Seafloor Electrical Resistivity Measurements

A seafloor dipoledipole electrical system has been developed recently to map the hydrate (Edwards, 1997; Yuan et al., 2000), and several successful surveys have been carried out near Site 889. In this "refraction" electromagnetic method, the traveltime for an electrical signal to diffuse through sediment from a seafloor source to a seafloor receiver is related linearly to the resistivity: the higher the resistivity, the shorter the traveltime. In practice, the phase difference between the transmitted electromagnetic signal and the received signal is viewed as a function of frequency. The measured phase differences are compared to theoretical values calculated for a resistivity-depth model. The general form of the model was based on smoothed resistivity measurements at Site 889 (Fig. 9), in which the 100 m interval above the BSR has constant resistivity while resistivities above this layer decrease uniformly to the seafloor. The seafloor electromagnetic data provide a scaling factor to determine the magnitude of the resistivity relative to a reference. For the best-fit model at a source-receiver separation of 493 m (Fig. 10), resistivities near 1.8 ohm m were obtained for the layer above the BSR., which was within 10% of the log resistivities measured at Site 889.

Figure 10. Results of towed seafloor EM system near Site 889. The observed phase differences (dots) best match those for a model in which resistivity above the BSR is 2.0 ohm m, just slightly less than that determined from the Site 889 resistivity logs.

6. SEAFLOOR COMPLIANCE MEASUREMENTS

Another independent method for estimating hydrate concentration is the measurement of seafloor compliance, since hydrate cementation increases the elastic moduli, especially the rigidity. Thus, this method is particularly sensitive to variations in seismic shear velocity. Compliance is obtained by using a seafloor gravimeter and a precision seafloor pressure gauge to measure the seafloor deformation due to ocean surface gravity waves and longer period internal waves. Successful data have been obtained from eight sites near ODP Site 889 (Willoughby & Edwards, 2000). The measured compliances increased uniformly with frequency, and the basic character of the data could be fit with a model in which seismic velocities increased logarithmically with depth. The addition of a hydrate layer with increased shear and compressional velocity above the BSR improved the fit. However, the absolute resolution of the hydrate layer is limited by the low frequencies of the gravity waves for the water depths found at Site 889. Improved relative resolution may be achieved by comparing compliances near Site 889 with measurements made at locations where no hydrate is expected.

7. ESTIMATES OF HYDRATE AND FREE GAS CONCENTRATION

We conclude this brief summary of geophysical investigations on the northern Cascadia margin by presenting results of various methods to estimate quantitatively the hydrate concentrations within the sediments. Most methods are geophysical, derived from a variety of techniques to determine seismic velocity and electrical resistivity, and these are the only means currently available for estimating concentrations away from the region of the ODP drill holes. However, for completeness, we also present estimates of hydrate concentration determined from geochemical techniques, specifically from measurements of pore fluid chlorinities in recovered core samples. In all estimation methods, we emphasize the importance of determining the reference profile for the parameter being considered, representing its value for sediment containing no hydrate and no gas.

7.1 Seismic Velocity

We have used two simple model approximations to relate the amount of hydrate concentration in the pore space to sediment velocities. One is to obtain a velocity for the combination of pure hydrate and sediment matrix (i.e., no pore fluid), and then to combine that composite matrix with water-saturated sediment to determine an overall model velocity (Yuan et al. 1996, 1999; Lee et al., 1993). For the observed increase in velocity of about 250 m/s relative to the reference velocity of 1650 m/s, this method suggests that about 10-25% of the sediment pore space is occupied by hydrate.

Figure 11. Hydrate concentration near ODP Site 889 estimated from (a) log resistivities and core salinities, and (b). velocity data from sonic logs, VSP and MCS

Figure 11. Hydrate concentration near ODP Site 889 estimated from (a) log resistivities and core salinities, and (b). velocity data from sonic logs, VSP and MCS

The other method assumes that the observed velocity increase relative to the reference (Fig. 5) is due simply to a reduction in porosity as the hydrate fills the pore spaces (Yuan et al., 1996). We have used the velocity-porosity of Hyndman et al. (1993) to calculate inferred porosities and hydrate concentrations for three velocity-depth datasets near Site 889: (1) downhole velocity logs, (2) VSP velocities, and (3) MCS velocities. As shown in Fig. 1 la, concentrations reach a maximum of 20-25% of the pore space above the BSR.

To determine the amount of free gas below the BSR, the major constraint is the reduction in P velocity from about 650 m/s tol400-1500 m/s, as determined from VSP data and from foil waveform velocity inversions (Yuan et al. 1999). Models and laboratory measurements indicate that a reduction to <1000 m/s is produced with a gas saturation of only 5%, while further increase in saturation makes little change. Thus, a velocity reduction of only ~200 m/s must be produced by a very small quantity of gas, 1% or less of the pore space.

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