Passive seismic imaging of subwavelength natural fractures: theory and 2-D synthetic and ultrasonic data tests

Abstract

I present a source-independent fracture imaging method to use passive seismic data for mapping subwavelength natural fractures. Unlike conventional source-dependent imaging that often adopts reflection-type seismic imaging with known source that is not available in passive seismic surveys, the proposed fracture imaging approach relies on the transmission and diffraction data without the need for source information. I assume that passive seismic data can be decomposed into two types of data: primary transmission wave data and diffraction (coda) wave data. The imaging formula states that primary waves should coincide with coda waves at scatterer points at the time of scattering. Instead of generating source wavefields in the conventional imaging method, the proposed method only need to propagate transmission wave data and diffraction wave data from the receiver arrays and apply an imaging condition to produce an image of fractures. This imaging procedure can be used for processing P wave or S wave. In synthetic examples, I evaluate the proposed method in several aspects: inaccurate source location, inaccurate velocity model, sparse receivers and irregular receiver spacing, elastic data and joint surface and borehole acquisitions. I found that the proposed approach performed well (or even better) comparable to source-dependent fracture imaging when assuming exact source information is known. With perturbed source locations with random shifts (e.g. estimated source location with errors), however, fractures were missing in the source-dependent fracture imaging results but the proposed approach was not influenced. In the presence of velocity errors and sparse and irregular receiver spacing, the proposed method produces better fracture images than the source-dependent imaging results.

Passive seismic imaging of subsurface natural fractures: application to Marcellus shale microseismic data

“…Best estimation of the source location may still suffer from errors of tens or hundreds of meters. Previous studies with 2-D synthetic models (Zhu 2019) show that the magnitude of 50 m of location error leads to defocusing of fractures in the seismic image. Consequently, the conventional SDFI technique may not be optimal to process passive seismic data.…”

“…Then the scattered waves propagate forwards following the transmitted wave. In terms of the reciprocity of wave propagation, both transmitted waves and scattered waves have spatial and temporal coincidence at the scatterer if we reconstruct the two wavefields by back-propagating (or ray tracing) the two data sets separately (Zhu 2019). Since the principle of seismic imaging is to find the spatial or temporal coincidence between two or more wavefields at each possible imaging location (Claerbout 1985), the scatters can be imaged by zero-lag cross-correlation of the two back-propagated wavefields expressed as:…”

“…However, accurate predictions of the source location in passive seismic applications (microseismic or earthquakes) are still challenging, and the errors of the source location are unavoidable, even with sophisticated source-location methods (Waldhauser & Ellsworth 2000;Zhang & Thurber 2003;Huang et al 2017). Defocusing or missimaging of the pre-existing fractures can be caused by the inaccurate source locations (Zhu 2019), which makes the existing SDFI methods not applicable to passive seismic data directly.…”

An introduction to seismic diffraction

“…Simple controlled simulations likewise suggested that there is order in the coda and that multiple scattering and its systematic study by means of auto-correlation can lead to the interferometric extraction of inter-scatterer traveltimes, which could, for example, be utilized for linking individual measurements and efficiently constraining tomographic inversion in depth. Owing to the multi-directional radiation of diffractions, joint method development and the linking of observations in earthquake seismology and controlled-source exploration might eventually enable an integrated view of the Earth's crust (compare, e.g., Zhu, 2018). While previous studies often suffered from poor data quality, the rise of dense seismic acquisition strategies across the scales, as, for example, provided by fibre-optic strain sensing (Daley et al, 2013;Lindsey et al, 2017;Jousset et al, 2018), promise an exciting future for exploring faults, fractures and unconformities with the weak signatures of seismic diffraction.…”

Recent Advances and Challenges of Waveform‐Based Seismic Location Methods at Multiple Scales

“…In this method, no source information is required, and zero‐lag cross correlations of the back‐propagated direct and scattered wavefields are used to image the scatters and fractures. Numerical examples and an ultrasonic data set have indicated the approach's improved performance in imaging fractures compared to standard RTM, while a field microseismic data from the Marcellus Shale demonstrated the feasibility of the proposed method in imaging subsurface natural fractures (T. Zhu, ; Huang & Zhu, ).…”