Abstract. Compliance is defined as the transfer function between the vertical displacement of the seafloor and the corresponding pressure expressed as a function of frequency. It is sensitive to the elastic parameters of the underlying sediments, particularly the shear modulus. We have measured normalized compliance from 0.001 to 0.049 Hz, using ocean surface gravity waves as a source, at sites in Cascadia near the Ocean Drilling Program (ODP) hole 889B. A differential pressure gauge, datalogger and self-levelling gravimeter were lowered to the seafloor and each site was occupied for eight hours. The compliance estimates are reproducible and are consistent with other available data and simple models of sediment physical properties. Shear strength is increased from a normal profile in the uppermost few hundred meters, possibly an effect of the presence of a known hydrate layer, and is decreased between 1.2 and 2 km below the seafloor, suggesting the presence of a low velocity zone.
S U M M A R YMethane hydrates are solid, non-stochiometric mixtures of water and the gas methane. They occur worldwide in sediment beneath the seafloor and estimates of the total mass available there exceed 10l6 kg. Since each volume of hydrate can yield up to 164 volumes of gas, off-shore methane hydrate is recognized as a very important natural energy resource.The depth extent and stability of the hydrate zone are governed by the phase diagram for mixtures of methane and hydrate, determined by ambient pressures and temperatures. In sea depths greater than about 300 m, the pressure is high enough and the temperature low enough for hydrate to occur at the seafloor. The fraction of hydrate in the sediment usually increases with depth. The base of the hydrate zone is a phase boundary between solid hydrate and free gas and water. Its depth is determined principally by the value of the geothermal gradient, and stands out on seismic sections as a bright reflection. The diffuse upper boundary is not as well marked so that the total mass of hydrate cannot be determined by seismic measurements alone.Ocean surface gravity waves induce a low-frequency, horizontally propagating pressure field which deforms the seafloor. The displacement of the seafloor depends on the oceanic crustal density and elastic parameters, particularly the shear properties. Seafloor compliance is the transfer function between seafloor deformation and pressure as a function of frequency. Compliance measurements made at specific frequencies are tuned to structure at specific depths. Methane hydrate, like ice in permafrost, changes the physical properties of the material in which it is found, decreasing the density while increasing the compressional and especially the shear velocities. We apply the method of Crawford, Webb & Hildebrand (1991) and show how the addition of compliance data, which is particularly sensitive to changes in shear velocity, can aid in the evaluation of the resource.Two exploration scenarios are investigated through numerical modelling. In the first, a very simple example illustrates some of the fundamental characteristics of the compliance response. Most of the properties of the section including the probable regional thickness of the hydrate zone, 200 m, are assumed known from seismic surveys and spot drilling. The amount of hydrate in the available pore space is the only free parameter.Hydrate content expressed as a percentage may be determined to about i 28 given compliance measurements with E per cent error. The rule holds over the complete range of anticipated hydrate-content values.In the second, less information is assumed available a priori. The complementary compliance survey is required to find both the thickness and the hydrate content in hydrate zones about 200 m thick beneath the seafloor, which contain up to 20 and 40 per cent hydrate in the available pore space, respectively. A linear eigenfunction analysis reveals that for these two models the total mass of hydrate, the product of hydrate content and thickness, ...
Integrated Ocean Drilling Program Expedition 311 is based on extensive site survey data and historic research at the northern Cascadia margin since 1985. This research includes various regional geophysical surveys using a broad spectrum of seismic techniques, coring and logging by the Ocean Drilling Program Leg 146, heat flow measurements, shallow piston coring, and bottom video observations across a cold-vent field, as well as novel controlled-source electromagnetic and seafloor compliance surveying techniques. The wealth of data available allowed construction of structural cross-sections of the margin, development of models for the formation of gas hydrate in an accretionary prism, and estimation of gas hydrate and free gas concentrations. Expedition 311 established for the first time a transect of drill sites across the northern Cascadia margin to study the evolution of gas hydrate formation over the entire gas hydrate stability field of the accretionary complex. This paper reviews the tectonic framework at the northern Cascadia margin and summarizes the scientific studies that led to the drilling objectives of Expedition 311 Cascadia gas hydrate.
We find geodetically detectable events of significantly elevated microseismic noise in an eight day Scintrex CG‐3M continuous relative gravity occupation in the southern Upper Geyser Basin of the Yellowstone caldera. The noise is recorded as large excursions in the gravity data, with frequencies between 0.033 and 0.5 Hz, and durations of up to one hour. These microseismic excursions, with gravity root mean square [RMS] amplitudes of ∼50 to 350 μGal, are up to an order of magnitude larger than the observed background levels of noise. The noise is also recorded as elevated variability in the gravity RMS time‐series, with frequencies >0.033 Hz. We suggest that these periods of elevated and variable microseismicity reflect active hydrothermal processes in the geyser basin not captured previously.
[1] Seafloor compliance, the transfer function between pressure induced by surface gravity waves and the associated seafloor deformation, can address outstanding questions about the nature of seismic blank zones, identified as cold vents offshore Vancouver Island, near Ocean Drilling Program (ODP) Site 889 and the recent Integrated Ocean Drilling Program (IODP) Expedition 311. Despite extensive seismic studies, it is not known whether free gas or gas hydrate is primarily responsible for the seismic blanking, nor are the concentration and distribution of either component conclusively determined. Here we use the formation's shear modulus to distinguish between gas hydrate, which can strongly alter the shear modulus, and free gas, which cannot. The depth-dependent gas hydrate distribution and pore-space saturation are estimated from the frequency variation of seafloor compliance measurements sensitive to the formation's shear modulus. Compliance data are consistent with pervasive moderate hydrate concentration over a broad region and significantly higher cylindrical concentrations associated with the cold vents, perhaps as much as 9 Â 10 6 m 3 in the 400 m diameter Bullseye vent, in agreement with controlled-source electromagnetic data. Compliance data can help distinguish between different models for the structure of such gas hydrate-bearing cold vents.Citation: Willoughby, E. C., K. Latychev, R. N. Edwards, K. Schwalenberg, and R. D. Hyndman (2008), Seafloor compliance imaging of marine gas hydrate deposits and cold vent structures,
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