The Ground CO2 Mapper. An innovative tool for the rapid and precise mapping of CO2 leakage distribution

The recently developed Ground CO2 Mapper (“Mapper” for short) is an inexpensive, light, robust, and low power consuming tool for determining the distribution of CO2 at the soil-atmosphere contact as an indicator of CO2 leakage. The basic premise behind the Mapper is that the contact between the ground surface and the atmosphere represents an interval where CO2 leaking from the subsurface can accumulate in anomalous concentrations due to two mechanisms, the higher density of CO2 with respect to air and the tendency of wind speed (and thus mixing) to approach zero near the ground surface due to frictional drag. Because of its measurement target and the tool’s very rapid response time, Mapper surveys can be conducted very quickly at a high sampling density, yielding accurate maps of CO2 spot anomalies. The unit can be used by anyone and deployed within only 5-10 minutes after sensor and GPS signal warm-up. Here we describe the Mapper and present results from a site of natural diffuse CO2 degassing in central Italy

Spatial-temporal water column monitoring using multiple, low-cost GasPro-pCO2 sensors: implications for monitoring, modelling, and potential impact

Monitoring of the water column in the vicinity of offshore Carbon Capture and Storage (CCS) sites is needed to ensure site integrity and to protect the surrounding marine ecosystem. In this regard, the use of continuous, autonomous systems is considered greatly advantageous due to the costs and limitations of periodic, ship-based sampling campaigns. While various geochemical monitoring tools have been developed their elevated costs and complexities mean that typically only one unit can be deployed at a time, yielding single point temporal data but no spatial data. To address this the authors have developed low-cost pCO2 sensors (GasPro-pCO2) that are small, robust, stable, and which have a low power consumption, characteristics which allow for the deployment of numerous units to monitor the spatial-temporal distribution of pCO2, temperature, and water pressure in surface water environments. The present article details the results of three field deployments at the natural, CO2-leaking site near Panarea, Island. While the first consisted of 6 probes placed on the sea floor for a 2.5 month period, the other two involved the deployment of 20 GasPro units along a transect through the water column in the vicinity of active CO2 seeps over 2 – 4 days. Results show both transport and mixing processes and highlight the dynamic nature of the leakage-induced marine geochemical anomalies. Implications for monitoring programs as well as potential impacts are discussed

How do we see fractures? Quantifying subjective bias in fracture data collection

The characterisation of natural fracture networks using outcrop analogues is important in understanding subsurface fluid flow and rock mass characteristics in fractured lithologies. It is well known from decision sciences that subjective bias can significantly impact the way data are gathered and interpreted, introducing scientific uncertainty. This study investigates the scale and nature of subjective bias on fracture data collected using four commonly applied approaches (linear scanlines, circular scanlines, topology sampling, and window sampling) both in the field and in workshops using field photographs. We demonstrate that geologists' own subjective biases influence the data they collect, and, as a result, different participants collect different fracture data from the same scanline or sample area. As a result, the fracture statistics that are derived from field data can vary considerably for the same scanline, depending on which geologist collected the data. Additionally, the personal bias of geologists collecting the data affects the scanline size (minimum length of linear scanlines, radius of circular scanlines, or area of a window sample) needed to collect a statistically representative amount of data. Fracture statistics derived from field data are often input into geological models that are used for a range of applications, from understanding fluid flow to characterising rock strength. We suggest protocols to recognise, understand, and limit the effect of subjective bias on fracture data biases during data collection. Our work shows the capacity for cognitive biases to introduce uncertainty into observation-based data and has implications well beyond the geosciences

Real and virtual outcrops to characterize the internal structure of a carbonate-hosted fault zone: the Tre Monti fault case study in Central Italy

A fault zone is composed of one or multiple fault cores, which are located within a complex network of fractures and secondary slip surfaces (i.e., the damage zone) that determine the mechanical behaviour. For example, fractures within the damage zone control fluid circulation and have a strong impact on the elastic properties of the host rock. Furthermore, multiple fault cores within the same structure can host both foreshocks and aftershocks during seismic sequences. All these observations suggest that fault zone structure exerts a primary control on the hydromechanical properties of faults. To perform a detailed reconstruction of fault zone internal structure, we have integrated standard structural geology investigations with structural analyses obtained from a terrestrial laser-scanner survey. We have investigated the internal structure of the Tre Monti fault, a SSE dipping carbonate-hosted right-transtensional fault in the Central Apennines. The fault is exposed for a length of ~ 8 km and is ~ 1 km wide, with a throw of ~ 1500 m accommodated by at least three sub-parallel main slip surfaces. Structural data have been gathered from exceptional exposures preserved within a quarry. In particular, the intensity and the orientation of joint sets within the damage zone were obtained both using a classical approach (i.e., scanline surveys on the quarry wall) and a semi-automatic extraction from the laser-scanner point cloud. Fracturing data were integrated with a detailed mapping of the secondary faults. Fracture orientations extracted semi-automatically from the point cloud are consistent with data derived from scanlines, suggesting that once the appropriate calibration procedure is adopted, the laser scanner analysis is a valuable complementary tool in structural geology, capable of reproducing a huge amount of data in a short time. The integrated analysis shows that secondary faults exhibit various orientations, from low angle antithetic normal faults to nearly-vertical oblique-slip faults with direction orthogonal to the main fault surface. They control local fracture density and geometry so that fractures are heterogeneously distributed within the quarry, with a density ranging between 10 and 50 m-1 and do not show exponential increase approaching the main fault. In addition, a nearly-vertical joint-set with WNW-ESE to WSW-ENE direction is widespread. Such an orientation is consistent with the horizontal, approximately N-S trending, 3 inferred from the right-lateral transtensional slip observed on the main fault surface. Our preliminary data highlight a wide fault zone structure, formed by parallel fault cores that are surrounded by a complex network of fractures and minor faults with multiple orientations. Further studies on similar faults will be performed to develop a reference framework of fault-zone structure that can be useful for all the studies aimed at the characterization of the hydromechanical properties of carbonate-bearing faults

Fracture distribution within a carbonate hosted relay ramp: insights from the Tre Monti fault (Central Italy)

Fracture distribution controls fluids circulation in fault damage zones, with evident implications for fault mechanics, hydrogeology and hydrocarbon exploration. Being usually characterized by a strong damage and structural complexity, this is of particularly importance for relay zones. We investigated the fracture distribution within a portion of a relay ramp damage zone pertaining to the Tre Monti fault (Central Italy). The damage zone is hosted within peritidal carbonates and located at the footwall of the relay ramp front segment. We analysed the distribution of the fracture density in the outcrop adopting a multidisciplinary study, involving classical and modern structural geology techniques: (1) scanlines measured in the field, (2) oriented rock samples, and (3) scan-areas performed on a virtual outcrop model obtained by aerial structure from motion. Scanlines and virtual scan-areas show that fracture density increases with the distance from the front segment of the relay ramp. Moreover, all the methods highlight that supratidal and intertidal carbonate facies exhibit higher fracture density than subtidal limestones. This trend of fracture density has two main explanations. (1) The damage is associated with the relay ramp development. Approaching the centre of the relay ramp (i.e., moving away from the front segment) an increase in the number of subsidiary faults with their associated damage zones produces high fracture densities. (2) The increase in fracture density can be attributed to the increasing content in supratidal and intertidal carbonate facies that are more abundant in the centre of the relay ramp. Our results highlight structural and lithological control on fracture distribution within relay ramps hosted in shallow water limestones

Hydraulic characterization of a fault zone from fracture distribution

A quantitative assessment of how faults control the migration of geofluids is critical in many areas of geosciences. We integrated geological fieldwork, quantitative analysis of the fractures distribution and numerical modeling to build a geometrical representation of a fault zone and to characterize its hydraulic properties. Our target is a fault located in the Majella Mountain (Italy). We collected 21 scan lines across the fault profile in order to characterize its architecture. The numerical modeling of the fracture network of the damage zones and their hydraulic parameters was performed using both commercial (Move (R)) and open source software (dfnWorks and PFLOTRAN). Move (R) was used to build a representative model of the fault zone using fracture spacing as a proxy, and to model the hydraulic parameters of the different fault domains. dfnWorks and PFLOTRAN were employed to infer the hydraulic parameters of the damage zones of the fault and then upscale these properties to an equivalent continuum domain, suitable for fluid flow simulations through the whole fault zone. Our findings show how even in a relatively small area it is possible to describe changes in terms of hydraulic properties of a fault zone and to build models capable to represent these variations

Radon Hazard in Central Italy: Comparison among Areas with Different Geogenic Radon Potential

Radon (222 Rn) is a natural radioactive gas formed in rocks and soil by the decay of its parent nuclide (238-Uranium). The rate at which radon migrates to the surface, be it along faults or directly emanated from shallow soil, represents the Geogenic Radon Potential (GRP) of an area. Considering that the GRP is often linked to indoor radon risk levels, we have conducted multi-disciplinary research to: (i) define local GRPs and investigate their relationship with associated indoor Rn levels; (ii) evaluate inhaled radiation dosages and the associated risk to the inhabitants; and (iii) define radon priority areas (RPAs) as required by the Directive 2013/59/Euratom. In the framework of the EU-funded LIFE-Respire project, a large amount of data (radionuclide content, soil gas samples, terrestrial gamma, indoor radon) was collected from three municipalities located in different volcanic districts of the Lazio region (central Italy) that are characterised by low to high GRP. Results highlight the positive correlation between the radionuclide content of the outcropping rocks, the soil Rn concentrations and the presence of high indoor Rn values in areas with medium to high GRP. Data confirm that the Cimini–Vicani area has inhalation dosages that are higher than the reference value of 10 mSv/y