amongst oil companies and contractors working in the North
Atlantic. Phillips has employed this method in other areas of the UK and Irish
continental shelf with the aim of improving the sub-volcanic image. The results of
these lines show significant improvement in the interpretability of these areas.
Long offset acquisition can benefit these problem areas in several ways: Invoking
a P-S conversion at the top basalt allows for a more efficient conversion at postcritical angles; The large amount of refraction occurring at the top of a fast, thick
basalt might push the subsurface coverage to beyond those distances commonly
recorded in standard acquisition; Multiple energy at long offset occurs at a much
greater travel time, giving a window in which the weak reflection energy might be
visible, without the need of demultiple methods; Sampling rugose and
heterogeneous bodies with sub-horizontal travel paths may smooth out the travel
time errors that cause mis-stacking at short offset.