Bottom-simulating reflectors: Seismic velocities and AVO effects

We obtain the wave velocities of ice- and gas hydrate-beating sediments as a function of concentration and temperature, Unlike previous theories based on simple slowness and/or moduli averaging or two-phase models, we use a B lot-type three-phase theory that considers the existence of two solids (gra in and ice or clathrate) and a liquid (water), and a porous matrix containi ng gas and water. For consolidated Berea sandstone, the theory underestimates the value of th e compressional velocity below 0 degrees C. Including grain-ice interaction s and grain cementation yields a good fit to the experimental data. Strictl y speaking, water proportion and temperature are closely related. Fitting t he wave velocity at a given temperature allows the prediction of the veloci ty throughout the range of temperatures, provided that the average pore rad ius and its standard deviation are known. The reflection coefficients are computed with a viscoelastic single-phase c onstitutive model. The analysis is carried out for the top and bottom of a free-gas zone beneath a gas hydrate-bearing sediment and overlying a sedime nt fully saturated with water. Assuming that the bottom-simulating reflecto r is caused solely by an interface separating cemented gas hydrate- and fre e gas-bearing sediments, we conclude that (1) for a given gas saturation, i t is difficult to evaluate the amount of gas hydrate at low concentrations. However, low and high concentrations of hydrate can be distinguished, sinc e they give positive and negative anomalies, respectively. (2) Saturation o f free gas can be determined from the reflection amplitude, but not from th e type of anomaly. (3) The P to S reflection coefficient is a good indicato r of high amounts of free gas and gas hydrate. On the other hand, the ampli tude-variation-with-offset curves are always positive for uncemented sedime nts.