A new approach to self‐potential (SP) data interpretation for the recognition of a buried causative SP source system is presented. The general model considered is characterized by the presence of primary electric sources or sinks, located within any complex resistivity structure with a flat air‐earth boundary. First, using physical considerations of the nature of the electric potential generated by any arbitrary distribution of primary source charges and the related secondary induced charges over the buried resistivity discontinuity planes, a general formula is derived for the potential and the electric field component along any fixed direction on the ground surface. The total effect is written as a sum of elementary contributions, all of the same simple mathematical form. It is then demonstrated that the total electric power associated with the standing natural electric field component can be written in the space domain as a sum of cross‐correlation integrals between the observed component of the total electric field and the component of the field due to each single constitutive elementary charge. By means of the cross‐correlation bounding inequality, the concept of a scanning function is introduced as the key to the new interpretation procedure. In the space domain, the scanning function is the unit strength electric field component generated by an elementary positive charge. Next, the concept of charge occurrence probability is introduced as a suitable function for the tomographic imaging of the charge distribution geometry underground. This function is defined as the cross‐correlation product of the total observed electric field component and the scanning function, divided by the square root of the product of the respective variances. Using this physical scheme, the tomographic procedure is described. It consists of scanning the section, through any SP survey profile, by the unit strength elementary charge, which is given a regular grid of space coordinates within the section, at each point of which the charge occurrence probability function is calculated. The complete set of calculated grid values can be used to draw contour lines in order to single out the zones of highest probability of concentrations of polarized, primary and secondary electric charges. An extension to the wavenumber domain and to three‐dimensional tomography is also presented and discussed. A few simple synthetic examples are given to demonstrate the resolution power of the new SP inversion procedure.

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Summary This paper summarizes results of a study of porosity, permeability, microstructure and acoustic properties of volcanic and pyroclastic rocks from Campi Flegrei (CF) and Mt. Etna (ET), Italy. We have measured the hydraulic, transport and acoustics properties of 28 room‐dry samples at ambient conditions, 25 room‐dry samples under confining pressure (up to 60 MPa) and 5 brine saturated samples under pressure (up to 45 MPa effective pressure). We established the following range of porosity, permeability, and Vp and Vs variations as functions of mineralogy and differential pressure in CF and ET lithologies: 1 Porosity  CF pyroclastic rocks 30–60 per cent  CF and ET lava rocks 2–20 per cent 2 Permeability  CF pyroclastic rocks 10–1000 mD  CF and ET lava rocks 0.01–100 mD 3 Velocity and Quality Factor  CF pyroclastic rocks Vp 2–3 km s−1Vs 1–2 km s−1Qp 5–80  CF and ET lava rocks Vp 3.5–5.5 km s−1Vs 2–3 km s−1Qp 10–115 Mineralogy and microstructure govern the acoustic and petrophysical properties of these rocks under pressure. In pyroclastic rocks, changes in acoustic response are directly related to presence of zeolites and pumice and their reactions with the pore fluid. In dry conditions, collapse of the internal texture of pumice leads to decreasing velocity with pressure. In saturated conditions, the water–zeolite interactions compete with effects due to collapsing internal textures and so velocity change with pressure is not as pronounced. The microstructural changes were confirmed by analyses of optical, hydraulic and transport properties after pressure: CT‐scans show a macroscopically more compact structure; under optical microscopy, pumice and zeolitized pumice appear shredded. A significant reduction in porosity and permeability is measured after pressurization. In lava rock samples, acoustic velocities increase in function of pressure and the velocity–pressure relationships are characteristic of samples with rounded pores. Our results emphasize the importance of conducting velocity measurements at simulated in situ conditions. By constraining the computations using site‐ and depth‐ specific rock physics properties, differences between ground deformation models in volcanic areas can be assessed and predicted more reliably thus reducing volcanic hazard.

Probability tomography is a new concept reflecting the inherently uncertain nature of any geophysical interpretation. The rationale of the new procedure is based on the fact that a measurable anomalous field, representing the response of a buried feature to a physical stimulation, can be approximated by a set of partial anomaly source contributions. These may be given a multiplicity of configurations to generate cumulative responses, which are all compatible with the observed data within the accuracy of measurement. The purpose of the new imaging procedure is the design of an occurrence probability space of elementary anomaly sources, located anywhere inside an explored underground volume. In geoelectrics, the decomposition is made within a regular resistivity lattice, using the Frechet derivatives of the electric potential weighted by resistivity difference coefficients. The typical tomography is a diffuse image of the resistivity difference probability pattern, that is quite different from the usual modelled geometry derived from standard inversion.