NOVEL APPLICATION OF CONTINUOUS GRAVITY AND GNSS TO UNDERSTAND EFFECTS OF LIQUID SATURATION CHANGES ON YIELDING DEFORMATION

Abstract

Continuous microgravity monitoring and repeated micro-gravity measurements have been trialled at the centre of a site of anomalous deformation in the Wairakei-Tauhara geothermal system. When combined with continuous data from GNSS (GPS), horizontal strain data across a monitored crack, and subsurface pressure-temperature data from shallow monitor bores, the gravity data provides additional information on possible mechanisms that may be driving changes in localized deformation rates over time.
The overall shape of the deformation anomaly remains unchanged, while rates continue to vary smoothly with time. Deformation (i.e. subsidence) mechanisms involving changing conditions in two boiling (2-phase) aquifers, at about 50-80m and 125-175m depth respectively, are postulated. An additional driving mechanism involving incremental yielding and repeated load changes (from passing trucks) is also explored.
Highly compressible hydrothermal clays are hosted within a buried hydrothermal eruption crater, which is ~200m deep near the centre of the deformation zone (based on continuous drill-core) and inferred to be ~200m wide (based on Geerstma fitted parameters to its shape). Sliding on the clay-lined crater walls may account for the shape consistency over 20 years. Significant transitions in clay yielding parameters (yield stress) and pressure decline from cooling groundwater inflow at different depths within the upper groundwater and intermediate-level boiling aquifers, can account for the observed compaction rate variations. Transient loading changes may also partly explain the ongoing deformation through incremental yielding.