Seafloor Compliance Research Articles

Shear velocities in Cascadia from seafloor compliance measurements

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.

North Cascadia Deep Sea Gas Hydrates

Abstract: The Cascadia accretionary margin off Vancouver Island is one of the best studied margins world‐wide for the determination of in situ properties of marine gas hydrates. Most quantitative information has come from cores and downhole logs of the Ocean Drilling Program (ODP) Leg 146 and from extensive seismic and other geophysical surveys. As part of the ODP site surveys, large‐offset multichannel seismic lines outlined the regional distribution of hydrate, and seismic velocity analyses coupled with full waveform inversion provided estimates of the vertical distribution of hydrate and gas. High resolution single‐channel seismic surveys indicate correlations between hydrate/gas concentrations and topographic highs, and between geothermal flux and topography; these correlations provide insight into fluid and methane flow through the sediments. A deep‐towed multichannel seismic survey (DTAGS), together with other data from 20–600 Hz, provide constraints on gradients at the base of the hydrate and gas layer. Extensive heat flow measurements, from probes and from depths of the bottom simulating reflector (BSR), have been modelled numerically, including the regional effects of sediment thickening and advective fluid flow in the accretionary prism. Other geophysical surveys include several seafloor electrical sounding experiments and seafloor compliance measurements of hydrate. ODP drilling has provided valuable downhole log data, core physical property data, and detailed pore fluid chemistry and isotopes. Several semi‐independent estimates of hydrate and gas concentrations have been obtained; all are dependent on reference sediment properties for which no hydrate and no gas is present. From a vertical seismic profile and other seismic data, the velocity increase in a hydrated region is consistent with hydrate concentrations of 20–30% of the pore space in a 100‐m interval above the BSR. Similar values are determined from log resistivity data and from core pore fluid chlorinity.

Constraints on melt in the lower crust and Moho at the East Pacific Rise, 9°48′N, using seafloor compliance measurements

Seafloor compliance measurements across the East Pacific Rise at 9°48′N reveal low shear velocities throughout the crust and at the crust‐mantle boundary, with the lowest shear velocities centered beneath the rise axis. The compliance method uses the seafloor deformation under the loading of long wavelength ocean waves to probe the oceanic crust. The shape of the compliance function as a function of frequency is primarily controlled by regions of low shear velocity within the crust. At 9°48′N, the shear velocity is less than 20 m/s in the shallow on‐axis melt lens located 1.4 km beneath the seafloor, demonstrating that the melt lens at this site is fully melt rather than a connected crystal mush. The compliance data also require a second on‐axis melt lens 5.4 ± 1 km beneath the seafloor, with shear velocities slower than 50 m/s. This “deep” melt lens may be created by melt pooling at a permeability or density barrier at the crust‐mantle interface. The shear velocity in the lower crust between the two melt lenses averages 1.7 km/s, indicating 2.5–18% melt. Melt persists in the lower crust to at least 10 km off‐axis, where the top of the lower crustal low‐velocity zone is approximately 4 km beneath the seafloor. In seismic layer 2B, the ratio of shear to compressional velocity increases from 0.41 on‐axis to 0.58 by 10 km off‐axis, indicating that there are abundant thin cracks in the sheeted dikes on‐axis and that these cracks close away from the rise axis. High on‐axis porosity in layer 2B may allow hydrothermal circulation down to near the shallow melt lens.

Estimating shear velocities in the oceanic crust from compliance measurements by two‐dimensional finite difference modeling

The deep seafloor deforms under the pressure loading of linear ocean surface gravity (water) waves at low frequencies (0.003 to 0.04 Hz). The ratio of seafloor displacement to pressure loading as a function of frequency, known as the seafloor compliance function, depends on the shear velocity structure of the oceanic crust and upper mantle. Compliance measurements are used to estimate oceanic crustal structure, particularly within low shear velocity regions such as sediments, fractured rock, and partial melt. Compliance calculated from laterally homogeneous (one‐dimensional, 1‐D) crustal models shows that a buried low‐velocity zone (LVZ) causes a peak in the compliance function at wavelengths 4 to 6 times longer than the LVZ depth, and that the compliance amplitude depends primarily on the LVZ shear velocity. A new numerical code allows forward modeling of compliance for two‐dimensional oceanic crustal models. The new code demonstrates that the peak in the compliance function directly over a finite width LVZ reaches a maximum value at higher frequency, and is of smaller amplitude, than predicted from 1‐D modeling. The compliance maximum persists outside of the region underlain by the LVZ but diminishes in amplitude and shifts to lower frequencies with increasing distance from the LVZ. The numerical models indicate that significant peaks in the compliance function indicate crustal LVZs, but that multiple compliance measurements are necessary to independently constrain the depth, location, and shear velocity of these features.

On the resource evaluation of marine gas-hydrate deposits using seafloor compliance methods

SUMMARY Methane 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 1016 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 ±2ɛ given compliance measurements with ɛ 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 eigen-function analysis reveals that for these two models the total mass of hydrate, the product of hydrate content and thickness, may be estimated to an accuracy of about 2.81ɛ and 1.83ɛ per cent, respectively, given compliance measurements with an accuracy of ɛ per cent.

Seafloor compliance observed by long‐period pressure and displacement measurements

Ocean surface waves with periods longer than 30 s create periodic, horizontally propagating pressure fields at the deep seafloor. Seafloor displacements resulting from these pressure fields depend on the density and elastic parameters of the oceanic crust. The displacement to pressure transfer function, the seafloor compliance, provides information about ocean crustal density and elasticity, and we outline a linearized inversion method to determine ocean crustal shear velocity from the compliance. By computing compliance partial differences with respect to changes in ocean crust shear velocity, we provide estimates of inversion stability and of the compliance sensitivity to crustal properties. Seafloor compliance, measured from pressure and acceleration spectra, is presented for two different sites: Axial Seamount on the Juan de Fuca Ridge and the West Cortez Basin in the California continental borderlands. The compliances and inverted structure for these two sites show significant differences; in particular, a zone of low shear strength is observed at depth within Axial Seamount, suggesting the presence of at least 3% partial melt within the upper 2500 meters of the ediface. These results suggest that the method provides a useful new geophysical prospecting tool.