B048 Seabed Properties Derived from Ambient Noise

Transcription

1 B048 Seabed Properties Derived from Ambient Noise E. Muyzert* (Schlumberger Cambridge Research) SUMMARY An extensive study of low-frequency ambient noise wavefields in the seabed has resulted in a new approach to determine the near-surface shear velocity. 33 minutes of continuous ambient noise recorded by an ocean-bottom cable (OBC) system without using an active source were analyzed for seafloor compliance and Scholte waves. Seafloor compliance was not observed and requires longer recording time. Scholte waves were observed in the frequency-wavenumber spectrum. The Scholte wave phase-velocity was modeled with a realistic one-dimensional near-surface model. The horizontal over vertical spectral ratio of the Scholte waves was also modeled and shown to be very sensitive to the near-surface shear velocity model. Applications of the method include shear-wave statics estimation and geohazard prediction.

2 Introduction The shear velocity is an important parameter depending on the material strength and is used as an indicator for geohazards such as overpressure and shallow water flows. Shot-generated Scholte waves have been used to determine the near-surface shear velocity (Muyzert et al., 2002). This approach is usually limited to shallow water, as the conversion of acoustic energy into Scholte waves depends on the distance of the airgun array to the seabed. Mallick and Dutta (2002) obtained a near-surface shear-wave profile from a non-linear prestack waveform inversion of towed streamer data, but this approach is computationally expensive and poorly constrained. Here, we assess three techniques for estimating a shear velocity profile using ambient noise recorded by a four-component ocean bottom cable system. The first method is seafloor compliance, which is the response of the seafloor to pressure variations in the water column (Crawford, 2000). Seafloor compliance is driven by low-frequency, long-wavelength infragravity waves or sea-surface waves. A shear velocity model can be obtained from the inversion of the spectral ratio of pressure and the vertical component of the seabed acceleration. The second method is inversion of the phase velocity of Scholte waves as observed in the ambient noise records. The third technique we evaluated is related to the horizontal over vertical (H/V) spectral ratio for the surface wave amplitudes (Nakamura, 1989). Here, we estimate the H/V spectral ratio of the Scholte waves present in the ambient noise dataset and compare it to modeled H/V spectral ratios calculated for realistic nearsurface velocity models. We applied the three techniques to a unique data set, 33 minutes of ambient noise recorded by the WesternGeco Q-Seabed OBC system. The data were recorded in the North Sea in a water depth of 270 m by 448 four-component receivers with 12.5 m spacing. Figure 1. The ultralow-frequency part of the frequency-wavenumber spectrum of the ambient noise records. (left) pressure component, (right) vertical component.

3 Seafloor compliance Seafloor compliance is driven by infragravity waves or sea-surface waves. Infragravity waves were observed in the fk spectrum of the pressure component of the ambient noise dataset at frequencies between 0.02 Hz and 0.07 Hz (Figure 1). The velocity of the infragravity waves follows the well-known deepwater dispersion relation: c 2 = g / k. The amplitude cutoff that is observed around 0.07 Hz is in agreement with the theory and depends on water depth and velocity. The dispersion curve extends to higher frequencies at the positive wave numbers, likely due to the prevailing wind direction. In the fk spectrum of the vertical component, no corresponding compliance signal was observed (Figure 1). Note that compliance is usually only observed using longer noise records (1-2 days) obtained by purpose-built low-frequency ocean bottom seismometers (Crawford, 2000). Scholte wave phase velocity Figure 2 shows the pressure, vertical, and crossline component fk spectra of the ambient noise records for frequencies up to 2.5 Hz. These fk spectra show three bands of energy. The ultralow-frequency compliance has already been discussed. The fastest energy band has a velocity of 1500 m/s and is related to water-borne noise. It extends well outside the plotting range over frequencies of 2.5 Hz. The slower energy band shows dispersion and its velocity varies from around 150 m/s at 2.5 Hz to over 700 m/s at 0.5 Hz. This band is interpreted as Scholte wave energy. The Scholte waves are visible on the vertical, inline (not shown), and pressure component fk spectra where they have the same velocities. The Scholte waves propagate in the vertical-radial plane with elliptical particle motion. The vertical motion in the seabed results in a pressure wave in the water because of the continuity of vertical stresses at the sea-bottom interface. Figure 2. FK spectra of the pressure component (left), vertical acceleration (middle), and crossline acceleration component (right) for frequencies up to 2.5 Hz. The theoretical seasurface dispersion curve is indicated with the black line. The theoretical Scholte and Love wave dispersion curves are indicated by the dotted lines.

4 Phase velocities were picked in the fk spectra on the vertical, inline, and crossline component for frequencies between 0.5 Hz and 2.8 Hz. Phase velocities were modeled using the Thompson-Haskell propagator matrix approach (Aki and Richards, 1980) and compared with observed velocities (Figure 3). The first model tested was Hamilton s empirical shear velocity model (Hamilton, 1976), which is given by: Vs ( z) = 128* z This model has been successfully used in the shallow North Sea (Muyzert et al., 2002). The phase velocities calculated for the Hamilton model are too fast for frequencies above 1.5 Hz. After some testing, a model with a low velocity gradient zone in the top 30 m of the near surface was found to fit the phase velocities well (Figure 3). The Scholte wave phase velocities for this model are too fast to explain the observed energy on the crossline component (Figure 2). However, the Love wave phase velocity dispersion curve calculated for this model fit the crossline fk spectrum well. It is concluded that the ambient noise dataset shows a strong directional propagation of Scholte waves and Love waves parallel with the cable direction. It is well known that the noise field on the sea bottom is dominated by Scholte waves through the interaction of acoustic noise in the water layer related to bad weather with the seabed (Webb, 1998), but the observation of Love waves requires further explanation. This is not caused by the OBC system, as for the same deployment, excellent vector fidelity was reported (Kragh et al., 2004). H/V amplitude The spectral ratio of the horizontal over vertical component of the ambient noise field is commonly used to estimate the near-surface shear-wave velocity from land seismic data (Nakamura, 1989). Here, we apply the H/V spectral ratio method to seabed data. As the spectra between frequencies of 0.5 Hz and 4 Hz are dominated by Scholte waves, the spectral ratio is a local measure of the Scholte wave amplitude, independent of source and propagation effects. Synthetic H/V spectral ratios have been calculated for the velocity models shown in Figure 3. The modeling shows that the H/V spectral ratio is highly dependent on the nearsurface shear velocity model (Figure 3). Asymptotes in the spectral ratio occur when the vertical displacement function is zero. The observed peak in the H/V spectral ratio around 0.8 Hz may be interpreted as an asymptote. Such an asymptote was found by models with with a low-velocity near-surface layer (Figure 3) Figure 3. Phase velocity (left) and H/V spectral ratio (middle) calculated for three shear velocity models (right). The data are shown in black.

5 Conclusions It has been demonstrated that the low-frequency part of the spectrum of ambient noise recorded during an ocean bottom survey is dominated by Scholte waves. Both the Scholte wave phase velocities and spectral ratios have been modeled with a one-dimensional velocity model. The resulting shear velocity model shows low shear velocities in the top 30 m when compared with other well-established models for the same region (Muyzert et al., 2002). As the H/V method is a single-station method, extension to two dimensions is straightforward. A specific feature of the method is that it has been applied to data recorded in deeper water, at 270 m water depth without the use of an active source. The dataset has also been analyzed for seafloor compliance. The amplitude of the seafloor compliance is not large enough to be detected in these records, and it is thought that longer records might be required to observe this phenomena. Acknowledgements I thank Satish Singh and Wayne Crawford for discussion on compliance measurements. Leendert Combee, Nicolas Goujon, Ed Kragh, Robert Laws and Karin Schalkwijk are thanked for discussions. References Aki, K. and Richards, P.G. [1980] Quantitative Seismology: Theory and Methods. W.H. Freeman, San Francisco. Crawford, W. [2000] Seafloor compliance measurements: application for hydrocarbon exploration. Lithos Science Report, 2, Hamilton, E.L. [1976] Shear-wave velocity versus depth in marine sediments: a review. Geophysics, 41, Kragh, E., Vigner A., Buizard, S., Stroemmen-Melboe A., Horne, S., Robertsson J., Combee L., Iranpour K., Goujon N., Gaiser J., Caprioli P., Muyzert E. and Martin, J. [2004] Vector fidelity characterization of a marine multi-component acquisition system. 66th Meeting, European Association of Geoscientists and Engineers, Expanded Abstract. Mallick, S. and Dutta, N.C. [2002] Shallow water flow prediction using prestack waveform inversion of conventional 3D seismic data and rock modelling. The Leading Edge, 21, Muyzert, E., Kommedal, J., Iranpour, K. and Olofsson, B. [2002] Near surface S-velocities, Statics and Anisotropy estimated from Scholte Waves. 64 th Meeting, European Association of Geoscientists and Engineers, Expanded Abstracts. Nakamura, Y. [1989] A method for the dynamic characteristics estimation of subsurface using microtremor on the ground surface. QR. of RTRI, 30, Webb, S.C. [1998] Broadband seismology and noise under the ocean, Reviews of Geophysics, 36,