Seismic Imaging of High Impedance Mineral and Gas Hydrate Deposits

应用常规地震勘探技术对深部矿藏的天然气水合物聚集进行成像带来了一个问题：由于岩石物理性质的强烈局部收缩而引起的地震波散射。块状硫化物表现出不同的纵波和横波速度，但密度对比度往往超过50%。天然气水合物聚集的特征是高纵波和横波对比(高达100%)、高度可变的衰减(Q因子)和没有明显的密度对比。 这种局部高阻抗(速度-密度积)对比的建模研究需要与所有可用井眼地球物理、地质和构造数据相一致的准确的3D地质/岩石物理模型。当岩石物理性质存在强烈的局部收缩时，地震波被非均质性散射。可以观察到两种效应：传播(直接)波的衰减和地震尾波(P-S波转换)的激发。强地震波阻抗对比的稳健模拟能力对于更好地理解小尺度结构中的波传播效应是重要的。高阻抗勘探目标(块状硫化物和天然气水合物)的三维弹性/粘弹性模拟表明，局部高阻抗对比导致强烈而复杂的散射响应，通常以转换横波为主。此外，由沉积物形状(标度系数)引起的幅度聚焦将导致明显的幅度-炮检距/方位角趋势。因此，必须仔细选择地震数据采集的几何形状和震源频率，以便观察到特征的幅度和相位变化。如果观察到，散射响应可以作为分类地震勘探目标组成的诊断工具。3D地球/勘探模型将基于全波形声波和密度测井、VSP、2D/3D地震数据和来自地球上一些最大的块状硫化物和天然气水合物矿床(阿拉斯加的红狗锌铅矿床、汤普森和萨德伯里的镍铜矿床、麦肯齐三角洲(NWT)的天然气水合物聚集)的大量钻孔数据。

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)).