Extended Meq Hectohertz Coda Wave Energy in the Ambient Crust -- Implications for Surface Seismic Array Flow Structure Imaging

Abstract

We study the scattering of hectohertz (hHz) seismic emissions (f ~ 500 ± 200 Hz) from microseisms (Meqs) induced by fluids injected at 6km depth in crystalline basement as recorded by sensors 4km above the events. Some 90% of Meq hHz seismic emission energy is scattered into multi-second coda wavetrains with negligible intrinsic attenuation (Q ~ 3000). Coda amplitudes decline as hyperbolic function H(t) ~ 1/(a+bt)1/b, 1 less than b less than 2. Numerical 3D acoustic wave scattering replicates decline profiles with 1 less than b less than 2 for “pink noise” spatially correlated wave speed fluctuations attested by well-log power-law scaling empirics; codas for higher/lower wave speed fluctuation correlation (“red/white noise”) decline with b ~ 1; b = 0 gives exponential decline. Meq hHz waveform spectral modelling shows induced dislocation slip velocities to be slow/erratic along reactivated ambient crust poroperm structures; we note that these crustal flow structures are observed to drain according to hyperbolic well production decline timelines. Coda wave scattering thus links crustal reservoir Meq seismic emissions to reservoir flow activity. Using this link, long duration coda waves recorded at decaHz frequencies on surface seismic arrays above crustal reservoirs can be processed into signal energy stacks that map persistent Meq emission flow-connectivity structures in accordance with seismic imaging conducted at stimulated shale formations.