Seismic Imaging of High Impedance Mineral and Gas Hydrate Deposits

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Applying conventional seismic exploration techniques to image gas hydrate accumulations of deep mineral deposits poses a problem: seismic wave scattering caused by strong local contracts in physical rock properties. Massive sulfide exhibit variable compressional and shear wave velocities but density contrasts often exceed 50%. Gas Hydrate accumulations are characterized by high compressional and shear wave contrasts (up to 100%, highly variable attenuation (Q-factor) and no significant density contrasts. Modeling studies of such local high impedance (velocity-density product) contrasts require accurate 3D geological /petrophysical models consistent with all available borehole geophysical, geological and structural data. when strong local contracts in physical rock properties are present, seismic waves are scattered by the inhomogeneities. Two effects can be observed: the attenuation of the transmitted (direct) wave, the excitation of seismic coda (due to P-S wave conversion). Robust modeling capabilities of strong seismic impedance contrasts are important for a better understanding of wave propagation effects in small scale structures. 3D elastic/visco-elastic modeling of high impedance exploration targets (massive sulfide and gas hydrates) revealed that a local high impedance contrasts lead to strong and complex scattering responses, often dominated by converted shear waves. In addition, amplitude focusing caused by the shape of the deposit (scale factors) will lead to pronounced amplitude versus offset/azimuth trends. Thus seismic data acquisition geometries and source frequencies must be carefully selected in order to observe characteristic amplitude and phase variations. If observed, the scattering response may serve as a diagnostic tool to classify the composition of the seismic exploration target.The 3D earth/exploration models will be based on full waveform sonic and density logs, VSP, 2D/3D seismic data and extensive drill hole data from some of the largest massive sulfide and gas hydrate deposits on earth (The Red Dog Zn-Pb deposit in Alaska, Ni-Cu deposits at Thompson and Sudbury, gas hydrate accumulations in the Mackenzie Delta (NWT)).