Abstract We propose an FWI strategy that makes use of transmitted waves as input data and utilizes both global and local optimization methods to estimate the P-wave velocity model of the subsurface. We envisage that our approach may be applicable to difficult seismic land data, like those from geothermal areas characterised by complex geological structures. As a test case, we considered the CROP/18A seismic reflection profile that crosses the geothermal field of Larderello (southern Tuscany, Italy). The aim is to estimate the P-wave velocity model down to a few kilometres depth below the surface that could be used as complementary information to the standard seismic reflection image which, in this case, does not show interpretable reflections in a range of depths accessible to industrial drillings. One innovative aspect of the inversion we propose with respect to conventional FWI approaches is its independence of a starting model that, ideally, should reproduce the true long wavelength velocity structure of the subsurface and that may be rather difficult to obtain in case of low quality data and complex geology. We lessen the dependence on knowledge of a suitable starting model by performing a sequence of two inversions. First, we employ a genetic-algorithm (GA) based inversion, a global optimisation method that does not require any specific starting model, resulting in a long wavelength, low-resolution velocity model. This model then becomes the starting model for a second FWI, driven by a local optimisation algorithm, aimed at bringing in the fine details of the subsurface velocity structure. The reliability of the final model is checked by comparing observed and predicted waves for many common shot gathers along the seismic line and through the matching between the velocities measured by check shots in two nearby wells and the FWI velocities in the same locations. Many details of the velocity field, likely related to metamorphic and igneous formations, become apparent and may complement the interpretation of the standard reflection image. From these results, it appears that the use of transmitted waves and of the FWI approach discussed here may effectively improve the information for geophysical interpretation of challenging seismic land data, such as those that characterise many areas of geothermal exploration.

中文翻译：

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通过传输波的全波形反演估计速度模型：以意大利托斯卡纳南部地热区地震剖面为例

摘要 我们提出了一种 FWI 策略，它利用传输波作为输入数据，并利用全局和局部优化方法来估计地下的 P 波速度模型。我们设想我们的方法可能适用于困难的地震陆地数据，例如来自具有复杂地质结构的地热区的数据。作为测试案例，我们考虑了穿越 Larderello（意大利托斯卡纳南部）地热场的 CROP/18A 地震反射剖面。目的是估计深达地表以下几公里的 P 波速度模型，该模型可用作标准地震反射图像的补充信息，在这种情况下，标准地震反射图像未显示可访问深度范围内的可解释反射到工业钻探。我们针对传统 FWI 方法提出的反演的一个创新方面是它独立于起始模型，理想情况下，该模型应该再现地下的真实长波长速度结构，并且在低质量数据的情况下可能很难获得和复杂的地质。我们通过执行两个反演的序列来减少对合适起始模型知识的依赖。首先，我们采用基于遗传算法 (GA) 的反演，一种不需要任何特定起始模型的全局优化方法，从而产生长波长、低分辨率速度模型。然后，该模型成为第二个 FWI 的起始模型，由局部优化算法驱动，旨在引入地下速度结构的精细细节。最终模型的可靠性是通过比较沿地震线的许多常见炮点集的观测波和预测波，以及通过附近两口井的检查炮测得的速度与相同位置的 FWI 速度之间的匹配来检查的。速度场的许多细节，可能与变质和火成岩地层有关，变得明显，并可能补充标准反射图像的解释。从这些结果来看，使用传输波和这里讨论的 FWI 方法似乎可以有效地改进具有挑战性的地震陆地数据的地球物理解释信息，例如那些表征许多地热勘探区域的数据。