1.•0 lOCI. Interpretation of shallow gas-charged sediments on seismic records. Martin Hovland (Statoil, P.O. Box 300, N-4(}01 Stavanger, Norway) Porewater and gas seepages through the seafloor have become the target of frontier research over the last decade. Such seepages occur worldwide and at water depths to over 5000 m. The seep locations are often identified by use of high-resolution shallow seismic systems. Data examples and case studies from the North Sea, the Skagerrak, offshore Mid

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... y, and the Persian and Mexican Gulfs are presented and discussed. Such terms as "acoustic turbidity," "acoustic voids," "enhanced reflectors," and "wipe out zones" will also be discussed. 1:20 IOC2. Anomalous acoustic bebavior exhibited by gas-rich sediments of the subaqueous Mississippi Sediments of the subaqueous Mississippi River Delta contain high concentrations of free gas, as manifested by anomalous physical and acoustic behavior. Much of the near-delta offshore area is a seismic "no-data zone," impenetrable to conventional profiling techniques. Specialized techniques developed for direct measurement of sediment acoustic properties reveal that velocities of less than 1000 ft/s (300 m/s} routinely occur in these muds, and acoustic energy at frequencies greater than 100 Hz is almost completely absorbed by only a few tens of feet of very gassy sediment. Severity of the acoustic anomalies typically decreases with depth beneath the seafloor, with the most severe anomalies generally occurring within the first 200 ft subbottom. Trapping of acoustic energy within these near-bottom, gas-charged layers is also observed. The nature and severity of the acoustic anomalies correlate well with both sediment physical properties, and the unstable system of collapse and mudflow features that typify the seafloor morphology of the deltaic no-data zone. Much of the seafloor in the immediate vicinity of the Mississippi Delta consists of sedimentary material containing high concentrations of free gas. Specialized techniques were developed for direct measurement of the acoustic properties of these gas-charged sediments, to subbottom depths of the order of 300 ft (100 m). These techniques included seafloor receiving arrays, modified checkshot profiles conducted in conventional engineering borings, and a seafloor-penetrating, hydrophone-instrumented rod used in the study of the shallowest subbottom layers. Collection of corresponding sediment samples was carefully integrated with these seismic experiments, to better determine the geologic nature and mechanical properties of the gas-charged materials. Final data acquisition practice was influenced both by the unusual physical properties of the materials being investigated, and the presence of strong and rapidly changing currents from the Mississippi River. Results of these in situ measurements demonstrate the widespread occurrence of anomalously low acoustic velocities (less than 1000 ft/s, or 300 m/s) and related extreme attenuation of higher frequency energy (greater than 100 Hz). 2:10 1OC5. Sound propagation in a shallow fresh-water aquaculture pond. Joe R. Zagar and Kenneth E. Gilbert (Natl. Ctr. for Physical Acoust., University, MS 38677) Measurements made as early as 1943 have shown that gassy freshwater sediments can be excellent reflectors of sound and behave acoustically as pressure-release surfaces. A simple propagation model is presented, based on bottom-reflection coefficients, that indicates one should expect strict mode propagation and sharp cutoffs in a horizontally stratified environment having gassy sediments. Continuous-wave measurements taken in a commercial aquaculture pond arc presented and compared to the propagation model. The data show that, between 600 Hz and 3.0 kHz, a gassy sediment layer does behave as a nearly pressurerelease surface. A simple mode-inversion technique is used to determine bottom-reflection coefficients for the sediments in the pond. Typical .values for the bottom-reflection coefficient are found to be in the range of -0.85 to --0.95. These values, as well as the estimates of the sediment sound speed, are in excellent agreement with those found by others. Additional measurements taken in depths as shallow as 7 cm are presented. These additional data suggest that the acoustic properties of the aquaculture pond that was studied are typical of all shallow water ponds possessing gassy sediments. [Work supported by ONR and USDA.] 1:55 IOC4. Effects of sediment gas on chirp sonar reflection profiles. Chirp sonar reflection profiles of gassy and gas-free seabeds are quantitatively compared to show the effects of sediment gas on normal incidence backscattering measurements. Acoustic FM pulses that sweep over the band of 2 to 10 kHz are generated by the chirp sonar and compressed using a correlation filter to generate a bandlimited impulse response of the seabed. The amplitude spectrum of the ideal wavelet (the unattenuated, compressed FM pulse) and the spectrum of wavelets backscattered from gassy sediments are used to investigate the frequency dependence of backscattering from gassy seabeds. [Work supported by ONR.] 2:25 1OC6. Gas bubbles in ocean sediments and high-frequency acoustic backscattering strength. The influence of trapped gas bubbles on the high-frequency acoustic bottom backscattering strength of ocean sediments will be examined. Statistical trends in the extant backscatter database at shallow grazing angles suggest that gas bubbles may be a dominant factor. The effect may be modeled in terms of an increase in the volume scattering strength of the sediment. Results from model and laboratory studies will he presented. [Work supported by ONR under NOARL management.] 2:40-2:50 Break Invited Papers 2:50 IOCU. Near-surfaee gas hydrates in deep-sea sediments. Roger D. Flood (Marine Sci. Res. Ctr., SUNY, Stony Brook, NY 11794-5000), Patricia L. Manley (Middlebury College), and Mary I. Scranton (MSRC-SUNY, SB) Recent studies of depositional processes in deep-sea continental margin and abyssal settings show that early diagenesis of organic-rich sediments can significantly alter physical properties in the upper 5-50 + m of the sediment column. Apparently, in situ methane production leads to sediment methane levels that may be high enough to allow hydrates to form while high CO 2 levels may cause localized carbonate precipitation. 1852 These diagenetic processes can give rise to a series of near-surface reflecting horizons (and associated physical properties) that are not related to primary alepositional processes. Where best studied (in the Argentine Basin at 5000-to 5500-m water depth [P. L. Manley and R. D. Hood, Deep-Sea Res. 36, 611-623 (1989)]), sediment velocities as low as 1.35-1.42 km/s overlie velocities as high as 1.8-3.0 km/s within 30 m of the sediment surface. While the high velocities appear to be related to near-surface hydrate formation, the origin of the low velocities is less well understood. Similar reflecting sequences have been observed in other areas, particularly in the North Atlantic. 3:10 1OC8. Seismic reflection velocity study of a gas-hydrate zone on the continental slope offshore South The acoustical and physical significance of bottom-simulating seismic reflections (BSR's} remains an observational challenge to geophysical methods. A common depth point (CDP} seismic reflection profile using a 240-channel, 6000-m array with a 177-liter (5-to 60-Hz) source was collected along the continental rise off the eastern U.S. where a BSR reflection is observed along a small portion of the line at about 3200-m depth. These data provide some velocity estimates in the vicinity of the BSR. The CDP data were transformed to the domain of vertical delay time and horizontal ray parameter for velocity analysis purposes. Even so, the resulting velocity profiles have limited vertical resolution (about 200 m } due to the distribution of interpreted sedimentary reflections used in the vertical delay time velocity analysis. Even with this admittedly low vertical resolution, the velocity above the BSR is at least 2000 m/s in an approximately 200-m zone, while the predicted velocity based on the extrapolation of regional gradients indicates that normal sediments should have a velocity of about 1850 m/s. A velocity of 2000 m/s suggests on average about a 50% substitution of hydrate in the pore spaces but the actual vertical concentration gradient is not constrained. This velocity anomaly also extends into other areas just above the theoretical phase boundary position, but where there is no detectable BSR. Beneath the BSR, even with the relatively low vertical resolution, a velocity decrease to about 1700 m/s is detected. This low velocity is observed only in zones with a detectable BSR. It is not observed beneath the theoretical phase boundary position elsewhere. This suggests that the origin of the BSR is not a simple boundary between hydrated and nonhydrated, normal sediments below. Initial investigations of amplitudes indicate significant increase in amplitude with offset. Full waveform, offset modeling of the data is underway. 3:30 1OC9. Estimation of amounts of gas hydrate in marine sediments using amplitude reduction of seismic reflections. William P. Dillon Gas hydrates (solid, crystalline water-gas mixtures} exist in sediments just below the seafloor. In seismic profiles, hydrate cementation creates zones of increased velocity and reduced amplitudes of stratal reflections (blanking}. By using sediment velocities (estimated by an inversion method), known sediment porosity, and pure hydrate velocity, the amount of hydrate in the highest velocity and most intensively blanked sediments off the southeastern U.S. is calculated; this represents maximum hydrate cemcntation. To create seismic models of the range of possible blanking effects, ordinary, nonhydrated sediments across a reflecting boundary (caused by a porosity change) are "replaced" with this maximum-hydrate end member in various proportions. Three classes of blanking are defined; class boundaries represent a change in reflection amplitude by a factor of 2, and the classes are relatable to the amounts of hydrate in bulk sediment. In order to estimate the amount of hydrate, these classes are mapped in a grid of reflection profiles processed to preserve relative amplitude, and the first semiquantitative estimate of gas hydrates in deep-sea sediments is produced. Outer Ridge. A bottom-simulating reflector (BSR) is observed in these data at the same depth that it is found in surface-tow multichannel seismic sections from approximately 150 km to the east, near Deep Sea Drilling Project (DSDP) sites 102, 104, and 533. Analysis of the highresolution (• 10 m) DTAGS data confirms that gas hydrate as well as unfrozen gas are concentrated within these sediments. Distinctive properties of the acoustic signal are combined with sediment sound speeds derived from these data to estimate the distribution and extent of the gas hydrate and regions of unfrozen gas. Results of analysis of DTAGS data extend findings from DSDP Leg 11, DSDP sites on the Middle America Trench, and the Black Sea. These data provide a means for determining mechanisms for the concentration of methane gas and the formation of gas hydrate. [Work supported by the Office of Naval Technology.] 4:05-4:15 Break Poster Paper The author will be at his poster from 2:40 to 2:50 p.m. and from 4:05 to 4:15 p.m. 10(211. Biogenic gas in shallow water sediments as indicated by high-resolution seismic records. Douglas N. Lambert (Naval Oceanographic and Atmospheric Res. Lab., Code 361, Stennis Space Center, MS 39529-5004} A 15-kHz narrow-beam ( 12 ø) seismic system, designated the Acoustic Seafloor Classification System (ASCS), has been used to identify areas of shallow sea floor gassy sediments. The ASCS was designed to remotely classify seafloor sediments by quantitatively and qualitatively measuring the echo return amplitude and pulse character in ten adjustable time windows that correspond to depth increments in the sediment. In addition, the ASCS produces a vcry high-rcsolution analog seismic record of the upper few meters of the seafloor on which the amplitude of the echo return (echo strength) for each of the depth inercments is also indicated. This unique combination of data provides an excellent means of identifying sediments containing biogcnie gas. Examples of identified gassy sediments shown on ASCS records range from a "speckled" appearance indicating random and isolated small gas concentrations to "white outs" that indicate large gas concentrations that completely attenuate the acoustic signal. Other records show highly reflective sediment layering that is due to a concentration of gas along bedding planes alternating with large pockets of 1ow-rcfiectivity, degassed sediment that appears to have been "homogenized" by gas movement. These degassed sediment pockets are occasionally accompanied by vertical gas escape conduits seen in the sediment and by the presence of vertical reflectors in the water column indicative of escaping gas bubbles.