Acquisition and inversion of Love wave data to measure the lateral variability of geo-acoustic properties of marine sediments

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Abstract

A towed sledge system has been utilised to generate and receive Love waves at the seabed. Due to unique deployment procedures, the system is capable of acquiring data both rapidly and efficiently over an extensive area. An experiment was undertaken to assess the capability of the system to measure the lateral variation of the shear wave velocity in unconsolidated near-surface sediments along a 4-km survey line. The site chosen was known to display significant variations in sediment characteristics over relatively short distances and could therefore provide a suitable test. A parametric approach was used to obtain phase-velocity dispersion curves from the Love wave data sets. This approach enabled the fk-spectra to be resolved to a sufficiently high level using a limited number of receivers. Finally, the shear wave velocity profile for each record was estimated with respect to a reference model using a non-linear least squares inversion algorithm. Results indicated that the shear wave velocity field varied significantly along the survey line. The shear wave velocity at a depth of 30 cm below the seabed changed from 30 to 55 m/s over the length of the survey line. The velocity variations correlated well to geotechnical data acquired from the area, suggesting that Love waves acquired from only five seafloor receivers can successfully be used to construct near-surface models of seafloor shear wave velocity in unconsolidated near-surface sediments, with a lateral resolution of up to 25 m and a depth measurement range of up to 4 m.

Introduction

In the marine environment, the geo-acoustic properties of near-surface unconsolidated sediments play a key role in predicting a number of physical properties. More specifically, the knowledge of near-surface shear wave velocity (Vs) values are of great importance in both geophysical and geotechnical applications. The shear wave velocity in unconsolidated sediments is primarily a function of the number and area of grain-to-grain contacts and the effective stress, σ, across those contacts. As such, it is a sensitive measure of the fabric of the sediment and thus can be used in the prediction of geotechnical parameters. A number of empirical relationships have been formulated linking Vs to σ and the void ratio (e) Hardin and Richart, 1963, Hamilton, 1976, Bryan and Stoll, 1989. Being empirical, such relationships have not proven universally applicable. However, these relationships can be used to predict void ratio from in situ shear wave velocity–depth data (Huws et al., 2000). The void ratio is defined as the ratio of pore space volume to solid grain volume and is, by proxy, a measure of the nature of the particle contacts and it, in turn, can be used to predict an approximate mean grain size (Hamilton, 1974). Knowledge of the shear wave velocity field has also been useful in the areas of sediment liquefaction-potential studies Robertson et al., 1992, Pyrah et al., 1998 and seafloor classification (Davis et al., 1997). On a larger scale, it is likely that the near-surface shear wave velocity field, if accurately modelled, could be used in the prediction of shear wave statics in commercial shear wave data.

Since the shear wave velocity is essentially a sensitive measure of the fabric of the sediment, any minor variation within the sediment fabric may result in a significant change in Vs. In near-surface marine sediments, the shear wave velocity can therefore vary significantly both vertically and laterally. In situ shear wave velocities of less than 20 m/s at depths of around 30 cm have been reported for silty-clays (Richardson et al., 1991), but for sands at similar confining stresses, the velocity can be higher than 100 m/s. Variability of this scale is large when compared to equivalent change in compressional wave velocity. A transition from silty-clays to sands on the continental shelf can occur over a relatively short distance (<1 km). In engineering terms, a transition of this magnitude would correspond to a significant change in the geotechnical properties of the sediments. In geophysical terms, the combination of low Vs and high variability associated with such a transition would result in large static errors in mode converted shear wave data. Thus, measuring the lateral variability of shear wave velocity in near-surface sediments has potentially very important and obvious applications.

The measurement of the shear wave velocity profile in marine sediments is often logistically difficult due to the nature of shear waves but a number of studies have succeeded in accurately measuring near-surface velocity profiles through various techniques. Richardson et al. (1991) used an approach whereby bender elements were directly inserted into soft sediment to measure the shear wave velocity between transmitter and receiver probes. Davis et al. (1989) describes the development of a seafloor shear wave refraction system capable of acquiring refraction profiles at discrete points along a survey line. Unlike alternative systems, it was able to measure the lateral variability in the topmost sediments over large distances. However, depending on the velocity–depth structure of the sediment, the system could exhibit limited vertical penetration. An extensive number of studies have concentrated on indirectly measuring the shear wave velocity profile by the inversion of interface waves of both the Scholte and Love type Caiti et al., 1994, Bautista and Stoll, 1995. The inversion of interface wave data as a method of estimating the shear wave velocity profile has the advantage over other approaches that it is able to obtain shear wave data in areas of velocity inversion and potentially to greater depths. Stoll and Bautista (1994) developed a source–receiver system designed to generate Sh waves and Love waves at the seabed. This source was deployed on the seafloor with a large receiver array from a ship at anchor and was used to collect a single Love wave data set. Using an individual seismic trace, the multiple filter technique was used to derive the average shear wave velocity profile between the source and the receiver. By repeating this procedure for each receiver, an indication of the lateral variability of the sediment could be made.

This paper describes the development of new bottom dragged instrumentation capable of acquiring Love wave data both rapidly and efficiently over large distances. In addition, it tests the feasibility of the system as a tool for observing the variation of shear wave velocity both with depth and distance in marine sediments. The technique is based on the estimation of the shear wave velocity profile from Love wave dispersion characteristics using a non-linear least squares inversion algorithm. Previous shear wave velocity values using this technique in land based experiments have proven reliable when compared with additional measurements.

Section snippets

Source

In collaboration with the British Geological Survey, a sledge mounted shear wave source was designed to excite Sh and Love waves in the seafloor as a result of a horizontal pulse applied to the sledge frame. The source is a refinement of a simple operating principle originally used by Schwarz and Conwell (1974) and then subsequently developed by the University of Wales, Bangor (Davis et al., 1989) whereby a high voltage electromagnetic hammer imparts a horizontal force relative to the seabed.

Description of experiment

An assessment of the capability of the system to detect lateral variations in shear wave velocity was made by means of a field survey. A site approximately 1 km offshore at Red Wharf Bay, Anglesey, UK (Fig. 2), was chosen since it was known to have a sand bank in the middle of the bay which provided a marked lithological transition from the surrounding silts over a relatively short distance. Specifically, the survey line, measuring approximately 4 km long, crossed from a sandy-silt facies into

Dispersion analysis

One of the most common approaches to measuring the dispersion characteristics of surface wave data is the fk-spectrum method (e.g. Gabriels et al., 1987). The analysis involves the transformation of surface wave data to the fk-domain. The data can then be used to create a phase-velocity-frequency spectrum using a simple relationship between the frequency (f), the wavenumber (k) and the phase-velocity (c) (c=2πf/k). However, tests have shown that the performance of this method is reduced in

The inversion algorithm

The final step in estimating the shear wave velocity profile is to invert the phase-velocity dispersion curve with respect to a reference model. The reference model consists of a number of layers each with specified shear wave velocity, compressional wave velocity, Vp, density, ρ, and thickness, dz. Synthetic phase-velocity values, calculated for the reference model, are used to invert the field Love wave dispersion curves. The inversion process is least sensitive to ρ and Vp. Nevertheless,

Shear wave velocity profiles

The Love wave records acquired along the designated survey line displayed a relatively limited frequency range. It is likely that a greater source–receiver offset combined with a greater number of receivers would have increased the frequency range for which phase-velocity values could be calculated. For each record, the frequency band of the phase-velocity dispersion curve that fulfils the criteria stated was used in the inversion. The phase-velocity dispersion curve calculated using values

Discussion

From preliminary inspections it is apparent that there are differences in the shear wave velocity profiles obtained from each of the facies. Shear wave velocity profiles from the silt region display higher surface velocity values than those obtained from the areas with higher sand content. It would be expected that for a pure silt, the shear wave velocity would be lower than in sands since Vs is essentially a function of the void ratio (Richardson et al., 1991). Published inter-relationships

Conclusions

The Love wave profiling system developed by the University of Wales, Bangor has demonstrated that surface wave data can be collected both rapidly and efficiently due to its unique deployment procedure. The low voltage electromagnetic hammer has proved to be a suitable source for the acquisition of such data. The low voltage principal behind the development of the source has also removed the need for an on-board power supply thereby simplifying the operation of the system.

Experiments carried out

Acknowledgements

We would like to thank the crew of the RV Prince Madog for their help in acquiring the data, Schlumberger Cambridge Research for part-funding of the project and the British Geological Survey for their collaborative work in developing the low voltage shear wave source used in this study.

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