Active and Passive Seismic Interferometry in Directionally Biassed Wavefields

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.