Geophysical evidence of gas seepage and mass movement in the Laacher See volcanic lake, western Germany

<p>The Laacher See caldera lake, formed by a series of phreatomagmatic and Plinian eruptions around 12,900 years BP, has been receiving increased attention lately with several studies investigating the present-day volcanic and geodynamic activity in the eastern Eifel, a densely populated area in western Germany. Volcanic activity beneath Laacher See is most notably evidenced by several gas seeps in the lake and its surrounding shore, emitting CO<sub>2</sub> of magmatic origin. During a 2019 survey, several geophysical techniques were used to investigate the CO<sub>2</sub> seeps at the lake floor. Here, we present results from multibeam echosounder and sub-bottom profiler data showing the presence of gas in both the water column (i.e. gas flares) and the lake sedimentary infill. Enhanced seismic reflections and acoustic blanking illustrate different levels at which free gas is accumulated in the lake sediments. Additionally, several stratigraphic horizons containing mass-transport deposits (MTDs) are observed in the laminated lake infill. The origin of these MTDs remains unclear, yet possible causes of slope failure in Laacher See might include seismic shaking, anthropogenic lake level fluctuation, and an increased fluid/pore pressure in the sediment due to free gas. Our results give a first indication of free gas in the lake infill, with further research needed to investigate the possible link between gas presence and mass movement in the lake. The monitoring of gas seeps at Laacher See and a further understanding of its gas-laden sedimentary infill can ultimately contribute to a better volcanic hazard assessment in the area.</p>

Linear poroelasticity

This chapter presents the theory of linear poroelasticity. The formulation is modeled after the theory of Biot and has applications in geomechanics, petroleum engineering, and hydrogeology. Relations for force and fluid balance are presented along, with constitutive relations coupling mechanical skeletal motion to fluid pore pressure and vice versa. Microstructural considerations are discussed, and the theory is demonstrated via application to geomechanical consolidation and to incompressible gels.

Low frequency seismic source investigation in volcanic environment: the Mt. Vesuvius atypical case

We present a detailed analysis of the low frequency seismicity occurred at Mt. Vesuvius in the time range 2003–2018. This kind of seismicity is atypical for the volcano and poorly studied, therefore we characterized it in terms of spectral analysis, waveform cross-correlation, location and polarization properties. The different decay patterns of the spectra, the existence of both earthquake families as well as single events, the relatively wide seismogenic volume inferred from the locations and polarization features, indicate that the events are caused by distinct source mechanisms: slow brittle failure in dry rocks and resonance of fluid-filled cracks. On these basis, we classified the earthquakes as Low Frequency (LF) and Long Period (LP). Despite the differences between the two classes, both the event types are ascribable to the dynamics of the deep hydrothermal reservoir which induces variations of the fluid pore pressure in the medium. The fluid amount involved in the generation process, as well as the physical-chemical properties of the surrounding rocks are the essential factors that control the occurrence of a mechanism rather than the other.

Seismicity induced by fluid migration in the Main Ethiopian Rift

<p>Faults can act as preferential degassing pathways for fluids of deep origin. Their migration and consequently variation of fluid pore pressure can cause a reduction of normal stress on the fault planes and trigger earthquakes. This can generate not only microseismicity but also events with significant magnitude. To understand this phenomenon, we studied the spatial, temporal and waveform characteristics of local seismicity from the northern sector of Main Ethiopian Rift (MER) of East Africa near Fentale and Dofen volcanoes. The seismic database contains events occurred in the MER from October 2001 to January 2003, and acquired by the Ethiopia Afar Geoscientific Experiment (EAGLE Project). The recorded events have been relocated with NLLoc using a new 3D velocity model derived from a wide-angle controlled source experiment. The relocated catalog contains a total of 1543 events with magnitudes between 0 and 4. The seismicity is mainly concentrated in two areas: near the border faults of the Ethiopian plateau and within the rift. On the border faults, events mostly occur down to 20 km depth, with an average depth of ~ 12 km. Within the rift, the events mostly happen down to 15 km depth, with an average depth of ~ 9 km. The seismicity is divided into several clusters aligned parallel to the rift direction, and in profile sections the clusters are mostly dipping steeply sub-vertical and dipping consistent with Andersonian normal faults. The analysis of the temporal-spatial distribution of earthquakes shows that some of the clusters are strongly concentrated in time and in space, and therefore swarm-like. To understand if the different clusters have been conditioned by fluid migration we have also analyzed the frequency content, release of seismic moment, and b-val is cut out. The link between earthquakes and fluid migration has also been explored by interpreting the distribution of seismicity using remote sensing mapping of faults, fumaroles and hydrothermal springs. Understanding where and how the fluid migration occurs will aid geothermal exploration efforts in the region, also improved knowledge of where geothermal activity is linked to seismicity has implications for seismic hazard estimation, which is very important for this densely and economically active areas.</p>

A two-phase dilatant debris flow model based on full scale shear stress and pore pressure measurements

<p>Since 2004, observations of shear and normal stresses have been collected at the base of naturally-triggered debris flows at the Illgraben observation station (Wallis, Switzerland) [1].   Because flow height and the normal force are simultaneously measured, and limited observations of basal fluid pore pressure are available, it is possible to investigate how the solid/fluid contents of the flow influence the measured shear stress.  The experimental results have emphasized two debris flow properties: (1) Debris flows are characterized by rocky or boulder-rich front, following by a fluidized tail. Consequently, the mass density varies from large values at the front of the flow to lower values towards the tail. A comparison between different debris flow events, however, likewise reveals that the streamwise change in density can vary dramatically between two different events. (2) The relationship between the measured shear and normal tress is highly non-linear. </p><p>Operating on the assumption that the streamwise change in density (or equivalently change in streamwise composition) is primarily responsible for the observed non-linear stress behavior, we develop a rheological model describing two-phase debris flow motion. The underlying idea behind the model is that the granular content of the flow can dilate, changing the solid/fluid composition of the flow, and thereby alter the bulk flow density. The model allows us to estimate the correct debris flow composition for different classes of debris flow varying from granular to muddy fluid. Based on these results, we are then able to reproduce the measured shear stress data when we simulate the measured events numerically.  The results appear to confirm dilatant-type flow models proposed by Takahashi [2], and later developed in detail by Iverson and George [3]. The model is used to back-calculate recent debris flow events that occurred near Davos Switzerland in 2018/2019.</p><p> </p><p> </p><p> </p><p>REFERENCES</p><ol><li>McArdell, B.W., Bartelt, P. and Kowalski, J. (2007): Field observations of basal forces and fluid pore pressure in a debris flow, Geophysical Research Letters, Vol. 34, No. L07406.</li> </ol><p> </p><ol><li>Takahashi, T. (2007): Debris flows: mechanics, prediction and countermeasures, Taylor and Francis / Balkema, 448pp.</li> </ol><p> </p><ol><li>George, D. L., & Iverson, R. M. (2011). A two-phase debris-flow model that includes coupled evolution of volume fractions, granular dilatancy, and pore-fluid pressure. Italian journal of engineering geology and Environment, 43, 415-424.</li> </ol>

The Influence of Coupled Thermomechanical Processes on the Pressure and Temperature due to Cold Water Injection into Multiple Fracture Zones in Deep Rock Formation

A technique to produce geothermal energy from deep rock formations at elevated temperatures consists of drilling two parallel deep boreholes, the second of which is directed so as to intersect a series of fractures produced by hydraulic fracturing in the first borehole. Then, the first borehole is used for injection of cold water and the second used to produce water that has been heated by the deep rock formation. Some very useful analytical solutions have been applied for a quick estimate of the water outlet temperature and injection/production pressures in this enhanced geothermal system (EGS), but they do not take into account the influence of thermomechanical and hydromechanical effects on the time evolutions of the pressure and temperature. This paper provided help for the engineering design of the EGS based on these analytical solutions, by evaluating the separate influences of the thermal (T), hydromechanical (HM), thermo-hydro-mechanical (THM) effects on the fluid pore pressure and temperature. A thermo-hydro-mechanical (THM) model was developed to simulate the heat extraction from multiple preexisting fracture zones in the hot rock formation, by considering permeability changes due to the injection pressure as a function of changes in the mean effective stress. It was found that the thermal effects (without coupling with mechanical effects) led to a decrease of the transmissivity of the fracture zones and a consequent increase in the injection pressure, by a maximum factor of 2. When the temperature is constant, the influence of the hydromechanical effects on the fluid pore pressure was found to be negligible, because in such scenario, the variation of the mean effective stress was 3 MPa, which was associated with a maximum increase in the initial permeability of the fracture zone only by a factor of 1.2. Thermo-hydro-mechanical effects led to a maximum increase in the permeability of the fracture zones of approximately 10 times the initial value, which was associated with a decrease in the fluid pore pressure by a maximum factor of 1.25 and 2, when hydrological and thermohydrological effects were considered, respectively. Changes in temperature were found not to be affected significantly by the thermomechanical and hydromechanical effects, but by the flow rate in the fracture zones. A sensitivity analysis was conducted to study the influence of the number, the initial permeability, the elastic modulus and the residual porosity of the fracture zones, and the elastic modulus of the confining intact rock, on the simulation results. The results were found to be the most sensitive to the number and the initial permeability of the fracture zones.

Non - linear concentration waves of pollutants in fluid - saturated porous rocks: constitutive equations approach and some examples of numerical solutions in the phase plane

The dynamics of fluid - saturated porous rock is a particularly interesting problem in geophysics and hydrology, nevertheless it is very important in environmental protection as well since it is able to describe, in a lot of situations, the propagation of pollutant substances dissolved in porous environmental matrices. It has been show that, for these systems, the nonlinear effects can be very important to consider. In this paper we'll discuss two novel approaches to the study of the above dynamics: one based on constitutive and conservation equations, the other considering the mass - balance equations for the solute flow and for adsorption rate of solute on the poroelastic matrix. We have shown the first model correctly describes the nonlinear behavior of the fluid pore pressure and pollutants concentration while the second gives the nonlinear term describing advection of which some numerical solutions has been calculated.

One-Dimensional Consolidation Modelling for Unlayered Soil

In this work, one-dimensional problem has a well-known linear solution and, thus, provides a simple verification of the consolidation capability using numerical solution. The coupling is approximated by the effective stress principle, which treats the saturated soil as a continuum, assuming that the total stress at each point is the sum of an effective stress carried by the soil skeleton and a pore pressure in the fluid permeating the soil. This fluid pore pressure can change with, and the gradient of the pressure through the soil that is not balanced by the weight of fluid between the points in question will cause the fluid to flow: the flow velocity is proportional to the pressure gradient in the fluid according to Darcy's law. A typical case is a consolidation problem. Here the addition of a load to a body of soil causes pore pressure to raise initially; then, as the soil skeleton takes up the extra stress, the pore pressures decay as the soil consolidates. The Terzaghi problem is the simplest example of such a process. For illustration purposes, the problem is treated with and without finite-strain effects. The numerical solution agrees reasonably well with the analytical solution, with some loss of accuracy at later times.

Characteristics of Gas Transport within Uniaxial Compression of Granite Sample

The physical processes triggering the fluid flow within the stressed rock are highly complex and not fully understood. The granite sample obtained from Creighton mine, Canada, was subjected to the temperature-pressure effects using a special rock mechanic testing machine equipped with a high precision gas monitor. It is shown that when the sample approached to the peak stress during the uniaxial compression test, the connective cracks instantaneously occurred accompanied by a swarm of AE activities, which suddenly decrease the fluid pore pressure. This change can be able to drive the gas back to the newly emerging crack due to the formation of gas pressure gradient within the damage zones. It is indicated that the different permeabilities among the zones can dominate the suction-exhaust proceeding of pore fluids within rock mass. Beyond the volumetric strain at null, the deformation of the stressed rock leads to a reversely change in pore pressure of closed pores. The feature for the gas emission determined by the changes in pore structure of rock is also discussed and analyzed.