Tiziana Vanorio

Three‐dimensional seismic tomography from P wave and S wave microearthquake travel times and rock physics characterization of the Campi Flegrei Caldera

[1] The Campi Flegrei (CF) Caldera experiences dramatic ground deformations unsurpassed anywhere in the world. The source responsible for this phenomenon is still debated. With the aim of exploring the structure of the caldera as well as the role of hydrothermal fluids on velocity changes, a multidisciplinary approach dealing with three-dimensional delay time tomography and rock physics characterization has been followed. Selected seismic data were modeled by using a tomographic method based on an accurate finite difference travel time computation which simultaneously inverts P wave and S wave first-arrival times for both velocity model parameters and hypocenter locations. The retrieved P wave and S wave velocity images as well as the deduced V p /V s images were interpreted by using experimental measurements of rock physical properties on CF samples to take into account steam/water phase transition mechanisms affecting P wave and S wave velocities. Also, modeling of petrophysical properties for site-relevant rocks constrains the role of overpressured fluids on velocity. A flat and low V p /V s anomaly lies at 4 km depth under the city of Pozzuoli. Earthquakes are located at the top of this anomaly. This anomaly implies the presence of fractured overpressured gas-bearing formations and excludes the presence of melted rocks. At shallow depth, a high V p /V s anomaly located at 1 km suggests the presence of rocks containing fluids in the liquid phase. Finally, maps of the V p *V s product show a high V p * V s horseshoe-shaped anomaly located at 2 km depth. It is consistent with gravity data and well data and might constitute the on-land remainder of the caldera rim, detected below sea level by tomography using active source seismic data.

The effect of chemical and physical processes on the acoustic properties of carbonate rocks

Carbonate rocks have major economic significance; 60% of the worlds oil reserves lie in carbonate reservoirs and the potential for additional gas reserves is huge. However, the relationship in carbonates between measured geophysical data and rock properties is complex, due to the large variety of textures that arise during postdepositional diagenesis and to the chemical processes (i.e., dissolution and replacement by newly formed phases) that characterize carbonate-forming minerals. Most experimental and theoretical rock physics research has focused on siliciclastic and shaly rocks. However, applying insights gained from clastics to carbonates is rarely straightforward, and thus is questioned in the literature.

Laboratory measurements of porosity, permeability, resistivity, and velocity on Fontainebleau sandstones

The relations among the resistivity, elastic-wave velocity, porosity, and permeability in Fontainebleau sandstone samples from the Ile de France region, around Paris, France were experimentally revisited. These samples followed a permeability-porosity relation given by Kozeny-Carman’s equation. For the resistivity measurements, the samples were partially saturated with brine. Archie’s equation was used to estimate resistivity at 100% water saturation, assuming a saturation exponent, n 2. Using self-consistent SC approximations modeling with grain aspect ratio 1, and pore aspect ratio between 0.02 and 0.10, the experimental data fall into this theoretical range. The SC curve with the pore aspect ratio 0.05 appears to be close to the values measured in the entire porosity range. The elastic-wave velocity was measured on these dry samples for confining pressure between 0 and 40 MPa.Aloading and unloading cycle was used and did not produce any significant hysteresis in the velocity-pressure behavior. For the velocity data, using the SC model with a grain aspect ratio 1 and pore aspect ratios 0.2, 0.1, and 0.05 fit the data at 40 MPa; pore aspect ratios ranging between 0.1, 0.05, and 0.02 were a better fit for the data at 0 MPa. Both velocity and resistivity in clean sandstones can be modeled using the SC approximation. In addition, a linear fit was found between the P-wave velocity and the decimal logarithm of the normalized resistivity, with deviations that correlate with differences in permeability. Combining the stiff sand model and Archie for cementation exponents between 1.6 and 2.1, resistivity was modeled as a function of P-wave velocity for these clean sandstones.

Emerging methodologies to characterize the rock physics properties of organic-rich shales

The high price of conventional oil and the ensuing focus on the security of hydrocarbon supplies is a clear signal for broader uses of unconventional sources of oil and gas. Organic-rich shales represent an enormous potential unconventional resource; the U.S. Energy Information Administration (EIA) estimates the world supply of oil shale at 2.6 trillion barrels of recoverable oil. The U.S. owns by far the largest amount (1–1.2 trillion barrels), mainly near the common borders of Wyoming, Utah, and Colorado, where much of the resource occurs at a saturation of about 10% by weight in layers that are 30–300 m thick (National Petroleum Council, 2007).

Texture analysis of a turbostratically disordered Ca-montmorillonite

Abstract Turbostratic disorder, consisting in a disorder in which different layers have different rotations with respect to an axis, is commonly found in montmorillonite. The effect of this kind of disorder on diffraction profiles is significant and must be taken into account, especially in quantitative phase analysis. The effect of the turbostratic disorder in textured materials has never been investigated. In the present work, we have developed a strategy to perform quantitative texture analysis on turbostratically disordered Ca-montmorillonite aggregates that were uniaxially compressed. Synchrotron diffraction images were analyzed with a Rietveld method and disordered and ordered models are compared. The method proved to be reliable and ready for further applications.

Confocal laser scanning and atomic-force microscopy in estimation of elastic properties of the organic-rich Bazhenov Formation

Shales are one of the most complicated and intriguing natural materials on Earth. Their multiphase composition is continually evolving over various scales of length and time, creating the most heterogeneous class of rocks in existence. The heterogeneities manifest themselves from the submicroscopic scale to the macroscopic scale, and all contribute to a pronounced anisotropy and large variety of shale macroscopic behavior (Ulm and Abousleiman, 2006). Moreover, the effects of the multiphase composition are amplified within organic-rich shales that contain varying amounts of kerogen. Despite significant research into the properties of kerogen, fundamental questions remain regarding how the intrinsic rock physics properties of the organic fraction affect the macroscopic properties of host shales.

Laboratory measurements of the acoustic and transport properties of carbonate rocks and their link with the amount of microcrystalline matrix

Laboratory data and computer tomography (CT) scan image analysis of carbonate rocks were combined demonstrating a quantitative link between acoustic and transport properties, the fraction of microcrystalline calcite (micrite) in the matrix, and the grain-to-micrite matrix ratio. Samples from the Monte Acuto formation, Gargano, Italy, a transgressive system tract (TST) whose microstructure ranges from basal pelagic mudstones to coarse calciturbidites, were analyzed. Results indicate that high micrite rocks with low grain-to- micrite matrix ratios, typical of low-energy depositional settings, are relatively stiffer and more impermeable. In contrast, lower amounts of micrite, due to high-energy depositional settings or its removal from intergranular macropores by progressive leaching of the matrix, leads to high grain-to-micrite matrix ratios. This process will increase porosity and reduce rock stiffness, both of which reduce velocity. Decreasing micrite content correlates well with increasing macroporosity....

The rock physics basis for 4D seismic monitoring of CO2 fate: Are we there yet?

Monitoring, verification, and accounting (MVA) of CO2 fate are three fundamental needs in geological sequestration. The primary objective of MVA protocols is to identify and quantify (1) the injected CO2 stream within the injection/storage horizon and (2) any leakage of sequestered gas from the injection horizon, providing public assurance. Thus, the success of MVA protocols based on seismic prospecting depends on having robust methodologies for detecting the amount of change in the elastic rock property, assessing the repeatability of measured changes, and interpreting and analyzing the detected changes to make quantitative predictions of the movement, presence, and permanence of CO2 storage, including leakage from the intended storage location.

The influence of pore fluids and frequency on apparent effective stress behavior of seismic velocities

Although poroelastic theory predicts that the effective stress coefficient equals unity for elastic moduli in monomineralic rocks, some rock elastic wave velocities measured at ultrasonic frequencies have effective stress coefficients less than one. Laboratory effective stress behavior for P-waves is often different than S-waves. Furthermore, laboratory ultrasonic velocities almost always reflect high-frequency artifacts associated with pore fluids, including an increase in velocities and flattening of velocity-versus-pressure curves. We have investigated the impact of pore fluids and frequency on the observed effective stress coefficient for elastic wave velocities by developing a model that calculates pore-fluid effects on velocity, including high-frequency squirt dispersion, and we have compared the models predictions with laboratory data. We modeled a rock frame with penny-shaped cracks for three situations: vacuum dry, saturated with helium, and saturated with brine. Even if the frame modulus depends only on the differential stress, the saturated-rock effective stress coefficient is predicted to be significantly less than one at ultrasonic frequencies because of two effects: an increase in the fluid bulk modulus with increasing pressure and the contribution of high-frequency squirt dispersion. The latter effect is most significant in soft fluids (helium in this experiment) in which the fluid-bulk modulus is less than or comparable to the thin-crack pore stiffness.

The deep structure of the Larderello‐Travale geothermal field from 3D microearthquake traveltime tomography

[1] With the aim of exploring the deep structure of the Larderello-Travale (LT) geothermal field, a high resolution 3-D tomographic inversion of microearthquake traveltimes has been performed. Results show that the deep part of the Larderello-Travale field is characterized by the presence of a structure having a velocity range of 6.0–6.5 km/s and a convex shape deepening towards the northeastern and the southeastern sides of the field. Earthquakes are mostly concentrated on the top of the high velocity structure and below the ‘K horizon’ implying a transition of rheological properties at depth. The reported dependence on time of ts-tp observed at one station located above an earthquake cluster suggests that the variation in pore fluid pressure might be responsible for the transition of rheological properties along the contact. In such an area, changes in pore fluid pressure might be related to time-dependent hydraulic mechanisms that are very effective in crustal rocks at elevated temperatures. INDEX TERMS: 7215 Seismology: Earthquake parameters; 7280 Seismology: Volcano seismology; 8045 Structural Geology: Role of fluids; 8180 Tectonophysics: Tomography; 8424 Volcanology: Hydrothermal systems. Citation: Vanorio, T., R. De Matteis, A. Zollo, F. Batini, A. Fiordelisi, and B. Ciulli (2004), The deep structure of the Larderello-Travale geothermal field from 3D microearthquake traveltime tomography, Geophys. Res. Lett., 31, L07613,