Measurements of P- and S- wave velocities, and the estimation of hydrate concentration at sites in the continental margin of Svalbard and the Storegga region of Norway

High-resolution seismic data from arrays of closely spaced four-component ocean-bottom seismic recorders, acquired from two sites off western Svalbard, and from one site on the northern margin of the Storegga slide, off Norway, show S waves, generated by P-S conversion on reflection, in addition to P waves. The P- and P-S- waves were inverted jointly to provide P and S velocity models, using 3D travel-time tomography, 2D ray tracing and 1D waveform inversion. At the NW Svalbard Site, positive Vp anomalies above a BSR indicate the presence of gas hydrate. A layer up to 150-m thick, containing free gas, beneath the BSR is indicated by a large reduction in Vp without a significant reduction in Vs. At the Storegga slide site, the lateral and vertical variation in Vp and Vs and the variation in amplitude and polarity of reflectors indicate a heterogeneous distribution of hydrate that is controlled by stratigraphically mediated migration of gas. Several methods for the estimation of hydrate content from the Vp and Vs velocity models were employed, using different models for the way in which hydrate affects the seismic properties of the sediment host and different approaches for estimating the background velocity of the sediment host. Two of these, a Biot-theory-based method and an effective-medium-based method, predict low concentrations of hydrate of up to 5% and 11% of pore space, respectively, in a zone about 100-m thick at the site off NW Svalbard. At the Storegga site, there is a heterogeneous distribution with concentrations of up to 10% or 20%, depending on the model for hydrate cementation.