A probabilistic geologic model of the Krafla geothermal system constrained by gravimetric data

Abstract

The quantitative connections between subsurface geologic structure and measured geophysical data allow 3D geologic models to be tested against measurements and geophysical anomalies to be interpreted in terms of geologic structure. Using a Bayesian framework, geophysical inversions are constrained by prior information in the form of a reference geologic model and probability density functions (pdfs) describing petrophysical properties of the different lithologic units. However, it is challenging to select the probabilistic weights and the structure of the prior model in such a way that the inversion process retains relevant geologic insights from the prior while also exploring the full range of plausible subsurface models. In this study, we investigate how the uncertainty of the prior (expressed using probabilistic constraints on commonality and shape) controls the inferred lithologic and mass density structure obtained by probabilistic inversion of gravimetric data measured at the Krafla geothermal system. We combine a reference prior geologic model with statistics for rock properties (grain density and porosity) in a Bayesian inference framework implemented in the GeoModeller software package. Posterior probability distributions for the inferred lithologic structure, mass density distribution, and uncertainty quantification metrics depend on the assumed geologic constraints and measurement error. As the uncertainty of the reference prior geologic model increases, the posterior lithologic structure deviates from the reference prior model in areas where it may be most likely to be inconsistent with the observed gravity data and may need to be revised. In Krafla, the strength of the gravity field reflects variations in the thickness of hyaloclastite and the depth to high-density basement intrusions. Moreover, the posterior results suggest that a WNW–ESE-oriented gravity low that transects the caldera may be associated with a zone of low hyaloclastite density. This study underscores the importance of reliable prior constraints on lithologic structure and rock properties during Bayesian geophysical inversion.

Compaction of Hyaloclastite from the Active Geothermal System at Krafla Volcano, Iceland

“…Compaction may be associated with multiple phenomena (Figure 9): from an increase in effective pressure which can be caused by either local increases in external applied stress (e.g., due to burial from subsequent deposits 11 Geofluids and lava emplacement or from magma intrusion) or local decreases in pore pressure (e.g., if fluids are drained, excessively extracted, or if the fluid density fluctuates). During simulated burial, the surface hyaloclastite transitions from elastic to inelastic compaction at an effective pressure of 22 MPa (i.e., P * ; Figure 2), corresponding to a depth of 1.3 km, assuming that the top part of the reservoir is made of layers of basalt and hyaloclastite with a nominal rock density of ~2500 kg • m −3 and a fluid density of 800 kg • m −3 (after [68]). Thus, it is likely that some hyaloclastites in the geothermal system at Krafla (logged at depths of at least 1362 m in IDDP-1; [36]) have undergone inelastic compaction at pressures exceeding P * due to burial.…”

Petrophysical Property Modifications by Alteration in a Volcanic Sequence at IODP Site U1513, Naturaliste Plateau

“…Subsequently, quantifying alteration and its correlation with petrophysical properties has faced obstacles in similar studies (e.g., Broglia & Moos, 1988;Delius et al, 2003;Greenfield et al, 2020;Helm-Clark et al, 2004;Jerram et al, 2019;Planke, 1994;Pola et al, 2014). Our study builds on recent studies that have begun establishing the quantitative correlations between petrophysical properties and alteration (e.g., Delayre et al, 2020;Lévy et al, 2018;Rossetti et al, 2019;Scott et al, 2019). Although similar investigations regarding the relationships between petrophysical variation and diagenesis in sedimentary rocks have been extensively studied (e.g., Bjørlykke, 2014;Giles, 1997;, the results are not applicable to altered volcanic rocks particularly due to changes in mineralogy and associated mineralization (Aspandiar & Eggleton, 2002;Chesworth et al, 2004;García-Romero et al, 2005;Giggenbach, 1984;Schlegel & Heinrich, 2015).…”

Thermal Liability of Hyaloclastite in the Krafla Geothermal Reservoir, Iceland: The Impact of Phyllosilicates on Permeability and Rock Strength

“…Triaxial testing was used to constrain the behaviour of the hyaloclastite at a confining pressure of 5 MPa (Figure 6(b)), which is representative of~300 m depth in the geothermal reservoir, assuming nominal rock and water densities of 2500 kg m -3 and 800 kg m -3 , respectively [62]. Under such a confinement, the compressive strength of the as-collected hyaloclastite is~30 MPa, exhibiting a small stress drop upon rupture.…”