Development of frequency-dependent traveltime tomography for controlled-source seismic data.

大约10年前，全球地震学中发展了一种理论，用于解释地震数据的有限频率性质，由此推导出相对于一维参考地球模型的延迟时间，称为有限频率走时断层扫描（FFTT）。通过考虑频率，可以更准确地估计速度，因为可以更准确地考虑波传播的物理特性。FFTT通常不适用于可控源数据，因为没有必要的地壳或近地表参考速度模型，能够产生与记录的地震图足够接近的真实合成波形，以估计有意义的延迟时间。因此，需要一种逆方法，该方法使用沿总路径计算的频率相关旅行时间，而不是延迟时间，以便对受控源（选择）旅行时间数据进行建模。我将开发一种频率相关的旅行时断层扫描（FDTT），其目标与全球地震学中使用的FFTT理论相同，即更准确的速度估计。FDTT是基于一种新的正演计算方法，称为波长相关速度平滑（WDVS）。虽然FDTT适用于所有可控源数据，但它最重要的应用将是近地表研究（100米以上），因为这些数据相对于地下非均质长度尺度而言通常具有较长的地震波长，因此射线（无限频率）理论最有可能失效。FDTT已经针对首次到达时间和二维模型进行了开发和测试。该合同将允许对合成数据和真实数据进行进一步测试，扩展到3D模型，处理反射时间，对WDVS算法进行全波场检查和校准，提高计算速度，并创建一个免费学术分发的软件包。地震走时层析成像是用来成像地球的？通过确定弹性波从表面到内核的速度的空间变化来确定S内部。在20世纪70年代，地震学家从医学界改编了断层扫描，直到最近，走时断层扫描的正演模拟部分一直是射线理论，一种波传播的无限频率近似值。这是因为射线理论计算效率高，适用于任意异质介质。在过去的10年里，全球地震学家已经证明，通过利用记录的地震图的频率内容，可以更准确地估计地球的三维速度结构。地幔。与地震数据相反，可控源数据是在地球应用研究中获得的。地壳和近地表在构造、石油、环境和工程方面的研究。由于地壳和近地表比地球更不均匀，因此需要一种不同的理论来利用受控源数据的频率内容。因此，没有事先已知的参考模型，从中微小的扰动就足以匹配实际数据。根据该奖项开发的新方法和相应的软件将在社会相关的新构造（风险）、环境和工程领域产生最大的影响。这将包括为地震危险评估成像浅层断层，协助修复地下水污染地点，划定未绘制的废物处置地点的边界，以及在建造桥梁和水坝等大型结构之前评估地下的能力。该方法将作为软件免费提供给学术界。

About 10 years ago, a theory was developed in global seismology to account for the finite-frequency nature of seismic data from which delay times relative to a 1D reference Earth model are derived, known as finite-frequency traveltime tomography (FFTT). By taking frequency into account, a more accurate estimation of velocity is possible because the physics of wave propagation is more accurately being taken into account. FFTT is generally not applicable to controlled-source data because there is no requisite reference velocity model of the crust or near-surface that is capable of yielding realistic synthetic waveforms that are close enough to the recorded seismograms to estimate a meaningful delay time. As a result, an inverse method that uses a frequency-dependent traveltime calculated along the total path, as opposed to a delay time, is needed in order to model controlled-source (picked) traveltime data. I will develop a frequency-dependent form of traveltime tomography (FDTT) that has the same objective as the FFTT theory used in global seismology, namely more accurate velocity estimation. FDTT is based on a new method of forward calculation called wavelength-dependent velocity smoothing (WDVS). Although applicable to all controlled-source data, FDTT's most important application will be in near-surface studies (upper ~100 m) since these data often have long seismic wavelengths relative to the length scale of subsurface heterogeneities, and hence the greatest potential for ray (infinite-frequency) theory to be invalid. FDTT has already been developed and tested for the case of first-arrival times and 2D models. This award will allow for further testing on synthetic and real data, extension to 3D models, treatment of reflection times, a full-wavefield check and calibration of the WDVS algorithm, improved computational speed, and creation of a software package for free academic distribution. Seismic traveltime tomography is used to image the Earth?s interior by determining the spatial variation in the velocity of elastic waves from the surface to the inner core. In the 1970's, seismologists adapted tomography from the medical community and until recently, the forward modeling component of traveltime tomography has always been ray theory, an infinite-frequency approximation of wave propagation. This is because ray theory is computationally efficient and applicable to arbitrarily heterogeneous media. In the past 10 years, global seismologists have shown that by exploiting the frequency content of the recorded seismograms, it is possible to obtain a more accurate estimation of the 3D velocity structure of the Earth?s mantle. Controlled-source data, as opposed to earthquake data, are acquired in applied studies of the Earth?s crust and near-surface in tectonic, petroleum, environmental and engineering studies. A different theory is needed to exploit the frequency content of controlled-source data since the crust and near-surface are much more heterogeneous than the Earth?s mantle, and therefore there is no reference model known in advance from which small perturbations would be sufficient to match the real data. The new method and corresponding software developed under this award will have its greatest impact in the societally relevant neo-tectonic (risk), environmental and engineering fields. This will include imaging shallow faults for earthquake hazard assessment, assisting in the remediation of a groundwater contamination site, delimiting the boundaries of unmapped waste disposal sites, and assessing the competence of the subsurface before large structures such as bridges and dams are constructed. The methodology will be made freely available as software to the academic community.