Pierre Morat

Radon-222 signatures of natural ventilation regimes in an underground quarry

Radon-222 activity concentration has been monitored since 1999 in an underground limestone quarry located in Vincennes, near Paris, France. It is homogeneous in summer, with an average value of 1700 Bq m(-3), and varies from 730 to 1450 Bq m(-3) in winter, indicating natural ventilation with a rate ranging from 0.5 to 2.4 x 10(-6) s(-1) (0.04-0.22 day(-1)). This hypothesis is supported by measurements in the vertical access pit where, in winter, a turbulent air current produces a stable radon profile, smoothly decreasing from 700 Bq m(-3) at 20 m depth to 300 Bq m(-3) at surface. In summer, a thermal stratification is maintained in the pit, but the radon-222 concentration jumps repeatedly between 100 and 2000 Bq m(-3). These jumps are due to atmospheric pressure pumping, which induces ventilation in the quarry at a rate of about 0.1 x 10(-6) s(-1) (0.009 day(-1)). Radon-222 monitoring thus provides a dynamical characterisation of ventilation regimes, which is important for the assessment of the long-term evolution of underground systems.

Field characterization of the relationship between electrical potential gradients and soil water flux

Electrical potential differences between electrodes installed vertically at four depths (0.3, 0.5, 0.7 and 0.8 m) were monitored continuously during a 2-month period in a measurement site under natural fallow. Simultaneously, changes in soil water content and in hydraulic head were measured on a daily basis at different depths of the soil profile at the same site. They were analysed to obtain daily values of the soil water flux at the depth z = 0.4 m. This was in particular carried out over a 10-day period following a rainfall event. At that depth the water flux was first oriented downwards (infiltration), then shifted progressively upwards (evaporation). It is clearly shown that there exists a very significant linear correlation between the electric potential gradient at that level and the value of the flux. Owing probably to electrode potential problems, there is a residual value when the flux is null. If the relationship is legitimately forced through the origin, it becomes clear that electrode measurements could be used to infer water circulation in the soil in terms of direction and amount of flow.

Pressure induced temperature variations in an underground quarry

Abstract Temperature measurements have been performed in the rock and atmosphere of an underground limestone quarry in Vincennes, France. In the atmosphere, temperature variations associated with variations of the atmospheric pressure are continuously observed, with an average coupling of 70 m°C/hPa at a period of 4 min, 35 m°C/hPa at 1 h, and 3 m°C/hPa at 1 day, and an increasing phase advance for increasing period. At the highest frequencies, the coupling is close to its adiabatic value of 80 m°C/hPa. The reduction of coupling with decreasing frequency is interpreted as diabatic thermal stabilisation of the atmosphere by heat flow and coupled feedback due to phase changes of water. During extreme events, such as the tempests that occurred in France in December 1999, additional effects were observed in the atmosphere and in the rock material as well. Pressure induced temperature variations may be important to study in underground monitoring experiments, and may be a useful tool for the characterisation of nuclear waste storage sites.

Variation of the electrical resistivity of large rock samples with stress

Many experimental and theoretical efforts have been devoted to the study of the electrical properties of rocks (e.g., Brace et al., 1965; Madden, 1982; Parkhomenko, 1982; Yamazaki, 1966). Special attention has been paid to possible relationships between variations of mechanical stresses inside the rock and variations of its electrical properties, in reference to a possible technique of predicting earthquakes (Raleigh et al., 1977). These studies were generally performed on small (decimeter sized) samples, but the complexity of the physical processes involved and the heterogeneity of rocks in the field make it necessary to try to get results from larger blocks, with dimensions of several decimeters (Sobolev et al., 1983) or of meters (this study). It also appears quite desirable to look for information on the electrical properties of rocks at different scales (from a centimeter to a kilometer, say) to see if some scaling laws could be experimentally established. These scaling laws would allow inference of electrical properties at a given scale from the knowledge of the electrical properties at a smaller scale (Madden, 1976). Scaling-law methods seem to be promising in seismogenesis studies (e.g., Allegre et al., 1984; Turcotte, 1986).

Long-term thermal evolution and effect of low power heating in an underground quarry

Vertical profiles of temperature have been monitored in the atmosphere of an underground quarry since June 2001. A long-term temperature increase of the order of 0.1 °C per year is observed. Three heating experiments with 100-W sources, performed to study plume dynamics, can also be used to investigate the effect of visits on this increase. Ten days after the 24-h heatings, the initial state is not restored. Artificial contributions to the observed long-term temperature trend, triggered by internal heat release, thus cannot be ruled out. This experiment defines strict criteria for the underground monitoring of global warming or the preservation of painted caves. To cite this article: C. Crouzeix et al., C. R. Geoscience 335 (2003).

Heat and water transport by oscillatory convection in an underground cavity

Temperature, relative humidity and air pressure were recorded in the gallery of an underground quarry and the self-potential was recorded in the walls of the gallery. Periodical variations, with a period of the order of one minute, consistent for all sensors, were observed. They are interpreted as manifestations of oscillatory convective motions of the air of the gallery, driven by the geothermal gradient and transporting water, as well as heat, from the floor to the roof of the gallery. Some consequences of this phenomenon, common in underground cavities, are presented.

First results of using combined mass and temperature measurements to study the water flow at the rock–atmosphere interface

The preservation of historical monuments requires both a detailed understanding of water circulation inside construction materials, and a deep comprehension of the associated physical and chemical effects on building structure. Combining mass and temperature measurements may be a new powerful tool to achieve this purpose, as first results obtained on a wall of the Saint-Gatien cathedral in Tours, France, indicate. The proposed method has been evaluated with dedicated experiments in a climatic chamber, regulated in both temperature and humidity, using three types of stones: Tours tuffeau, Baumberg limy sandstone, and Ruthen sandstone. Climatic chamber data indicate that temperature gradients between rock surface and atmosphere provide a meaningful and practical estimate of the water flow, which controls the heat exchange between rock and atmosphere because of the high enthalpy of vaporisation of water. Continuous measurements of the mass of a sample confirm this assumption and allow the calibration of temperature data with respect to water flow estimations. Temperature gradients between points on the stone surface are also dominated by changes in the water flux, and they provide a sensitive estimate of local variations in the heat and water transport properties of non-saturated stones. Combined mass and temperature measurements therefore appear as a promising method to estimate in situ the global and local water flow between the rock and the atmosphere and thus to diagnose the state of stone degradation, or to estimate quantitatively the efficiency of treatment processes, both in the field and in the laboratory.

SP coseismic signals observed on an electrodes array in an underground quarry

Electric self-potentials (SP) and strains were measured on the surface of a pillar of the abandoned underground Meriel limestone quarry (France). The time variations of strains and SP are interpreted in terms of crack instability. The dynamic mapping technique shows that the most intensive SP variations, corresponding to maximum stress variations estimated from strain data, occur in the vicinity of active crack tips or crack intersections. The location of these maxima changes with the stress distribution variations. The synchronous stacking technique allows to detect significant pressure variations through electrodes installed in the fractured zone as well as through outside electrodes close to the medium discontinuities (petrological boundaries and pillar edges).