Fracture characterization by means of attenuation and generation of tube waves in fractured crystalline rock at Mirror Lake, New Hampshire

Abstract

Results are presented from experiments carried out in conjunction with the U. S. Geological Survey at the Hubbard Brook Experimental Forest near Mirror Lake, New Hampshire. The study focuses on our ability to obtain orientation and transmissivity estimates of naturally occurring fractures. The collected data set includes a four‐offset hydrophone vertical seismic profile, full waveform acoustic logs at 5, 15, and 34 kHz, borehole televiewer, temperature, resistivity, and self‐potential logs, and borehole‐to‐borehole pump test data. Borehole televiewer and other geophysical logs indicate that permeable fractures intersect the Mirror Lake boreholes at numerous depths, but less than half of these fractures appear to have significant permeability beyond the annulus of drilling disturbance on the basis of acoustic waveform log analysis. The vertical seismic profiling (VSP) data indicate a single major permeable fracture near a depth of 44 m, corresponding to one of the most permeable fractures identified in the acoustic waveform log analysis. VSP data also indicate a somewhat less permeable fracture at 220 m and possible fractures at depths of 103 and 135 m; all correspond to major permeable fractures in the acoustic waveform data set. Pump test data confirm the presence of a hydraulic connection between the Mirror Lake boreholes through a shallow dipping zone of permeability at 44 m in depth. Effective fracture apertures calculated from modeled transmissivities correspond to those estimated for the largest fractures indicated on acoustic waveform logs but are over an order of magnitude larger than effective apertures calculated from tube waves in the VSP data set. This discrepancy is attributed to the effect of fracture stiffness. A new model is presented to account for the mechanical strength of asperities in resisting fracture closure during the passage of seismic waves during the generation of VSPs.

Tube-wave Generation Due to Permeable Layers in a VSP Experiment - A New Model Elucidating the Effect of Dip Angles

“…Similar to the open fracture model (Hardin et al, 1987;Bakku et al, 2013), we assume that the dynamic layer thickness L(t) is determined from the layer compliance Z L (m/Pa) and the effective stress in the layer as…”

“…Here we assume that the stress field is uniform everywhere along the layer (Hardin et al, 1987), which is the main difference from the foregoing study (Li et al, 1994). It enables us to obtain the explicit connections between the permeable layer model and the open fracture model, which is discussed in a later section.…”

“…Conventionally, the models of tube-wave generation have considered the fluid flow due to the deformation of (1) a parallel-wall open fracture (Beydoun et al, 1985;Hardin et al, 1987;Ionov, 2007;Bakku et al, 2013) or (2) a permeable layer (Li et al, 1994). The former is found to be useful to estimate the permeability corresponding to µm-to-mm scale fractures (e.g., Hardin et al, 1987;Hornby et al, 1989), whereas the latter is useful for larger-scale (cm-to-m) geological faults (e.g., Li et al, 1994;Kiguchi et al, 2001).…”

“…The former is found to be useful to estimate the permeability corresponding to µm-to-mm scale fractures (e.g., Hardin et al, 1987;Hornby et al, 1989), whereas the latter is useful for larger-scale (cm-to-m) geological faults (e.g., Li et al, 1994;Kiguchi et al, 2001). …”

“…Furthermore, they assume that the fluid pressure perturbation at the fracture-borehole intersection is negligibly small (Beydoun's boundary condition), which is valid for small-amplitude tube waves. Hardin et al (1987) extended Beydoun's approach by considering that the incident stress is uniform everywhere along the fracture so that they can correctly include the stiffness of the fracture in the deformation process. Ionov (2007) removed the small-amplitude assumption of the Beydoun's approach by considering a realistic boundary condition that the fluid pressure at the fracture-borehole intersection is equivalent to the tube-wave generation amplitude (Ionov's boundary condition).…”