The non-geometric¯P Swave in high-resolution seismic data: observations and modelling

Abstract

Summary In data from a high‐resolution seismic survey conducted over a near‐surface environment consisting of homogeneous soft clay, we consistently observe a distinct seismic phase arriving between the direct compressional wave and the Rayleigh wave. This phase is characterized by high amplitudes at near offsets and a phase velocity corresponding to about twice the shear wave velocity. Based on analytical and numerical analyses, this signal could be unambiguously identified as a non‐geometric wave, which is excited if the source is located near the Earth’s surface and the Poisson’s ratio in the vicinity of the source is unusually high. To date there are only a few speculative observations of this particular non‐geometric seismic wave phenomenon. However, given the commonly very high Poisson’s ratio of surficial materials, we expect this phase to be present, albeit unidentified, in many near‐surface seismic surveys. The presence of this non‐geometric wave increases the complexity of the seismic record, and failure to identify it may result in misinterpretations, particularly of high‐resolution seismic reflection data as well as of shear wave and surface wave data. Potential applications of this seismic phase may arise from its high sensitivity to the shear wave velocity in the immediate source region.

Nongeometrically converted shear waves in marine streamer data

“…Subsequently, their findings found theoretical support by a Cagniard-de Hoop analysis by Drijkoningen and Chapman (1988). In high-resolution seismic exploration data on land, Roth and Holliger (2000) also describe the occurrence of such a wave, and support their interpretation by a Cagniard-de Hoop analysis.…”

Feasibility of detecting near-surface feature with Rayleigh-wave diffraction

“…2a shows a synthetic shot gather for void Model 1 with a source to diffraction point distance (d) of 29 m. The non-dispersive direct surface wave traveling at 190 m/s is the surface wave in the homogenous half-space. A seismic event with a phase velocity (∼400 m/s) of about twice the S-wave velocity is also modeled, which is called nongeometric¯P S wave and due to a high Poisson's ratio (Roth and Holliger, 2000). Diffractions with weak energy can be identified from the unfiltered data (Fig.…”

“…a) A synthetic shot gather of a 2 m × 2 m void at the depth of 2 m in a homogeneous half space with the nearest offset of 1 m and a trace interval of 1 m. Three waves are noticed: the non-dispersive direct surface waves traveling at 190 m/s; the non-geometric¯P S wave due to a high Poisson's ratio(Roth and Holliger, 2000) traveling at a phase velocity (∼400 m/s) of about twice the S-wave velocity; and diffracted surface waves. b) FK filtered data reveal diffracted surface waves traveling at 190 m/s (a straight line).…”

Shear Waves in Streamer Data via Non-geometrical Conversions

“…Its reason lies in the fact that the arrival includes an evanescent exponentiallydecaying part for which this ray path cannot be drawn. Examples of such waves have been reported in the past, like on land in global seismology (Daley & Hron, 1983) and in shallow land seismic (Roth & Holliger, 2000), called S* and a P S-wave respectively. Recently, such a non-geometric wave has also been discovered in a marine environment by Allouche et al (2011), where an evanescent P-wave in the water converts to a propagating S-wave in the water bottom.…”