Inversion of temporal gravity changes by the method of local corrections: A case study from Mayon volcano, Philippines Peter Vajda a, ⁎, Ilya Prutkin b , Robert Tenzer c , Gerhard Jentzsch b a Geophysical Institute, Slovak Academy of Sciences, Dubravska cesta 9, 845 28 Bratislava, Slovakia b Institute of Geosciences, Jena University, Burgweg 11, D-07749 Jena, Germany c National School of Surveying, Division of Sciences, University of Otago, 310 Castle Street, Dunedin, New Zealand abstract article info Article history: Received 25 October 2011 Accepted 13 June 2012 Available online 21 June 2012 Keywords: Monitoring volcanoes Gravity change interpretation Mass change Mayon The 3D inversion method based on local corrections has been introduced by Prutkin to invert potential field data. It has been applied to gravity data inversion on local, regional, and global scales. Here we introduce the application of this method to invert temporal gravity changes. The inversion procedure is demonstrated by a case study on gravity changes observed at the Mayon stratovolcano between campaigns spanning a period of 1992–1996. Residual gravity changes are compiled from the observed ones. No significant surface deforma- tion was observed within the accuracy of the campaign GPS measurements during the considered period. Re- sidual gravity changes were first inverted in terms of sources represented by 3D line segments. The line segment approximation facilitates the second step of the iterative non-linear inversion based on local correc- tions. In the second step, the residual gravity changes were inverted in terms of 3D star-convex homogenous bodies representing sources of subsurface mass/density changes. Published geological evidence indicates a shallow magma system at Mayon. The absence of significant deformation accompanying the gravity changes indicates that this system was nearly open during 1992–1996. We hypothesize that the sources of the gravity signal represent mass transport, namely injection of magma into voids of the shallow system. Inversion re- sults for campaign differences 3–1 and 4–1 are presented. For both epochs the inversion results in a source located at a depth of about 4.4 km (± 0.3 km) below sea level. The mass of injected magma was estimated at 0.35 (± 0.10) MU and 0.12 (± 0.04) MU for the two epochs, respectively. We also compare our results with previous interpretations of the same data. © 2012 Elsevier B.V. All rights reserved. 1. Introduction Joint monitoring and interpretation of crustal deformations and tem- poral gravity changes observed on the surface in volcanic areas contrib- utes valuable insights into the very complex underground physical and chemical processes associated with magmatic and/or hydrothermal sys- tems (see Dzurisin, 2003; Battaglia et al., 2008; and references therein). Temporal gravity changes are sensitive to subsurface mass transport and to temporal changes in the subsurface density distribution. They can detect such changes and processes at depth often prior to any other precursors of unrest. Therefore their monitoring and interpretation should be indispensable in mid- or long-term hazard assessment. The role of gravity changes in volcano monitoring and risk mitigation pro- grams become particularly important in cases where the subsurface pro- cesses are accompanied with no measurable deformations (e.g., Battaglia et al., 2008; Johnson et al., 2010). The gravity time series collected around Teide volcano (Gottsmann et al., 2006) in 2004/5 represent a good illus- tration of such a case. In this work, we introduce the application of the method of local cor- rections, described in Section 2, to invert temporal gravity changes. This is accomplished on data from the Mayon volcano. In Section 3 we de- scribe the observations and how they were processed, including the cor- rections made to compile residual gravity changes, and what the error budgets are. Our Mayon campaign data are particular in that significant gravity changes of up to 150 μGal (1500 nm/s 2 ) are accompanied with no observed vertical deformation within the ±4 cm observational accu- racy. Difficulties associated with accounting for the hydrologic compo- nent in the gravity signal during the campaigns at Mayon are described. Next we present the results of the inversion of the time-lapse gravity changes for selected observation campaigns during the period of 1992–1996. We also compare our results to previous interpretations of the same data. We highlight potential benefits originating from the use of the described methodology along with, or compared to, other in- terpretation or inversion techniques. 2. Method of local corrections This 3D inversion method has been already applied to interpret gravity data on local, regional, and global scale (Prutkin, 2008; Prutkin Journal of Volcanology and Geothermal Research 241–242 (2012) 13–20 ⁎ Corresponding author. Tel.: +421 2 5941 0603; fax: +421 2 5941 0607. E-mail address: Peter.Vajda@savba.sk (P. Vajda). 0377-0273/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.jvolgeores.2012.06.020 Contents lists available at SciVerse ScienceDirect Journal of Volcanology and Geothermal Research journal homepage: www.elsevier.com/locate/jvolgeores