Waveform analysis of Scholte modes in ocean sediment layers

In an effort to determine the characteristics of seismic noise on the ocean bottom and its relationship to the structure of the sea-floor, we have adapted the method of nonlinear waveform fitting to accommodate multidimensional models (shear velocity beta and shear damping Q(s)), and have applied it to invert several records of interface waves (Scholte 1958) from the THUMPER experiment off southern California. Waveform fitting is a very powerful tool to determine the S velocity in the top few metres of the sediment. Starting from beta = 30 m s(-1) at the top clay layer, the S velocity increases with a gradient of 2.8 m s(-1) m(-1) over the first 150 m of sediment. A theoretical estimation of the source strength gives coherent estimates of Q(s) as a function of depth for distances between 400 and 1070 m from the source. The Q(s) models are characterized by very low values (10-20) in the top three metres, but by values in excess of 100 below that level. The results confirm the identification of the noise as harmonics of interface waves. In the area of this experiment, the largest noise amplitudes belong to the fundamental mode and penetrate to a depth of about 20 m into the sediment. The overtone energy can be appreciable too, and is noticeable to about 80 m depth. The Q(s) structure confirms the strong influence that the sea-floor structure has on the noise spectrum. The high attenuation at frequencies above 3-4 Hz suppresses noise propagation and produces low noise at higher frequencies. (Similarly, high attenuation in the asthenosphere suppresses noise propagation below 0.1 Hz.)