Surface wave analysis for heterogeneous geological formations in geothermal fields: effect of wave propagation direction
Surface wave analysis for heterogeneous geological formations in geothermal fields: effect of wave propagation direction
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DOI：
10.1080/08123985.2019.1597497
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发表时间：
2019-04
影响因子：
0.9
通讯作者：
R. D. Andajani;Tatsunori Ikeda;T. Tsuji
中科院分区：
文献类型：
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作者：
R. D. Andajani;Tatsunori Ikeda;T. Tsuji
摘要 我们对多道和多次地震数据应用了先进的面波分析，以高横向分辨率估计横波速度结构。尽管在表面波分析中通常假设水平层状结构，但由于地热田等环境的地质构造不均匀，这种假设可能会被违反。通过共中点（CMP）互相关（CMPCC）分析可以有效地估计相速度的横向变化。在本研究中，我们在 CMPCC 分析工作流程中引入了两种额外的方法，以进一步提高相速度估计的横向分辨率。一种方法是窗口控制 CMPCC 分析，它对 CMP 道集应用空间窗口，同时保持相速度估计的准确性。在此分析中，我们发现很难改进较低频率下的相速度估计，因为必须保持更宽的空间窗口以保持色散曲线的准确性。因此，我们引入了另一种方法，其中我们考虑表面波从源到接收器的传播方向。在宽频率范围内，在存在横向速度变化的情况下，基于源-CMP 方向的互相关选择可显着提高色散曲线分辨率。我们对日本西南部九州岛山川地热田采集的地震数据进行了方向控制 CMPCC 分析，并获得了非均质地质环境中的频散曲线。然后，我们通过对频散曲线应用遗传算法反演来获得横波速度剖面。地热场中的横波速度剖面可解析反射地震剖面上无法识别的浅层和局部异质结构（例如火山灰、浮石和火成岩侵入体）。
ABSTRACT We applied advanced surface wave analysis for multichannel and multishot seismic data to estimate S-wave velocity structure with high lateral resolution. Although a horizontally layered structure is typically assumed in surface wave analysis, this assumption might be violated in environments such as geothermal fields because of their heterogeneous geological formation. The lateral variation of phase velocity can be effectively estimated with common midpoint (CMP) cross-correlation (CMPCC) analysis. In this study, we introduced two additional approaches into the CMPCC analysis workflow, to further improve lateral resolution of phase velocity estimates. One approach is window-controlled CMPCC analysis, which applies a spatial window for the CMP gathers while maintaining the accuracy of the phase velocity estimates. In this analysis, we found that it is difficult to improve phase velocity estimates at lower frequencies, as a wider spatial window must be kept to maintain the accuracy of the dispersion curve. Therefore, we introduced another approach in which we consider the direction of surface wave propagation from sources to receivers. Selection of cross-correlations based on source–CMP direction provides a significant improvement in dispersion curve resolution in the presence of lateral velocity variations, for a wide frequency range. We applied direction-controlled CMPCC analysis to seismic data acquired in the Yamagawa geothermal field, Kyushu Island, southwest Japan, and obtained dispersion curves in the heterogeneous geological setting. We then obtained S-wave velocity profiles by applying genetic algorithm inversion to the dispersion curves. The S-wave velocity profiles in the geothermal field resolve shallow and local heterogeneous structures (e.g. volcanic ash, pumice and igneous intrusions) that cannot be identified on reflection seismic profiles.