A free-geometry geodynamic modelling of surface gravity changes using Growth-dg software

Abstract

Globally there is abundant terrestrial surface gravity data used to study the time variation of gravity related to subsurface mass and density changes in different geological, geodynamical and geotechnical environments. We present here a tool for analysing existing and newly acquired, 4D gravity data, which creates new findings from its reuse. Our method calculates in an almost automatic way the possible sources of density change responsible for the observed gravity variations. The specifics of the new methodology are: use of a low number of observation points, relatively small source structures, low signal/noise ratio in the data, and a free 3D source geometry without initial hypothesis. The process is based on the non-linear adjustment of structures defined by aggregation of small cells corresponding to a 3D section of the sub-floor volume. This methodology is implemented in a software tool, named GROWTH-dg, which can be freely downloaded for immediate use, together with a user manual and application examples.

Microgravity Change During the 2008–2018 Kı̄lauea Summit Eruption: Nearly a Decade of Subsurface Mass Accumulation

“…The local free‐air gradient depends significantly on the source of deformation and may be different for, for example, post‐glacial rebound (Olsson et al., 2015) compared to volcano deformation involving subsurface fluid redistribution, where the free‐air gradient or Bouguer corrected free‐air gradient (Vajda et al., 2020, 2021) may be more suitable. Free‐body geometry inversions (Camacho et al., 2021) or coupled inversions of surface deformation and gravity (Nikkhoo & Rivalta, 2021) may provide an alternative to recover source parameters; however, mass accumulation without commensurate surface deformation that involves non‐elastic behavior, for example, density changes through degassing or the compressibility of gas‐rich magma (Rivalta & Segall, 2008), makes joint inversions of gravity and deformation nontrivial. Furthermore, because multiple processes and sources may have been active over the 2009–2015 period, we adopted a classical approach, applying a (theoretical) correction for the observed vertical surface deformation before completing point source gravity inversions.…”

Benefits and Limitations of the Growth Inversion Approach in Volcano Gravimetry Demonstrated on the Revisited 2004–2005 Tenerife Unrest

“…The same approach turned out to be applicable also to the inversion of time-lapse gravity changes (Camacho et al 2021b) or to the joint inversion of gravity changes and elevation changes (surface deformation) at benchmarks (Camacho et al 2011a generated by mass changes and pressure changes, respectively. A similar approach was developed for inverting surface deformation fields (such as those obtained from satellite DIn-SAR data), based on seeking not only pressure sources but also free geometry dislocation sources, namely dip slip, strike slip and tensile sources (Camacho et al 2020;Fernández et al 2021).…”

“…A similar approach was developed for inverting surface deformation fields (such as those obtained from satellite DIn-SAR data), based on seeking not only pressure sources but also free geometry dislocation sources, namely dip slip, strike slip and tensile sources (Camacho et al 2020;Fernández et al 2021). In the case of inverting spatiotemporal (time-lapse) gravity changes using the GROWTH-dg software application (Camacho et al 2021b), the sources represent subsurface spatiotemporal (time-lapse) density changes that will be called simply "differential density" in the sequel, understanding they represent a subsurface density change distribution over a time interval.…”

“…1) acquired and processed by Gottsmann et al (2006). We apply a free geometry, model exploration and subsurface cell population inversion approach called Growth (Camacho et al 2021b). Our objective is to analyse the benefits and limitations of the Growth inversion by analysing the Growth gravimetric model of the unrest in the context of the current geophysical knowledge of the unrest.…”

Interpretation of spatiotemporal gravity changes accompanying the earthquake of 21 August 2017 on Ischia (Italy)

“…To invert the residual gravity changes at benchmarks, we apply a "Growth" inversion approach (Camacho et al, 2011(Camacho et al, , 2021a, modified to working with sparse micro-gravity changes of low signal-to-noise ratio (Camacho et al, 2021b, Vajda et al, 2021, which makes no a priori assumptions about the source geometry. The application of the "Growth-dg" inversion tool enables to obtain, in a nearly automatic and non-subjective mode, a 3D model of subsurface temporal density changes given by means of aggregation of parallelepiped cells.…”