No abstract

A combination of geophysical techniques can yield estimates of several physical properties versus depth. An example is presented for a thick sequence of sediments in Middle Valley near the present spreading axis of the Juan de Fuca Ridge. We have computed the porosity, temperature, and thermal conductivity versus depth by combining the results of seafloor electrical and heat flow surveys. The electrical resistivity of the sediments is empirically related to the porosity of the medium and to the pore fluid electrical conductivity, which is in turn sensitive to the temperature. The temperature profile is determined from the heat flow and the thermal conductivity. The latter is determined from calculated porosity and estimates of the constituent mineral and pore fluid thermal conductivities. The measured surface heat flow is assumed to be, and the resistivity has been determined to be, approximatly constant with depth. The computation of the variation of temperature, porosity, and thermal conductivity with depth yields physically realistic and reasonable results. Other physical properties (void ratio, bulk density, water content, compressional wave velocity) are estimated from the porosity. The shear wave velocity is calculated empirically from the compressional wave velocity, and Poisson's ratio is then computed from the compressional and shear wave velocities. The results are in good agreement with values derived for similar rapidly deposited sediment sequences elsewhere, but the model calculations indicate that Deep Sea Drilling Project measurements may be biased toward higher-porosity samples.

Two ocean bottom magnetometers have been deployed at sites on either side of the Juan de Fuca Ridge, 20 and 40 km from the crest, near latitude 47°N. Twelve days of data were recorded simultaneously at the two sites, and these data have been used to determine the electrical conductivity structure beneath the region. The vertical field variations at both stations are of very low amplitude in the period range 0.25 to 8 hours, with no detectable phase reversal across the ridge. Electric fields in the ocean layer have been estimated from the attenuation of sea floor horizontal fields with respect to Victoria, and the resulting impedance spectra inverted to find resistivity as a function of depth using the technique of Oldenburg (1979). Resistivity models which fit the data within the specified bounds are characterized by a comparatively resistive crust and upper lithosphere overlying a low resistivity zone between depths of 55 and 75 km which in turn appears to be underlain by a higher resistivity section. The average resistivities are used to estimate temperature and melt fraction on the basis of the effective medium theory of Shankland and Waff (1976) for a basalt melt fraction within an olivine matrix. For the low resistivity zone, and depending on the water content of the melt (0 to 10%), the estimated temperature range is 1400°C to 1250°C and the corresponding melt fraction 0.05 to 0.09. The data suggest that the melt fraction decreases below 80 km. The conductivity profile from magnetotelluric data on 72 Ma ocean floor in the North Central Pacific (Filloux, 1977) has similar characteristics to the 1 Ma old conductivity structure near the Juan de Fuca Ridge, but with the low resistivity layer at depths of 140 to 220 km. In both cases, the resistivity minimum should be equated with the region of maximum partial melt within the asthenosphere, rather than with the base of the lithosphere. The increase in depth to this zone with increasing lithosphere age is in general agreement with plate tectonic concepts.

In the summer of 1984 an electrical survey using magnetometric off-shore electrical sounding (MOSES) was conducted at two sites in Middle Valley, part of the northern Juan de Fuca h d g e complex. MOSES has been designed t o minimize the difficulties inherent in electrical surveys of the crust below the electrically conductive sea layer. Site 1 , at 48'32 'N, 128'42 'W, is in the central part of the turbidite-filled basin. Using a two-layer model of conductive sediments overlying a fractured basalt basement, the sediment resistivity and thickness were found t o be 0.82 * 0.06 S2m and 1800 f 300 m, respectively. The basement resistivity, although not well constrained by the data is consistent with the results obtained a t site 2. Site 2, located at 48'10'N, 128"5O'W, has a thinner sediment layer, which appears to vary with position. The sediment conductivity-thickness product is the parameter determined by the data. If the sediment resistivity were the same as at site 1, the sediment thickness would be 140 * 30 m to the SE of site 2 , and 240 f 55 m to the NW. The fractured basalt basement has a resistivity of 8.5 f 3.4 Qm and is a t least 1000 m thick.Using temperature-corrected pore fluid resistivity, the calculated porosity is found to vary from 62 per cent at the top t o 21 per cent at the base of the sediments and is 8 per cent in the basement. These values are in good agreement with estimates from seismic velocities for a thick turbidite sequence in a nearby sediment-filled basin and determined for layer 2A/B basalts in DSDP hole 504B, respectively.

Magnetotelluric sounding reveals a conductive zone beneath the Pacific Ocean at depths in excess of 60 km but does not resolve the resistivity of the lithosphere above this zone. Further resolution can be obtained by controlled source electrical methods. The simplest of these are the galvanic techniques. Dipole‐dipole resistivity sounding is not suitable because dipole separations of thousands of kilometers would be required to obtain values of the resistivity. A viable alternative is to measure on the ocean floor the magnetic field of a vertical bipolar source extending from the sea surface to the seafloor. Magnetometer transmitter separations of only a few kilometers are sufficient to determine the resistivity of a half space beneath the ocean. Sounding curves similar to those of the resistivity method may be constructed to resolve the resistivity of a layered lithosphere. The curves constructed are valid at alternating frequencies which are small compared with a skin frequency not in the ocean but in the lithosphere. The depth of penetration is of the order of half the transmitter‐receiver separation. Magnetic field amplitudes are in the range of picotesla for reasonable lithospheric resistivities and separations up to 10 times the length of the bipole. Modern instrumentation, modified for the ocean floor, can detect such signals at a range of 20 km at a frequency of about 0.02 Hz averaged over several hours.