Mud volcanoes in central Italy: Subsoil characterization through a multidisciplinary approach

Mud volcanoes are common geological phenomenons observed worldwide as in Italy. They are well described in the scientific literature for their morphological, tectonic and hydrogeological features. They represent not only a relevant geological and geomorphological interest but also not negligible elements of hazard and risk associated with the presence of soft and pliable sediments and with the possible unexpected emission of gas and mud. Therefore, the understanding of their structure and hydrogeological circuits in the subsurface is an important key to define hazard and risk conditions in the adjacent areas. This paper deals with a multidisciplinary study including geophysical and hydrochemical surveys undertaken at the Pineto (central Italy) mud volcano site to achieve an interpretative conceptual model explaining the shallow upward migration of deep mud fluids. Shallow electrical and seismic imaging of the mud volcano was obtained using two dimensional and three dimensional (2D-3D) electrical resistivity tomography and 2D reflection seismic surveys. The hydrochemical properties of the rising fluids were assessed by means of seasonal measurements of the chemical-physical parameters, the concentrations of major ions, and some natural isotopes. This mud volcano or mud lump appears as a dome of about 15 x 10 m in size. The height of the crater is 2 m approximately, while the diameter of the crater is 25 m. Emission of fluids and solids (cold brine, mud, gas) occurs from this crater. Upper Pliocene-lower Pleistocene foredeep pelitic deposits (over-compacted clays with silty-sandy levels) overlain by clayey-silty deposits crop out in the area. The survey results seem to reveal that the uprising of deep fluids does not occur exactly below the mud volcano at present. Instead, a high conductivity body is present within a fractured zone in the pelitic deposits at 60 m approximately to the ENE. The probable occurrence of a high permeability layer approximately between 20 and 30 m below ground level, confined by clay deposits, gives rise to an overpressured mud fluids reservoir. Mud fluids appear to be flowing toward the surface from the reservoir up to the mud volcano crater. The interpretative conceptual model proposed by the authors is a first attempt to explain the shallow upward migration of deep mud fluids in this central Italy mud volcano. The results can be used to identify the uprising of fluids with similar chemical-physical properties even in areas where the superficial and morphological evidence of the volcanic bodies have been obliterated or covered. (C) 2015 Elsevier B.V. All rights reserved.