| Project Detail |
Characterizing the properties of the Earths near surface zone (upper several meters to depths on the order of 100 meters or more) has a wide range of applications of great societal need. For instance, in building and underground construction such methods are used to map fault zones, bedrock layers and hazardous quick clay formations. In Switzerland, monitoring of unstable slopes is an important area, particularly as areas of permafrost can destabilize due to global warming. Mapping the near-surface is also an increasingly important area in the exploration of our solar system to for instance help unravel the depositional history in which erosion and meteor impacts have changed the landscape.Many of these applications rely on seismic methods to map subsurface structures and to derive meaningful elastic properties that in turn can be used to derive a geological interpretation of subsurface structures. Near-surface seismic imaging was pioneered in the early 90’s by groups at ETH-Zürich (Prof. Alan Green), the University of Kansas (Prof. Don Steeples) and others. The approach taken was to mimic seismic methods used in the exploration seismic community for hydrocarbon prospecting. Since the targets of interest were shallower, higher frequencies could be used and the acquisition geometry scaled proportionally, reducing the spacing of source and receiver locations. Although this approach has proven extremely successful, it has reached its limits limits as it is fundamentally limited by cost-constrained applications. In this proposal we describe a radically different approach to seismic imaging and monitoring of the near-surface zone that addresses this near-surface seismic paradigm. Our approach is enabled by recent developments in so-called DAS (Distributed Acoustic Sensing) seismic sensing technology and is based on two fundamental observations.First, we note that the acquisition geometry in near-surface seismic imaging and characterization is fundamentally different to that of exploration seismics in that sources and receivers are located right at the zone of interest. As a consequence, we argue that both passive and active seismic methods should be used together with methods such as wave equation inversion (note that this is fundamentally different and unrelated to the popular Full Waveform Inversion technique with entirely different objectives). We show that different quantities (related to dynamic strain) are needed for wave equation inversion as opposed to those available using conventional three component (3C) geophones. DAS technology partially already can provide these measurements. Second, since the late 90s it has become increasingly clear that the principal problem in exploration-scale seismic surveys is to record unaliased short wavelength noise, such as scattered ground roll or Rayleigh waves. As a result, modern exploration-scale land seismic surveys will involve several 100000 live recording channels, recording (locally) densely sampled data where the noise appears coherent and where sophisticated beam forming or polarization filtering techniques are applied to separate signal from noise. In contrast, state-of-the art near-surface seismic surveys still record spatially under-sampled noise and rely on crude noise attenuation strategies from the 70s or 80s to separate ground-roll from signal (e.g., 2D frequency-wavenumber domain filtering). However, inline well-sampled data, such as DAS, naturally lend themselves for wavefield reconstruction of 3D carpet-like receiver grids . We propose to extend recent advances in generalized sampling of wavefields for reconstruction of towed marine seismic data to the reconstruction of densely sampled 3C particle motion data from sparse configurations of DAS fibres augmented by sparse 3C geophone recordings. The resulting carpet-like multicomponent data will allow for much better strategies to separate noise (e.g., scattered ground-roll) from signal. These two observations, leveraging from DAS technology, represent nothing short of potential game changers for near-surface imaging and characterization. However, in order to fully benefit from these ideas, new wavefield sensors are needed measuring wavefield quantities that are not available using DAS. Therefore, in a third separate workpackage, we propose to develop new MEMS (micro-electromechanical systems) sensors designed to be sensitive to (combinations of) other strain components than those that DAS can detect, for instance shear strains.We will be working with a combination of data from an unstable slope field site, synthetic data as well we data that we will acquire in our Centre for Immersive Wave Experimentation (CIWE) laboratory. In CIWE we can acquire ultra-dense carpets of 3C particle velocity measurements using a 3C scanning Laser Doppler Vibrometer (LDV) together with DAS recordings. We plan to conduct experiments in a range of unconsolidated media with constitutive stress-strain relations that may sometimes not be well known. The CIWE laboratory thus critically bridges synthetic studies with field experimentation to allow us to fully explore benefits and limits of novel sensors as well as our near-surface imaging and characterization and methodology. |
