Poroelastic responses of confined aquifers to subsurface strain and their use for volcano monitoring

Karen Strehlow*, Joachim H Gottsmann, Alison C Rust

*Corresponding author for this work

Research output: Contribution to journalArticle (Academic Journal)

9 Citations (Scopus)
211 Downloads (Pure)

Abstract

Well water level changes associated with magmatic unrest can be interpreted as a result of pore pressure changes in the aquifer due to crustal deformation, and so could provide constraints on the subsurface processes causing this strain. We use Finite Element Analysis to demonstrate the response of aquifers to volumetric strain induced by pressurised magma reservoirs.

Two different aquifers are invoked - an unconsolidated pyroclastic deposit and a vesicular lava flow - and embedded in an impermeable crust, overlying a magma chamber. The time-dependent, fully coupled models simulate crustal deformation accompanying chamber pressurisation and the resulting hydraulic head changes as well as porous flow in the aquifer. The simulated strain leads to centimetres (pyroclastic aquifer) to meters (lava flow aquifer) of hydraulic head changes; both strain and hydraulic head change with time due to substantial porous flow in the hydrological system.

Well level changes are particularly sensitive to chamber volume, shape and pressurisation strength, followed by aquifer permeability and the phase of the pore fluid. The depths of chamber and aquifer, as well as the aquifer's Young's Modulus also have significant influence on the hydraulic head signal. While source characteristics, the distance between chamber and aquifer and the elastic stratigraphy determine the strain field and its partitioning, flow and coupling parameters define how the aquifer responds to this strain and how signals change with time.

We find that generic analytical models can fail to capture the complex pre-eruptive subsurface mechanics leading to strain-induced well level changes, due to aquifer pressure changes being sensitive to chamber shape and lithological heterogeneities. In addition, the presence of a pore fluid and its flow have a significant influence on the strain signal in the aquifer and are commonly neglected in analytical models. These findings highlight the need for numerical models for the interpretation of observed well level signals. However, simulated water table changes do mirror volumetric strain and wells are therefore a valuable addition to monitoring systems that could provide important insights into pre-eruptive dynamics.
Original languageEnglish
Pages (from-to)1207–1229
Number of pages23
JournalSolid Earth and Discussions
Volume6
Issue number4
DOIs
Publication statusPublished - 10 Nov 2015

Bibliographical note

Date of Acceptance: 21/10/2015

Keywords

  • Volcano deformation
  • Aquifers
  • Poroelasticity
  • Volcano monitoring

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