ALBERT

Description: The circulation of gases and aqueous fluids, as well as the abundance of gas hydrates in convergent margin scenarios, play a significant role in global mass balances. Hence, the study of these processes with various techniques has become a major objective in geosciences research during the previous decade. The main focus of the research expedition HYDGAS during cruise Sonne SO-150 was the qualification and quantification of massive gas hydrates in the sediment using seismic methods. Gas hydrates, or clathrates, are ice-like crystals of gases (predominantly methane) and water, which occur in various forms in subseafloor sediments. According to global estimates, the carbon fixed in such gas hydrates exceeds by far the amount of presently discovered oil and gas occurrences. Thus, gas hydrate research is of both scientific, economic, and environmental importance. The continental margin off Oregon, where the Juan de Fuca Plate is subducted beneath the North American continental plate, is a well studied area which is characterized by its wealth of clathrates and hydrate-related deposits. Within the accumulated sediment of the upper subduction zone, the so called Cascadia accretionary prism, massive gashydrates and associated chemical anomalies (brines), their chemical oxidization products (authigenic carbonate), and typical fauna assemblages related to methane-rich fluids have been previously reported. Apart from shedding more light on the principal understanding of gas hydrate environments, the main aim of the HYDGAS expedition is an estimate of gas hydrate quantities, their potential economic relevance in the future, and the risk of environmental hazards (submarine slides, release of greenhouse gases into the atmosphere) from gas hydrate dissociation. During cruise Sonne SO-150 , improved geophysical methods were used to identify and quantify the spatial variation of gas hydrates across Hydrate Ridge. Narrowly spaced receivers on the seafloor (approximately 200 m apart), shallow and deep towed systems, and a broad range of source frequencies (5 Hz to 2 kHz) were used to guarantee pristine data quality. Here, special attention will be given to parameters which will assist in gas hydrate identification and quantification. The new geophysical data will be compared and calibrated using the results from direct measurements on recovered core as well as along the borehole wall (downhole logging) at ODP Site 892. Calibration of the geophysical information will allow us to optimize data processing and interpretation. The results from cruise Sonne SO-150 will significantly broaden the understanding of seismic signals in hydrate-bearing sediments, and thus will add a quantitative component to the overall gas hydrate research. Five receiver assemblages in three locations, at Site 892 (ODP Leg 146) and at two future ODP drillsites (ODP Leg 204, scheduled for autumn 2002), provided a wealth of data from 5 different sources used. During intervals of data retrieval and OBH/OBS refitting, a total of 1490 km of seismic reflection profiles were acquired using five different source (broad frequency range) and three different streamer (surface and deep tow) configurations. The narrowly spaced grid of seismic reflexion and Parasound lines permits us to tie in the results from the receiver experiments for quantitative estimates.