Seismic No-Data Zone, Offshore Mississippi Delta: Part I-Acoustic Characterization

Abstract

Shallow acoustic velocity and attenuation profiles have been measured in different seafloor mass-flow features of the subaqueous Mississippi Delta. Velocities of less than 1000 ft/sec occur routinely in these gas-charged muds, and energy at frequencies above approximately 100 Hz is almost completely absorbed by only a few tens of feet of very gassy sediment.High resolution seismic profiles acquired on the seafloor show frequent contamination from seafloor "ground roll," "singing" phenomena, and other trapped energy events. These observed acoustic anomalies correlate well with the known local seafloor instability features.

Acoustic Properties of Shallow Seafloor Gas

“…Two of the more recent measurements of geoacoustic properties of gassy, sediment were carried out in the Gulf of Mexico (Edrington andCalloway, 1984, andTinkle, et at~, 1988. ) Although, in common with most actual measurements of gassy seafloor acoustic properties, neither of the two cited studies included an actual measurement of the amount of gas in the seafloor, they are nonetheless quite significant.…”

“…The' . measurements of Tinkle, et al (1988), provide one of the very few actual seafloor measurements of a reductio~of seafloor sound velocity by aI, order of magnltude fnom values f6r water aturated sediment. Although acoustic theory 1 ndicates the possi bil ity of sucb low values of sound velocity (on the order of 100 meters per second), they are rarely observed.…”

Preliminary mapping of void fractions and sound speeds in gassy marine sediments from subbottom profiles

“…1) or be inverted to estimate the bubble population. Bubble radii distributions from 10 µm to 20 mm, and void fractions as great as 9%, have been inferred from the inversion of either compressional wave data, [6][7][8][9][10] acoustic backscatter, [11][12][13] or two-frequency techniques. [14][15][16] While such inversions can be based on models of bubbles in water, a model incorporating geotechnical properties of the host sediment surrounding the bubbles (both before and after it is altered by the presence of bubbles) would be preferable.…”