Shallow water sediment properties derived from high‐frequency shear and interface waves

Abstract

Mthe scahed. stibhotton s elocit v/at tentation st ruectuLre is essential input bor predictive propation nmodels. To estiratec this structure. bottoin-mounted soutces and receivers %%ere used to make mecasuirments of shear ;and compressional wave ptopitgatioti in shallow water sedimnents of the continental shell'. usually where boreholes and high-resolution retlecuion profleks give substantial suipporting geologic infoaniation ;about the suhsuu1'ace. This colocation provides an opportunity ito comipate seismically determined estimates oft physical properties of' the seabed with the "gound truth-properties. Measuirenments were miade iti 1986 %kith source/detector otffsets up to 2(M) mn producing shear wave velocity versus depth proliles of, Ithe uippet ( So it) fiiii h seabed l;and P' wase proliles to lesser depths). Measuremients tn 1988 were made with smailer source devices designed to emphasiz.e higher f'requencies and recoirded by an array of 3(0 sensors spaced atf I-m intervals to imiprove spatial samnpling and resolution of s hallow structure. These investigations with shear waves have shown that significa~nt laiteraml and vertical variations in the physical properties of the shallow seabed are cotumion and tare principally created b.% erosional and depositional processes assoctated %%ith glacial cycles and sea level oscillations during the Quaternarv. When the seabed structure is relatively uniform over the length of the profiles. the shear wave fields aire well ordered, and the iuatching of the data with full waveform synthetics has been suceessl'ul. producing velocity/attenuation models consistent with the subsurface lithology indicated by coring results. Both body waves and LILow-frequency sound propagation in shallow water environments is not restricted to the water __ column but also involves the subbottom. Thus. as well as being important for geophysical description of the seabed. subbottom velocity/attenuation structure is essential input for predictive propagation models. To estimate this structure, bottom-mounted sources and receivers were used to make measurements of shear and compressional wave propagation in shallow water sediments of the continental shelf, usually where boreholes and high-resolution reflection profiles give substantial supporting geologic information about the subsurface. This colocation provides an opportunity to compare seismically determined estimates of physical properties of the seabed with the -'ground truth" properties. Measurements were made in 1986 with source/detector offsets up to 200 m producing shear wave velocity versus depth profiles of the upper 30-50 m of the seabed (and P wave profiles to lesser depths). Measurements in 1988 were made with smaller source devices designed to emphasize higher frequencies and recorded by an array of 30 sensors spaced at I-in intervals to improve spatial sampling and resolution of shallow structure. These investigations with shear waves have shown that significant lateral and vertical variations in the physical properties of the...

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

“…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. The use of impulsive sources such as a sledge mounted airgun (Ewing et al, 1992) to generate S h waves was found to also produce significant S v and P wave energy. The operating principle and the configuration of the electromagnetic hammer source greatly reduce the proportion of energy converted to P and S v waves.…”

The development of ocean test beds for ocean technology adaptation and integration into the emerging U.S. offshore wind energy industry

“…While there are no off-the-shelf acquisition kits that we know of for this type of survey, standard solid-state MCS streamers could be arranged on the seafloor to test the approach and sensitivities to positional accuracy. Seafloor sources exist, including a design for the S-wave source used by Ewing et al (1992) and Collins et al (1996), and a vibrating source recently developed by Norwegian colleagues ( Fig. 3; Vanneste, 2011).…”

Converted waves in a shallow marine environment: Experimental and modeling studies

“…We note that V S differs from P-wave velocity V P because of the sensitivity of V S to pore content and hence strong correlation with lithology variations ͑Gregory, 1976; Hamilton, 1976͒. From in situ and core measurements of the upper tens of meters of marine sediments, many authors ͑e.g., Theilen and Pecher, 1991;Ewing et al, 1992;Ayres and Theilen, 1999͒ report that V S increases rapidly with depth, whereas V P remains nearly constant. The availability of P-and S-wave information allows one to estimate lithological properties such as porosity and grain size through empirical relations ͑Domenico, 1984͒.…”