Dependence of seismoelectric amplitudes on water content

International audienceThe expectation behind seismoelectric field measurements is to achieve a combination of the sensitivity of electrical properties to water content and permeability and the high spatial resolution of seismic surveys. A better understanding of the physical processes and a reliable quantification of the conversion between seismic energy and electric energy are necessary and need to take into account the effect of water content, especially for shallow subsurface investigations. We performed a field survey to quantify the seismoelectric signals as the water content changed. We measured seismoelectric signals induced by seismic wave propagation, by repeating the observations on the same two profiles during several months. The electrical resistivity was monitored to take into account the water content variations. We show that the horizontal component of the seismoelectric field, normalized with respect to the horizontal component of the seismic acceleration is inversely proportional to the electrical resistivity ρ0.42±0.25 . Assuming that the observed resistivity changes depend only on the water content, this result implies that the electrokinetic coefficient should increase with increasing water saturation. Taking into account the water saturation and combining our results with the Archie law for the resistivity in non-saturated conditions, the normalized seismoelectric field is a power-law of the effective saturation with the exponent (0.42 ± 0.25)n where n is Archie's saturation exponent. Hydrogeophysics; electrokinetics; streaming potential; seismoelectric conversion; field observation; water conten

The structures beneath submarine methane seeps : seismic evidence from Opouawe Bank, Hikurangi Margin, New Zealand

The role of methane in the global bio-geo-system is one of the most important issues of present-day research. Cold seeps, where methane leaves the seafloor and enters the water column, provide valuable evidence of subsurface methane paths. Within the New Vents project we investigate cold seeps and seep structures at the Hikurangi Margin, east of New Zealand. In the area of Opouawe Bank, offshore the southern tip of the North Island, numerous extremely active seeps have been discovered. High-resolution seismic sections show a variety of seep structures. We see seismic chimneys either characterised by high-amplitude reflections or by acoustic turbidity and faults presumably acting as fluid pathways. The bathymetric expression of the seeps also varies: There are seeps exhibiting a flat seafloor as well as a seep located in a depression and small mounds. The images of the 3.5 kHz Parasound system reveal the ear-surface structure of the vent sites. While highamplitude spots within the uppermost 50 m below the seafloor (bsf) are observed at the majority of the seep structures, indicating gas hydrate and/or authigenic carbonate formations with an accumulation of free gas underneath, a few seep structures are characterised by the complete absence of reflections, indicating a high gas content without the formation of a gas trap by hydrates or carbonates. The factors controlling seep formation have been analysed with respect to seep location, seep structure, water depth, seafloor morphology, faults and gas hydrate distribution. The results indicate that the revailing structural control for seep formation at Opouawe Bank is the presence of numerous minor faults piercing the base of the gas hydrate stability zone

Seismic investigation of a bottom simulating reflector and quantification of gas hydrate in the Black Sea

A bottom simulating reflector (BSR), which marks the base of the gas hydrate stability zone, has been detected for the first time in seismic data of the Black Sea. The survey area is in the northwestern Black Sea at 44°–45°N and 31.5°–32.5°E. In this paper, seismic wide-angle ocean bottom hydrophone (OBH) and ocean bottom seismometer (OBS) data are investigated with the goal to quantify the gas hydrate and free gas saturation in the sediment. An image of the subsurface is computed from wide-angle data by using Kirchhoff depth migration. The image shows the BSR at 205–270 m depth below the seafloor and six to eight discrete layer boundaries between the seafloor and the BSR. The top of the hydrate layer and the bottom of the gas layer cannot be identified by seismic reflection signals. An analysis of traveltimes and reflection amplitudes leads to 1-D P-wave velocity–depth and density–depth models. An average S-wave velocity of 160 m s−1 between the seafloor and the BSR is determined from the traveltime of the P to S converted wave. The normal incidence PP reflection coefficient at the BSR is −0.11, where the P-wave velocity decreases from 1840 to 1475 m s−1. Velocities and density are used to compute the porosity and the system bulk modulus as a function of depth. The Gassmann equation for porous media is used to derive explicit formulae for the gas hydrate and free gas saturation, which depend on porosity and on the bulk moduli of the dry and saturated sediment. A gas hydrate saturation–depth profile is obtained, which shows that there is 38 ± 10 per cent hydrate in the pore space at the BSR depth, where the porosity is 57 per cent (OBS 24). This value is derived for the case that the gas hydrate does not cement the sediment grains, a model that is supported by the low S-wave velocities. There is 0.9 or 0.1 per cent free gas in the sediment below the BSR, depending on the model for the gas distribution in the sediment. The free gas layer may be more than 100 m thick as a result of a zone of enhanced reflectivity, which can be identified in the subsurface image

A method for determining gas-hydrate or free-gas saturation of porous media from seismic measurements

The occurrence of gas hydrate or free gas in a porous medium changes the medium’s elastic properties. Explicit formulas for gas-hydrate or free-gas saturation of pore space on the basis of the Frenkel-Gassmann equations describe the elastic moduli and seismic velocities of a porous medium for low frequencies. A key assumption of the model is that either gas hydrate or free gas is present in the pore space in addition to water. Under this assumption, the method uses measured P- and S-wave velocities and bulk density along with estimates of the moduli and densities of the solid and fluid phases present to determine whether gas or hydrate is present. The method then determines the saturation level of either the gas or the hydrate. I apply the method to published velocity and density data from seismic studies at the antarctic Shetland margin and at the Storegga slide, offshore Norway, and to borehole log and core data from Ocean Drilling Program (ODP) Leg 164 at Blake Ridge, offshore South Carolina. A sensitivity analysis reveals that the standard deviations of the gas-hydrate and free-gas saturations reach 30%–70% of the saturations if the standard deviations of the P- and S-wave velocities and of the bulk density are 50m∕s ..

Traveltime approximation for a reflected wave in a homogeneous anisotropic elastic layer

An approximation to the traveltime field is calculated for an elastic wave that propagates in a homogeneous anisotropic layer and is reflected at a plane boundary. The traveltime is approximated by a Taylor series expansion with the third derivative of the traveltime being taken into account. The coefficients of the series refer to the seismic ray, which is locally the fastest ray. Simple formulae are obtained for orthorhombic media in the crystal coordinate system, which relate the traveltimes of the reflected waves to the elastic constants of the medium. A numerical example is presented for wave propagation in orthorhombic olivine, which is a constituent of the Earth's mantle. A second example is given by an isotropic host rock with a set of parallel cracks, which is an important model for wave propagation in the Earth's crust. The elastic parameters can be determined by measuring the reflection times as a function of source–receiver offset. The approximate traveltime–distance curves are compared with traveltimes obtained from seismic ray tracing