The presence of hydrates also tends to lower the heat capacities ties of sediments, but these effects are coinparative'ysmall and not useful for exploration.

On Leg 78B, the Glomar Challenger returned to Hole 395A on the Mid-Atlantic Ridge and logged the upper 0.5 km of the crust, obtaining excellent resistivity, natural-gamma, and caliper logs throughout the section and reasonable density and velocity logs near the base. The logs show that the crust may be divided into two distinct geophysical units. The upper 400 m of the crust displays high porosities and low velocities, densities and resistivities. Below this, between 400 and 500 m sub-basement, the crust displays a low crack porosity (1-2%), high resistivities (up to 1000 öm), and high velocities (up to 6.0 km/s). For the lower unit, a value of 2.2 was obtained for the exponent m in Archie's Law, which is consistent with low permeabilities measured in the same interval with a packer (2 to 9 µdarcies). Since the boundary between these two units is marked by a sharp increase in alteration products and the lower unit behaves as if it were sealed, we conclude that Hole 395A penetrated a major geophysical discontinuity.

Deep Sea Drilling Project (DSDP) studies at Site 570 on the landward slope of the Middle America Trench off Guatemala allow for the first time a quantitative estimate of the methane hydrate content in the massive mudstones deposited there. Drilling across the Guatemalan transect on DSDP Legs 67 and 84 has resulted in the greatest number of visual observations of gas hydrate in any marine area. At Site 570, a 1.5-m-long section of massive methane hydrate was unexpectedly cored in an area where none of the usual signs of gas hydrate in seismic records were present. The sediment section is similar to that recovered at the other eight sites off Guatemala, but drilling at Site 570 may have penetrated through a fault zone that provided the space for accumulation of massive gas hydrate.The methane hydrate was analyzed using the following well logs: density, sonic, resistivity, gamma-ray, caliper, neutron porosity, and temperature. The density, sonic, and resistivity logs define a 15-m-thick hydrated zone within which a 4-m-thick nearly pure hydrate section is contained. The methane gas content ranges from 240 m 3 to 1400 m 3 per m 2 of lateral extent; and if the body extends a square kilometer, its total volume of stored gas could be from 240 × 10 6 m 3 to 1400 × 10 6 m 3 .Because the acoustic impedance of hydrate calculated from the sonic and density logs shows no anomalous values, the shape and extent of the hydrate body cannot be defined in seismic records. Thus the body is theoretically nonreflective in contrast to the base of the hydrate reflection. The base of the gas hydrate reflection is presumed to be the result of the velocity contrast between sediment containing gas hydrate and sediment containing free gas.

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