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Shake, Squeeze, and Rumble: Geophone, Hydrophone, and Microphone Observations and Physics of Engineered Geothermal System-Induced Hhz Microearthquakes.

Description: 

Three seemingly independent observations of 200-800 Hz signals from an EGS stimulation in Finland basement rock have a common source. The observations come from: a 1.9-2.4 km deep geophone array; a hydrophone array in a nearby shallow bay; and microphones at several places around the bay, all with sampling rates at or above 2 kHz. The common source is 5-6 km deep injection induced microearthquakes directly beneath these sensors. (An audible rumble was produced by wave energy trapped in the bay.) The full-duration spectra of these events appear as a symmetrical hill centered on 500 Hz, declining to 200 and 800 Hz on its flanks. Superimposed on the spectral hills are rapidly changing fluctuations originating in the ~4-second codas following the direct waves. Coda fluctuations are absent from the spectra of the direct P-wave displacement waveform.
For those familiar with the empirical Green’s functions of multiplet earthquakes, the MEQ spectra can be understood as the result of a sequence of closely spaced impulse emissions – the Shah Function of signal analysis. However, the actual source physics of induced hHz displacement signals is more novel: the displacement motions of the P wave arising from bi-directional slip in ambient crustal fractures - as initiated by EGS fluid injection. In other words, the initial radial source-volume dislocations are followed in a few milliseconds by reverse-direction radial source-volume dislocations. Here ‘ambient’ means fractures unrelated to planer faults – i.e., fractures in country rock far from active faults. The borehole array first motion P-wave displacement can be fit with a Haskell’s model of far-field displacement by summing over volumetric source dislocations. The hHz event-versus-size distribution follows the lognormal distribution of ambient crustal fractures. We suggest these events form a new class of fracture-permeability-related seismicity highly relevant to the development of Engineered Geothermal Systems.

Session: Understanding and Managing Induced Seismicity

Type: Oral

Date: 4/19/2023

Presentation Time: 03:00 PM (local time)

Presenting Author: Peter E. Malin

Student Presenter: No

Invited Presentation: 

Authors