Active and Passive Seismic Interferometry in Directionally Biassed Wavefields

关于地球地下的大多数信息（例如，岩层几何形状、温度、压力、成分、流体含量）要么来自地震，要么来自电磁波。这些声波通过地下传播，然后折射、折射或反射（回声）回到地面。在那里，它们被记录下来并被解释为地下性质。传统上，这种波来自活跃的能量源（地震学中的地震，或工业地下勘探环境中的活跃诱发地震或电磁源）。然而，在过去的五年中，一套革命性的新方法在“波场干涉测量法”的总称下发展起来，从根本上改变了地震学的性质。在其最流行的形式中，干涉测量允许来自被动来源的能量，如海浪，风和人为活动（以前被认为是背景噪声）被用来成像地球。干涉测量允许这种“噪声”场转换成看起来像来自有源的地震图的信号，即使没有这样的源发生。这些虚拟（想象）震源产生的地震图被用来描绘真实的地球结构。在短短五年内，这已成为地壳和上地幔表面波层析成像的标准技术，勘探行业也在开发类似的技术。事实上，在地震学领域，这种方法非常成功，以至于地震信号（以前的数据源）现在经常被忽略——只有背景能量场（以前被认为是噪声）被用于地下成像。这种方法存在一个局限性问题，在过去两年中才被充分阐明。理论上，当噪声场均匀地来自各个方向时，干涉测量就能工作。在地球上，无论是被动噪声场，还是在工业环境中使用的主动“定制”噪声场，都不会出现这种情况，主要是因为能量从地球表面或附近的源传播的主要形式是通过所谓的表面波，即在传播过程中环绕地球最外层的波。因此，表面波在某种程度上主宰了虚拟地震记录，淹没了所有体波信息。对于工业勘探来说，使用体波是非常必要的。由于干涉测量将为地下勘探打开新的大门，因此非常希望能够改变干涉测量方法，使其能够在偏置能量场中工作。我们的研究小组最近开发了一种称为“方向平衡”的方法，可以集成在波场干涉方法中，以纠正由于能量场方向性而产生的偏差（临时专利申请；手稿提交发表）。这种方法有望减少大约水平传播的表面波能量，同时将更垂直传播的体波提高到一个现实的水平。该方法要求能量被记录在一组接收器上（而不是像标准干涉测量法那样仅由一对接收器记录）。在工业地震中，接收器阵列总是可用的，因为它们构成了地震采集和处理系统的固有组成部分。因此，原则上定向平衡直接适用于工业地震数据，使用被动和主动能源。该项目将把定向平衡方法发展到工业应用的程度，并将其应用于工业合作伙伴提供的真实的、工业规模的地震数据集。通过增强体波相对于表面波的到达，这些方法有望使波场干涉测量技术适用于工业规模的地震，从而开辟新的研究和开发领域，并为地下勘探创造新的令人兴奋的可能性。

Most information about the Earth's sub-surface (e.g., rock stratal geometries, temperatures, pressures, composition, fluid content) comes from either seismic or electromagnetic waves. These propagate through the subsurface and either defract, refract or reflect (echo) back to the surface. There they are recorded and interpreted for sub-surface properties. Traditionally such waves emanate from active energy sources (earthquakes in seismology, or actively-induced seismic or electromagnetic sources in industrial subsurface exploration settings). However, in the past five years a revolutionary new set of methods has developed under the general name of 'Wavefield Interferometry', which have changed the nature of seismology fundamentally. In its most popular form, interferometry allows the energy from passive sources like ocean waves, wind, and anthropogenic activity (previously considered to be background noise) to be used to image the Earth. Interferometry allows this 'noise' field to be converted into signals that look like seismograms from active sources, even though no such sources occurred. The resulting seismograms from such virtual (imagined) sources are used to image the real Earth structure. In only five years this has become a standard technique in surface wave tomography of the Earth's crust and upper mantle, and similar techniques are under development for the exploration industry. Indeed, in the seismological community this has been so successful that signals from earthquakes (the previous data source) are now often ignored - only the background energy field (previously considered to be noise) is used for subsurface imaging. A limiting problem exists with such methods, which has only been fully illuminated over the past two years. Theoretically, interferometry works when the noise field comes equally from all directions. This is never the case on Earth for either passive noise fields, or even when active 'bespoke' fields are used in the industrial setting, principally because the dominant form of energy propagation from sources on or near the Earth's surface is through so-called surface waves, waves that hug the Earth's outermost surface as they travel. Surface waves thus dominate the virtual seismograms to an extent that swamps all body wave information. For industrial exploration it is strictly necessary to use body waves. Since interferometry would open new doors in subsurface exploration, it is highly desirable to be able to alter the interferometric methods to be able to work within biased energy fields. Our research group has recently developed a method, called 'directional balancing', that can be integrated within wavefield interferometric methods to correct biases due to the energy field directionality (provisional patents filed; manuscript submitted for publication). This method promises to reduce approximately-horizontally propagating surface wave energy, while enhancing the more vertically-propagating body wave arrivals to a realistic level. The method requires that energy is recorded on an array of receivers (rather than only by a pair of receivers as in standard interferometry). In industrial seismics, arrays of receivers are always available since they form intrinsic components of the seismic acquisition and processing system. Hence, in principle directional balancing is directly applicable to industrial seismic data, using both passive and active sources of energy. This project will develop the directional balancing method to the point of industrial application, and apply it to real, industrial-scale, seismic data sets provided by the industrial partner. By enhancing the body wave arrivals relative to surface waves, these methods promise to make wavefield interferometry techniques applicable to industrial scale seismics, thereby opening new fields of research, development and creating new and exciting possibilities for subsurface exploration.