Comparisons of seismic waveforms propagating through geological formations, before and during injection of microbubble water and/or CO2, are suggestive of dynamic effects that cannot easily be explained with normal linear elastic theory. A characteristic change in the coda, a strong loss of low frequency energy, and a moderate increase of high frequency energy have been noted. Rather than appealing to linear elastic wave theory coupled with an unrealistic level of new heterogeneity, we point out that homogeneous elastically-braced media produce all three of these features in a propagating waveform as first order effects. A modified Klein-Gordon equation is used to replicate behaviour in a published microbubble injection experiment, and similar features are sought in a raw VSP data set acquired before and during injection of CO2 at the CaMI-FRS in November 2018. Early indications are that the two spectral changes explainable via Klein-Gordon waves can indeed be seen in the VSP data, but issues such as source coupling repeatability must yet be eliminated as factors. If the new bracing parameter η introduced to reproduce these data features macroscopically quantifies transient fluid properties, determination and mapping of this parameter would provide a new mode of seismic characterization of injection.
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