The Naval Research Laboratory (NRL) has been conducting coordinated investigations of marine gas hydrates based on precise co-location of both direct sediment cores and data from a deep-tow multichannel seismic system. The seismic instrument (known as the Deep Towed Acoustics/Geophysics System (DTAGS)) was developed by NRL to support detailed studies of deep-ocean marine sediments by towing both the seismic source (220Hz - 1kHz, 200 dB // 1 ?Pa @ 1 m) and 48 channel hydrophone array ?300 m above the seafloor in up to 6 km deep water. This instrument has proven to be ideal for studies of marine gas hydrates. Data from this system have been used to study the impact of hydrate dissociation on sediment properties on and near the Blake Ridge and on the Cascadia Margin. Investigations in the Gulf of Mexico are scheduled for Spring 2005. Using bottom mounted acoustic transponders for long baseline (LBL) navigation, we have been able to co-locate the deep-tow seismic data with sediment cores, water samples, etc. to constrain models for processes that create and dissociate marine gas hydrates in much greater detail than previously possible. We show relationships between geologic features resolved with the seismic data and geochemical evidence for variability in methane flux through the seafloor. We also present preliminary results from lattice-gas numerical simulations of gas-fluid flow through complex sediments where parameters are predicated on results obtained with DTAGS and associated geochemical samples.
The Naval Research Laboratory (NRL) has been investigating natural marine gas hydrates for over a decade. The original work was predicated on data taken with a unique deep-tow multichannel seismic system that could resolve the detailed structure and physical properties of marine sediments within the gas hydrate stability zone, regardless of water depth. Here we discuss the evolution of NRL's investigations from seismic studies to integrated investigations of gas hydrates using a multidisciplinary approach. In the early 1980's the U. S. Navy developed sesimo-acoustic technology to support the study of marine sediment properties in the deep ocean1. The instrument that was developed is, essentially, an adaptation of conventional multichannel seismic technology with the source frequency band increased in order to the resolve structure and physical properties within the upper ?1 km of sediments. This requirement meant that both the source and receiver array would have to be operable at depths of at least 5 km below the surface of the ocean. We are now operating with a second generation deep-tow system that uses a digital array and improved source. The system is known the Deep Towed Acoustics/ Geophysics System (DTAGS).
DTAGS is designed around a Helmholtz resonator acoustic source. This seismic source provides a source-waveform that is independent of tow depth (to ?6 km). Currently, the source produces a linear 250 ms swept-frequency waveform from 220Hz to 1kHz with a Sound Pressure Level (SPL) of 201 dB // 1 ?Pa @ 1 m.