Acquisition and Inversion of Dispersive Seismic Waves in Shallow Marine Environments

Abstract

ZusammenfassungDie Kenntnis der elastischen Eigenschaften mariner Sedimente, insbesondere die Scherwellengeschwindigkeit, ist eine wichtige Information für die Untersuchung der Stabilität von Meeresböden. Dies ist insbesondere für geotechnische Anwendungen im flachmarinen Bereich von Bedeutung, aber auch für die Untersuchung von Hangstabilitäten der Kontinentalrän-der und für die Gashydratforschung von Interesse.Seismische Experimente in der Ostsee sowie die Analyse eines Datensatzes aus der Nordsibirischen Laptevsee konnten zeigen, dass dispersive seismische Wellenfelder mit Hilfe einer nahe der Meeresoberfläche geschleppten seismischen Quelle (Airgun) angeregt werden können. AbstractThe knowledge of the shear wave velocity structure of shallow marine sediments is an important information for the assessment of sediment stability. This is of interest for marine geotechnical applications as well as investigations of sea-floor stability of continental shelves and margins and in gashydrate research. The derivation of seismic velocities with special interest on the shear wave velocity from the analysis and inversion of dispersive seismic waves is investigated in this work.Dispersive waves are excited by surface towed airguns and the acquisition was achieved with two different configurations. Firstly, the stationary-receiver method comprises of an ocean bottom seismometer station (OBS) and excitation with a surface towed airgun. Secondly, a towed-acquisition system includes a streamer towed at ten meters above the sea bed and excitation with an airgun. Two different wave types could be observed with either of the two acquisition systems. Interface waves, namely of the Scholte wave type, propagating along the sea floor and guided waves in the water column, denoted as acoustic guided waves despite of their sediment interaction at the sea bed. The both wave types were observed in certain environments indicating the limitations in the acquisition of these wave types depending on the sediment properties. These limitations as well as the difference in dispersion sensitivity to variations of the seismic properties give rise for separate treatment of both wave types. While the dispersion sensitivity of Scholte waves as well as other interface waves is dominant for shear wave velocity variations, the variation of the dispersion of acoustic guided waves is affected by variations of compressional and shear wave velocity as well as density to a smaller extend. The difference in velocity and frequency range of interest for both wave types, i.e. the ranges of most variations in the dispersion characteristics, also affect the required acquisition configuration and parameters. While a long offset recording (exceeding 800 m) is required for acoustic guided waves, the fulfillment of the spatial sample criteria sometimes limits the feasibility to adequately acquire the dispersive wavefield. This is most dominant for the Scholte wave measurements in very soft marine sediments.The derivation of the shear wave velocity structure was inve...

Variability of Scholte-wave Dispersion in Shallow-water Marine Sediments

“…Multiple air gun shots situated along a line are recorded by stationary oceanbottom seismometers (OBS) or by geophones, buried approximately 0.5 m beneath sea bottom. The acquisition parameters for the study sites are listed in (Bohlen et al, 2004;Klein et al, 2005). A detailed discussion of optimal parameters for Scholte-wave acquisition is given by Klein et al (2005).…”

“…The acquisition parameters for the study sites are listed in (Bohlen et al, 2004;Klein et al, 2005). A detailed discussion of optimal parameters for Scholte-wave acquisition is given by Klein et al (2005). We analyze the recorded seismograms in a commonreceiver-gather (CRG) using the multichannel dispersion analysis described in the next section.…”

“…Recently, the multichannel analysis of surface waves (MASW) has been employed in marine applications (Allnor et al, 1997;Muyzert, 2000;Park et al, 2000;Ritzwoller and Levshin, 2002;Bohlen et al, 2004;Klein et al, 2005). This enables an analysis of lateral variation along continuous profiles.…”

“…This enables an analysis of lateral variation along continuous profiles. Multichannel Scholte-wave data can be acquired with a moving source and a single ocean-bottom-seismometer (Allnor et al, 1997;Bohlen et al, 2004), a single source and a multichannel geophone-cable deployed on the sea floor Muyzert, 2000), or, in favorable environments, with a towed source and a deep-towed multichannel hydrophone-streamer (Klein et al, 2005).…”

FINOSEIS: A new approach to offshore-building foundation soil analysis using high resolution reflection seismic and Scholte-wave dispersion analysis

“…We are now applying a similar technique in the offshore domain, i.e. multichannel dispersion analysis of liquid-solid surface waves, so called Scholte-waves, in the shallow marine environment as introduced by (Bohlen et al, 2004;Klein et al, 2005;Luke and Stokoe, 1998).…”