Multi-Level Gas Monitoring: A New Approach in Earthquake Research

Abstract

Fluid anomalies were often considered as possible precursors before earthquakes. However, fluid properties at the surface can change for a variety of reasons, including environmental changes near the surface, the response of the superficial fluid system to loads associated with the mechanical nucleation of earthquake fractures, or as a result of transients in fluid flow from the depths. A key problem is to understand the origin of the anomaly and to distinguish between different causes. We present a new approach to monitor geochemical and geophysical fluid properties along a vertical profile in a set of drillings from a depth of a few hundred meters to the surface. This setup can provide hints on the origin of temporal variations, as the migration direction and speed of properties can be measured. In addition, potential admixtures of fluids from a deep crustal or mantle origin with meteoric fluids can be better quantified. A prototype of a multi-level gas monitoring system comprising flow and pressure probes, as well as monitoring of fluid-geochemical properties and stable isotopes is being implemented in a mofette field with massive CO 2 (up to 97 tons per day) degassing. The mofette is believed a gas emission site where CO 2 ascends through crustal-scale conduits from as deep as the upper mantle, and may therefore provide a natural window to ongoing magmatic processes at mantle depth. Fluids from three adjacent boreholes-30, 70, and 230 m deep-will be continuously monitored at high sampling rates.

Insight Into Hartoušov Mofette, Czech Republic: Tales by the Fluids

“…This indicates a hydraulic connection between the boreholes. This hypothesis was also supported by Woith et al (2020), who documented a decrease in the gas flow and bubble fraction (Figure 4D) on the 30th of August. At the same time, the Rn concentration of the F3 drill mud reached its maximum.…”

“…This observation was further studied by Nickschick et al (2015), who hypothesized the existence of pull-apart basin-like structures inside the PPZ, and it was tested by Kämpf et al (2019), who instead identified en-echelon faults, which act as fluid channels to depth. Barometric pressures (note inverted scale in Figure 4E) are negatively correlated to the gas flow and bubble fraction at F1 (Woith et al, 2020). Fischer et al (2020) explained this by proposing that elevated barometric pressures contribute to the dissolution of CO 2 that in turn hampers degassing.…”

“…In August 2019, a third borehole (F3) was drilled to a depth of ∼238 m. Its principal aim is to investigate the relation between geogenic degassing and earthquake activity by combination of geochemical and geophysical techniques. Further details about the borehole configuration can be found in Woith et al (2020).…”

ICDP drilling of the Eger Rift observatory: magmatic fluids driving the earthquake swarms and deep biosphere

“…1). In particular, two existing monitoring wells, F1 and F2 (Bussert et al, 2017;Fischer et al, 2020), were complemented by the F3 drill hole; these three adjacent boreholes, F1 (30 m), F2 (70 m) and F3 (230 m), provide continuous monitoring of fluids at high sampling rates to acquire fluid parameters (gas flow, water temperature and water level/pressure) as well as chemical (CO 2 , Ar, N 2 , O 2 , He, H 2 and CH 4 ) and isotopic (δ 13 C CO 2 , δ 18 O CO 2 and 222 Rn) gas content (Woith et al, 2020). Additionally, samples for laboratory analysis of He, Ne and Ar isotopes are taken repeatedly (roughly every 2 months), as theses isotopes are useful tracers for constraining the fluid origins and mixing ratios of mantle components.…”

“…Ultimately, a borehole seismometer will be installed at the bottom of F3 and will be complemented by a capillary tube to collect "fresh" gases from the CO 2 horizon at depth, directly at the point where the fluids enter the borehole to avoid possible contamination or impact from external processes. Further details on the instrumentation of this mofette field with massive CO 2 degassing (up to 97 t d −1 ) as well as the first monitoring results are summarized in Fischer et al (2020), Woith et al (2020) and Daskalopoulou et al (2021). Once the novel monitoring system is fully operational, fluid transients will be able to be observed in great detail.…”