利用梯度结构张量刻画碳酸盐岩缝洞体边界

Research and application of carbonate fracture-cavity boundary characterization method based on gradient structure-tensor

碳酸盐岩缝洞体、河道及断层的地震响应均属于不连续异常,业界利用梯度结构张量(GST)刻画碳酸盐岩缝洞体边界的应用相对较少。为此,基于前人的研究,针对GST属性的详细计算方法及其几何意义,分析了各计算环节对计算结果的影响。技术原理分析及实际地震资料测试表明:①对张量元素的平滑计算是整个技术的关键,使结构张量矩阵不仅包含最大梯度能量方向的特征,而且还包含所有正交方向的变化特征;梯度元素的平滑对背景噪声具有一定压制作用,可以提高最终计算结果的信噪比。②第二特征值与第三特征值对碳酸盐岩缝洞体均具有一定刻画能力,反映了不同观测视角的缝洞体三维空间形态特征;优选或组合第二与第三特征值属性,在合适的低值截取与显示范围内可以有效刻画碳酸盐岩缝洞体边界。③利用钻井信息作为标尺刻度无量纲特征值,求解的特征值线性组合结果本质上是基于钻井信息的多重约束解,融合了第二与第三特征值的细节特征,并且有效解决了不同钻井刻度值不一致的问题。因此,GST属性与振幅变化率等常规属性相比,可更连续地刻画碳酸盐岩缝洞体,通常还能在一定程度上反映溶蚀规律。

The seismic responses of carbonate fracturecavity,channels,and faults are all discontinuous anomalies;however,the application of gradient structure-tensor(GST)to delineate carbonate fracture-cavity boundary is relatively rare.Therefore,the influence of each calculation link on the result is analyzed based on previous studies and according to the detailed calculation method of the GST attribute and its geometric significance.The results of technical principle analysis and actual seismic data tests are as follows① The smooth calculation of tensor matrix elements,as the key to the whole technique,makes the structure-tensor matrix contain the features in the maximum gradient energy direction and the changing features in all orthogonal directions.The smooth of gradient elements can suppress background noise to some extent and improve the signal-to-noise ratio of the final calculation.② Both the second and the third eigenvalues have an ideal ability to describe the carbonate fracture cavities,reflecting the three-dimensional morphological characteristics of the fracture cavities from different perspectives.By the selection or combination of the second and third eigenvalue attributes,the carbonate fracture-cavity boundary can be characterized effectively in the suitable range of low-value interception and display.③ With drilling information as a scale to the dimensionless eigenvalues,the linear combination of eigenvalues is essentially a multi-constrained solution,which incorporates the detailed features of the second and third eigenvalues and effectively solves the problem of inconsistent values for different drilling scales.In conclusion,compared with conventional attributes such as amplitude change rate,the GST attribute can describe carbonate fracture-cavity more continuously,thus reflecting the corrosion law to a certain extent.