Layer stripping of shear-wave splitting in marinePSwaves

Abstract

S U M M A R YThe properties of split S waves can be used to infer: (1) the state of stress and strain in the Earth; (2) the directional dependence of hydraulic conductivity and (3) small changes in pore-fluid pressure in the rock mass that occur in response to dynamic processes, such as the earthquake cycle. Measurements of split S waves are particularly useful in shallow (<1000 m subseabed) marine sediments, where S-wave splitting from an azimuthal elastic anisotropy is typically produced by the presence of near-vertical aligned cracks. Here we present a method of measuring small amounts of S-wave splitting in marine P-to-S mode-converted waves, and illustrate the technique with data from an ocean-bottom seismometer (OBS) deployed on the west Svalbard continental slope. The analysis applies a modified version of the Alford rotation and layer-stripping technique developed for zero-offset S-wave sources and treats PS waves that undergo mode conversion at reflectors that are close to the seabed in comparison with the overlying water depth. When the seismic record contains coherent signal on both the in-plane and out-of-plane components, the layer-stripping technique is capable of decoupling the Swave splitting from the effects of P-wave velocity anisotropy and reflector dip that influence the downgoing, P wave, part of the ray path. The amount of S-wave splitting in the data is small, however, and we find that this causes a greater practical problem for the analysis than the known theoretical limitations of the layer-stripping theory (such as use of a finite-offset source). For the analysis of the example data we develop a number of procedures that are necessary to mitigate the low signal-to-noise levels. These include using a wide range of shot-receiver azimuths to generate data redundancy, methods of identifying and rejecting poor measurements, and a predictive layer-stripping approach that minimizes the propagation of errors through the analysis that arise from scatter in the layer-by-layer results.With the PS waves of the example data, which have a dominant period about 30 ms, we find the technique is capable of measuring S-wave splitting to a precision of about 0.5 ms for 4 or 5 layers. The number of layers successfully treated would increase if the amount of S-wave splitting were larger than in these data, for which the total cumulative S-wave splitting was about 10 ms over a 450 m depth interval. The orientation of the fast split S wave was measured with a precision of about 15 • . Our results give an S-wave velocity anisotropy of 1-2 per cent in the shallowest 25-30 m subseabed that implies the presence of a differential horizontal stress at, or very close to, the seabed. The S-wave splitting accumulated throughout the investigated section at a rate that was consistent with predictions made for a single set of parallel, fluid-filled cracks with crack-density about 0.015. The fast S wave was found to be oriented at 75 ± 15 • for the uppermost 150 m, before drifting clockwise to an azimuth of 190-210 • . The ...

In situ stress state from walkaround VSP anisotropy in the Kumano basin southeast of the Kii Peninsula, Japan

“…Seismic anisotropy is a phenomenon that can be used to reveal in situ stress state [e.g., Crampin et al , 2008]. Seismic anisotropy within sediments is related to the amount of fracturing (or crack) and the dominant orientation of those fractures [ Kaneshima et al , 1988; Haacke and Westbrook , 2006; Haacke et al , 2009]. For rocks containing vertically aligned cracks (horizontal transverse isotropy; HTI), the fast velocity direction coincides with the direction of crack alignment; the degree of velocity difference provides information about crack density [ Crampin , 1985].…”

Geographical distribution of shear wave anisotropy within marine sediments in the northwestern Pacific

“…10a). The observed anisotropic structure within marine sediments has been explained by the presence of aligned cracks (e.g., Haacke and Westbrook 2006;Haacke et al 2009). In the outer rise region, because the plate is bending along the Japan and Kuril trenches, the tensional stresses are induced within the uppermost of the Pacific Plate nearly in the trench-normal direction.…”

Observation of shear‐wave splitting in the multicomponent node data from Atlantis field, Gulf of Mexico

“…Determination of this additional angle (θ) is generally straight forward in the ideal marine data and can be done with a hodogram analysis of the direct wave arriving from shots to a receiver though the water column (Alford 1986; Gaiser 1999). Another way of determining θ is application of different angles of rotation to the horizontal components till the minimum direct arrival energy is observed in the rotated transverse component (Haacke and Westbrook 2006; Haacke et al 2009). Both the methods return two values of θ (for the maximum positive and negative values of the radial component) and therefore the angle estimate is non‐unique.…”

“…After rotating the data, we examine the gather for typical signs of anisotropy induced S‐wave splitting both in the radial and transverse components. Those signs are: periodic variation of traveltime and amplitude in the radial component (cos 2 (ϕ−ϕ SYM ) type) and polarity flip in the transverse component at every 90° azimuth interval (Crampin 1985; Tatham et al 1992; Gaiser 1999; Simmons 2009; Haacke et al 2009; Maultzsch et al 2009). Note that in the process of data preparation, we never applied any typical data processing scheme (e.g., filtering, stacking or azimuthal binning).…”