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

Soil gas distribution in the main coseismic surface rupture zone of the 1980, Ms= 6.9, Irpinia Earthquake (southern Italy)

Soil gas measurements of different gas species with different geochemical behaviors were performed in the area of the Pecore Plain, a 200m × 300m sized, fault-bounded extensional basin located in the northernMount Marzanomassif, in the axial belt of the southern Apennine chain. The Pecore Plain area was affected by coseismic surface faulting during the Ms = 6.9, 1980 Irpinia earthquake, the strongest and most destructive seismic event of the last 30 years in southern Italy. The collected data and their geostatistical analysis provide new insights into the control exerted by active fault segments on deep-seated gas migration toward the surface. The results define anomalies that are aligned with the NW-SE trending coseismic rupture of the 1980 earthquake along the western border of the plain, as well as along the southern border of the plain where a hidden, E-W striking fault is inferred. Geospatial analysis highlights an anisotropic spatial behavior of 222Rn along the main NW-SE trend and of CO2 along the E-W trend. This feature suggests a correlation between the shape and orientation of the anomalies and the barrier/conduit behavior of fault zones in the area. Furthermore, our results show that gas migration through brittle deformation zones occurs by advective processes, as suggested by the relatively highmigration rate needed to obtain anomalies of short-lived 222Rn in the soil pore

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

Using near-surface atmospheric measurements as a proxy for quantifying field-scale soil gas flux

We present a new method for deriving surface soil gas flux at the field scale, which is less fieldwork intensive than traditional chamber techniques and less expensive than those derived from airborne or space surveys. The “open-field” technique uses aspects of chamber and micrometeorological methods combined with a mobile platform and GPS to rapidly derive soil gas fluxes at the field scale. There are several assumptions in using this method, which will be most accurate under stable atmospheric conditions with little horizontal wind flow. Results show that soil gas fluxes, when averaged across a field site, are highly comparable between the open-field method and traditional chamber acquisition techniques. Atmospheric dilution is found to reduce the range of flux values under the open-field method, when compared to chamber-derived results at the field scale. Under ideal atmospheric conditions it may be possible to use the open-field method to derive soil gas flux at an individual point; however this requires further investigation. The open-field method for deriving soil–atmosphere gas exchange at the field scale could be useful for a number of applications including quantification of leakage from CO2 geological storage sites, diffuse degassing in volcanic and geothermal areas, and greenhouse gas emissions, particularly when combined with traditional techniques

Mantle-derived CO2 migration along active faults within an extensional basin margin (Fiumicino, Rome, Italy)

Fluid migration along faults can be highly complex and spatially variable, with the potential for channeled flow, accumulation in capped porous units, fault cross-flow, lateral migration along strike, or complete sealing. Extensional basin margins can be important for such migration, given the associated crustal thinning and decompression that takes place combined with potential geothermal or mantle gas sources. One such example is near the urban area of Rome, situated along the active extensional continental margin of the Tyrrhenian back arc basin and surrounded by Middle-Upper Pleistocene K-rich and arc-related volcanoes. Recent research activities in the area around Fiumicino, a town 25 km to the west of Rome, has highlighted the close spatial link between degassing CO2 and the faults that provide the necessary vertical migration pathways. In particular, detailed soil gas and gas flux surveys have highlighted the release at surface of large volumes of asthenospheric mantle CO2 in correspondence with normal faults observed in a new seismic reflection profile acquired along the Tiber River. Detailed reconstruction of the Pleistocene–Holocene stratigraphy of the area dates fault activity from 20,000 to 9000 years BP. It is proposed that the gas migrates preferentially along the cataclastic tectonic breccias of the faults until it encounters recent, unconsolidated sediments; porous units within this shallow stratigraphy act as temporary secondary traps for the leaking gas,with local gas release at the ground surface occurring where the sealing of the overlying aquitards has been compromised. Degassing and active faults confirm the extensional tectonics affecting the area and the geodynamic scenario of a mantle wedge beneath the western Apennines, associated with ongoing W-directed subduction. Moreover, degassing highlights the potential geochemical and seismic risks for the highly populated urban areas near Rome