Shear-Wave Source for Offshore Geohazard Studies: A Pilot Project To Improve Seismic Resolution and Better Constrain the Shear Strength of Marine Sediments
Authors: Vanneste M. , Westerdahl H. , Sparrevik P. , Madshus C. , Lecomte I. , Zühlsdorff L. 
Published: October 2008

Most offshore geohazards involve features located within the first hundreds of meters of the seabed. Whereas conventional high-resolution P-wave seismic data are necessary to obtain a detailed image of the subsurface, they are not sufficient to extract important geotechnical information such as shear strength. Since the introduction of multi-component seismics in offshore investigations the last 10 years, it has been proved that P-to-S converted waves (also called C-waves) bring useful and complementary information to PP reflections, in terms of imaging, lithology and fluid identification to reservoir mapping. However, it comes at the expense of a more complicated processing sequence due to non-symmetric raypaths. Using pure SS reflections appears therefore very attractive, similar to PP processing in terms of symmetry, though S-waves must be generated and recorded at the seabed.

Despite their potential added value, few commercial or academic shear-wave sources exist. Therefore, a robust fully workable prototype shear-wave vibrator has been developed at the Norwegian Geotechnical Institute (NGI), aiming at reservoir levels. Even though the first testing results are promising, the prototype is a rather heavy electro-hydraulic source in a suction anchor, and is not suitable for quick surveying or hard seabed conditions. An improved and significantly smaller sledge-type version is currently under study, which will allow quick surveying (2D profiling), focus on shallower targets, and which will be easily deployed from smaller research vessels.

In parallel to the design and construction of the sledge-type shear-wave source, modeling studies are started up to define SS processing sequences, inspired by PP ones, multicomponent offshore acquisition techniques and land seismic data. It is important to address issues such as resolution as well as subsurface illumination, in order to constrain the imaging, whereas rock physics dedicated to unlithified sediments is paramount to establish proper relationships between sediments, rocks, fluids and elastic/physical parameters, and ultimately soil mechanics.

Objectives

The objectives of this paper are (1) to present a pilot technological project in which new shear-wave sources are being developed; (2) to discuss how these sources may contribute to a better understanding, improved imaging and illumination as well as characterization of the subsurface, from shallow horizons to reservoir depths; and (3) reflect on the use of shear-wave data for offshore geohazard investigations.

On the importance of shear-wave data as a complement to compressional wave data in offshore applications

P- and S-waves

Whereas conventional P-wave seismic data are essential to understand continental margin evolution and sediment dynamics, there are several instances in which either imaging is poor, or the data are inadequate in describing the in situ properties [1]. This is due to the fact that P-wave propagation senses both sediment matrix and pore fluids. Gas pockets or shallow gas in particular lead to degraded P-wave images [2], making it impossible to obtain details about either the extent and saturation of the gas accumulation, or draw conclusions about the underlying structures, stratigraphy and fault configuration.