METHANE HYDRATE AND FREE GAS ON THE BLAKE RIDGE FROM VERTICAL SEISMICPROFILING

Seismic velocities measured in three drill holes through a gas hydrate deposit on the Blake Ridge, offshore South Carolina, indicate that su bstantial free gas exists to at least 250 meters beneath the bottom-si mulating reflection (BSR). Both methane hydrate and free gas exist eve n where a clear BSR is absent. The low reflectance, or blanking, above the BSR is caused by lithologic homogeneity of the sediments rather t han by hydrate cementation. The average methane hydrate saturation abo ve the BSR is relatively low (5 to 7 percent of porosity), which sugge sts that earlier global estimates of methane in hydrates may be too hi gh by as much as a factor of 3.