On the uncertainty of geophysical data interpretation in volcanic areas through a case study: Ciomadul Volcano

Abstract

This study mainly deals with the consequences of inadequate data coverage on the interpretation of geophysical information. The results of the joint gravity and geomagnetic survey of the Ciomadul Volcano are discussed in connection with conclusions of previously performed magneto-telluric (MT) investigations.In the first step, inversion of both the potential fields and MT data apparently suggested the existence of a shallow (i.e., <5 km) magma chamber containing residual melt, or alternatively, highly hydrothermally altered rocks. However, strong arguments exist against both of these hypotheses such as lack of strong geothermal manifestations at the surface, or the too young age of the volcano, inconsistent with the assumed advanced alteration stage.During the next step, a brief study on the limitations of the inversion approach revealed significant shortcomings generated by uneven data distribution. Based on these conclusions, the gravity inversion results were reconsidered by substituting the initially assumed single magma chamber with a hypothetical swarm of narrow volcano conduits, each with size below the Nyquist threshold of the gravity survey as determined by an observations coverage density of less than one station/km2.The new 3D density-based model was obtained by forward modelling, unveiling the structure of the shallow part of the volcanic feeder system that gradually developed and branched bottom-to-top.Given the shortcomings of the inversion due to inappropriate data coverage, it was speculated that a former solution of MT inversion might have been biased also by data scarcity. That leads to the integration of local effects (caused by narrow, but highly conductive fluid-filled conduits) into a single electrical resistivity anomaly that is improperly interpreted.