Offshore Geotechnical Investigations with Shear Waves

Abstract Shear wave velocity methods are well established for geotechnical site investigations on land. Presently, acoustic based systems are the predominant means of seismically investigating offshore sites. There is great potential for applying shear-wave based methods to the offshore environment to measure the shear stiffness of sediments, a characteristic that is not measured with acoustic-based systems. As the offshore industry moves into water depths exceeding 1500 m, the demand for innovative methods of geotechnical site characterization will be more pressing. The present state-of-the-practice in offshore shear wave velocity measurements is reviewed. Limitations on operational water depths and subbottom profiling depths are discussed. Lastly, a brief examination of some potential applications of these methods in the offshore environment is presented. Introduction In situ shear wave velocity measurements have been widely used on land for geotechnical site characterization over the past three decades. In the realm of seafloor investigations, compression-wave based methods, such as sonar and subbottom profilers, presently dominate the in situ seismic characterization methods. These methods are commonly referred to as acoustic methods and involve detecting reflected acoustic wave energy from both the seafloor surface and from soil layers within the seafloor. The primary uses of acoustic profilers are to evaluate seafloor topography, subbottom layering, anomalies in layering or lateral continuity, and potential geohazards that can be estimated from such profiles. For many applications involving engineering design, these methods are only used peripherally because they provide no direct information on the engineering properties of the sediments. The major advantage of shear-wave based methods is that they measure a basic property of the sediment, the shear modulus of the material skeleton. The shear modulus, G, of the sediment is an important parameter which is used either directly in evaluations such as determining deformations in the seafloor from static and dynamic loads, or indirectly in evaluations such as estimating shear strength of cohesive soils, the liquefaction potential of granular soils, or the in situ density of gravelly soils. To understand the relationships for a soil sediment between in situ shear wave velocities, linear and nonlinear shear modulus and stress-strain behavior, consider Figs. 1 and 2. The stress-strain (?-?) relationship for a soil sediment is a nonlinear function as illustrated in Fig. 1 for monotonic loading. This ?-? relationship can be represented by an initial tangent shear modulus, denoted as Gmax, and subsequent secant shear moduli, denoted by G1, G2, etc., which exhibit decreasing values with increasing strain, as shown in Fig. 1. When this relationship is displayed in terms of the shear modulus versus the logarithm of shearing strain, as shown in Fig. 2, it is observed that there is a range of strains over which the value of G remains equal to Gmax. It is in this shearing strain range where in-situ shear wave velocity measurements are performed. These measurements represent effective stress measurements at the instant the measurement is performed, and they are equally applicable to very soft sediments (Vs = a few m/sec) as well as very stiff rock (Vs = several 1000 m/sec).