Interpreting Time-Varying Processes using Empirical Data

Time-varying processes present both challenges and opportunities when interpreting tomographic models. The challenges mainly arise from differences between a method’s temporal and spatial resolution, and the size and timing of the physical processes. A mismatch here will produce a weighted average of the subsurface changes in models using time-dependent tomography. Even when there is such a mismatch, there are clear benefits from studying time-varying processes.

Many different variables are packed into geophysical properties. Seismic velocities, for example, change due to porosity, fluid presence, fluid phase, fractures, and fracture properties, among other factors. This makes interpretation non-unique. Tomographic changes due to time-varying processes help reduce the possibilities for interpretation, especially when integrating multiple geophysical methods. Because tomographic methods can also produce artificial changes between models (Hobé et al., 2021), an important question in time-dependent tomography becomes:
 
What is the theoretical magnitude of changes in geophysical properties in a time-varying area?

As an initial answer, we present an overview of empirical data from the IMAGE project, which investigated the Reykjanes Peninsula, Iceland. This overview shows the inherent temporal variability of this volcano-tectonic region. In the lab, geophysical properties changed as much as 30% due to, e.g., fracturing, fracture healing, fluid-phase changes, and changes in fluid saturation. All these phenomena are common occurrences in this area, which hosts several volcanic systems with hydrothermal and seismic activity. These phenomena also have a secondary impact on how the empirical data should be interpreted, which is usually underestimated or overlooked in tomographic interpretations: These time-varying processes can strongly affect effective pressures, which is one of the main variables in the empirical data. To show the magnitude of this oversight, we present examples where effective pressure is affected, along with the theoretical changes in geophysical properties. Lastly, we show how the inclusion of effective pressures in interpretation can aid in the identification of time-varying processes.

 

Hobé, A., Gudmundsson, O., Tryggvason, A., and the SIL seismological group (2021): Imaging the 2010-2011 inflationary source at Krýsuvík, SW Iceland, using time-dependent Vp/Vs tomography, in Proceedings World Geothermal Congress 2020, Forthcoming