Acoustic energy dissipation in gas hydrate bearing sediments, ODP leg 204, Hydrate Ridge
Abstract
While it has been well documented that the presence of hydrate increases sonic velocity in the host sediment, recent work on sonic logging data from the Blake Ridge and the Mackenzie delta show that sonic waveforms amplitude are also significantly affected by the presence of gas hydrate. The data suggest that this effect is dependent not only on hydrate concentration, but also on the nature and on the frequency of the acoustic source used. Based on these observations, we use sonic waveform amplitudes and attenuation to investigate in situ the nature of the interaction between gas hydrate and its host sediment. During Leg 204, an extensive logging program was conducted in nine sites to characterize gas hydrate distribution near the crest of the Southern Hydrate Ridge. Six of the wells were logged with the Dipole Sonic Imager tool. In order to document the energy loss in these sediments, two sources (monopole and dipole) were each used at two different frequencies in every hole. We use a spectral method to calculate attenuation and quantify energy loss. The results show strong energy dissipation where hydrate concentrations are the highest, near the crest of the ridge (Sites 1250 and 1247). Monopole waveforms have the greatest attenuation. Attenuation of the high-frequency dipole waveforms is generally greater than at low frequency. At Site 1252, however, where hydrate concentration is low, energy loss is greater for the low-frequency dipole. Below the gas hydrate stability field, low energies of both monopole and dipole data are associated with free gas migration pathways. An elastic wave propagation model using percolation theory for frozen porous media and multiple-phase fluids can explain our observations at these frequencies in the Hydrate Ridge sediments.
- Publication:
-
EGS - AGU - EUG Joint Assembly
- Pub Date:
- April 2003
- Bibcode:
- 2003EAEJA....11089G