02668nas a2200337   4500000000100000000000100001008004100002260000800043653006500051653005600116653004300172653004000215653001300255653002200268653004800290653001300338653001600351653002700367653002800394653001300422100001500435700001900450700001500469700001800484245011300502856004300615300001100658490000800669520163900677020001402316       2004                            d  bAGU10a3022 Marine Geology and Geophysics: Marine sediments$\#$821210a3025 Marine Geology and Geophysics: Marine seismics10a3210 Mathematical Geophysics: Modeling10a4227 Oceanography: General: Diurnal10aseasonal10aand annual cycles10a4259 Oceanography: General: Ocean acoustics10aacoustic10aattenuation10agassy marine sediments10aprocesses and transport10avelocity1 aAngus Best1 aMichael Tuffin1 aJustin Dix1 aJonathan Bull00aTidal height and frequency dependence of acoustic velocity and attenuation in shallow gassy marine sediments  uhttp://dx.doi.org/10.1029/2003JB002748  aB081010 v1093 aRemote prediction of gassy marine sediment properties is important for geohazard assessment. Gas bubble resonance theory suggests that gassy sediments exhibit acoustic wave velocity-frequency and attenuation-frequency relationships that depend on gas bubble size, gas content, and sediment elastic properties. An acoustic monitoring experiment to investigate gas bubble resonance effects was undertaken at an intertidal site at Dibden Bay, Southampton, United Kingdom. A vertical hydrophone array was positioned to straddle the top of the gassy zone identified on acoustic reflection profiles at about 1 m below the seabed. A miniboomer in the seabed above the array was used to generate broadband (600 Hz to 3000 Hz) acoustic signals every 10 min during a 24 hour period with water depths varying between 0 m (subaerial exposure) at low tide and 2.35 m at high tide. The calculated frequency spectra of compressional wave attenuation coefficient show an attenuation maximum (over 200 dB/m) that shifts in frequency from 1050 Hz at low tide to 1250 Hz at high tide, thus for the first time providing direct evidence of in situ gas bubble resonance in marine sediments. Modeling suggests that effective gas bubble radii of 11 mm to 13 mm are responsible for the attenuation maximum, supported by X-ray computed tomography scan observations on a pressure core (which also indicate that bubble shape depends on sediment type). Modeling of bubble size fluctuations due to pressure equilibration cannot reproduce the observed frequency shift of the attenuation maximum, implying that gas diffusion and nonspherical bubbles are significant.  a0148-0227