• Main tunnel of the Low-Noise Laboratory
    Main tunnel of the Low-Noise Laboratory
  • Antiblast gallery of the Low-Noise Laboratory
    Antiblast gallery of the Low-Noise Laboratory
  • Flow points and field measurements
    Flow points and field measurements
  • Geophysical measurements
    Geophysical measurements
  • Entry of the Low-Noise Laboratory
    Entry of the Low-Noise Laboratory

 

 

Manager of the LSBB site : Konstantinos Chalikakis

1. Scientific Goals

In the heart of the karstic massif of Fontaine-de-Vaucluse, LSBB enables research to be undertaken at scales that are usually unobtainable, ranging from flowing fractures (61 currently) to a continuum of 14,000 m2 (beneath 30 to 519 m of cover rocks), with everything in between, in particular one block 5 m on a side and 20 m high (5 boreholes). It is thus possible to study problems of changing scales with a real ability for validation. The project’s ultimate goal is produce a digital model (taking into account the various spatial and temporal scales involved in the system) of its complex structure and associated flows, and to develop the methodologies needed to characterize the environment and for applying modeling tools.

The principal scientific objectives of current experiments and monitoring studies are the following:

  • To develop a multi-method, multi-scale approach to characterizing the environment (on surface, in boreholes, and underground by direct measurements in tunnels) that is suitable for imaging highly heterogeneous bedrock media.
  • To acquire the data needed to characterize the portions of the system available for storage and any variations in the quantity of water stored.
  • To acquire the data necessary for a structural characterization of the system, aimed at the creation of a 4-D geomodel for spatializing its geophysical and hydrogeological parameters.
  • To study the hydrodynamic operation (both for transmission and storage) of the unsaturated zones of karstic systems.
  • To study the geo-, hydro-, physico- (and even bio-) chemical exchanges induced by the hydrodynamic processes.
  • To acquire the data required for testing and validating hydrogeologic modeling methods designed for fractured environments.

2. Monitoring carried out and principal experiments conducted on the site

In this regard a number of monitoring and experimental programs are being carried out at LSBB in order to provide the relevant parameters, including time series and long-term experiments for the characterization and modeling of the transport of water and chemical elements in this complex, heterogeneous aquifer. The main monitoring campaigns involve:

  • Onsite climatic monitoring: Two Campbell meteorological stations were installed in 2008, at the top and bottom of the observatory (measurements of rainfall, solar radiation, wind strength and direction, atmospheric pressure, and temperature, at 15-minute intervals).
  • Hydrogeophysical and hydrogeodesic characterization of the site: the scientific approach adopted consists of combining a wide range of geophysical methods (electrical, electromagnetic, gravimetric, RMP, and seismic) to determine geophysical properties in and especially above the LSBB tunnel. Repeated measurements over time (monitoring) and space (on surface, in the tunnels, and in the boreholes) should lead to a detailed characterization of the environment (common parameterization of an integrative 4-D model), and to define the limits of resolution, of sensitivity, and of complementarity of the geophysical measurement techniques with regard to hydrogeological measurements for characterizing 4-D hydrodynamic parameters in a complex environment.
  • Hydrodynamic and hydrochemical monitoring of flows on the site: in contrast to the speleological approach, the LSBB tunnels explore the carbonate matrix in an arbitrary fashion; they therefore intersect an assemblage of cracks and fractures through which rainfall water flows naturally to reach the saturated zone. Sixty-one flow points have been identified, georeferenced, and bored into the rock. Five points, four of which flow perennially, have been monitored since 2003. The others have been identified more recently, after a change in the local rainfall pattern during the winter of 2008. The flows are the subject of semimonthly measurements, sampling, and analyses (measurements of discharge, pH, and electrical conductivity, and laboratory chemical analyses of major ions, total organic carbon, and certain isotopes: 13C and 180); the number of flows varies, depending on the hydrogeologic conditions.

3. Advanced projects

Characterization of structure and capacity

LSBB provides access from the surface to depths of several hundred meters in the infiltration zone of a carbonate reservoir whose porosity and fracturing are highly heterogeneous within the various facies encountered, both horizontally and vertically. Imagery of the reservoir, observation of its functioning, modeling of its response to natural stresses (seismic tremors, rains, etc,) and to anthropic ones such as injections of fluids (water, CO2), and the identification of temporal variations in the reservoir’s properties due particularly to variations in the saturation level of the unsaturated zone, are investigated by various geophysical techniques (seismic and seismology, radar, electrical, electromagnetic, gravimetry, deformation, tiltmetry, and density measurements), which supplement the hydrogeologic approach. A number of scales are considered, depending on the process being studied and the resolution capacities of the instruments, e.g., tens of meters for seismic to tens of centimeters for radar.This has all been achieved through the interaction of a number of teams (in particular EMMAH, GSRC, GEOAZUR, LEAT, UPPA, Géosciences Montpellier, and others) via various research programs (ANR HPPPCO2, MAXWELL, LINES, the INTERIMAGES and P&U T2DM2 programs, etc.) and theses (Emeline Maufroy, Benoit Derode, Pierre Jeanne, Jan Beres, Dikun Yang, Daryl van Vorst, Rob Eso, Sabrina Deville, and Simon Carrière) either recently completed or in progress. Overall coordination is provided under the LSBB program theme “Fractured porous carbonate reservoir, hydrophysical dynamics of processes, and interactions with waves”. A compilation and synthesis report on the various measurements is in progress as part of a thesis by Simon Carrière, initiated in late 2010. Some examples of measurement campaigns that have already been completed are shown below:

 

Figure 1: Seismics, INTERIMAGES 2006 Experiment (after Maufroy et al., submitted)

 

Figure 2: GPR imagery (UPPA, GEOAZUR, LEAT joint program) directed by Guy Sénéchal and Dominique Rousset, Université de Pau et des Pays de l’Adour (RST, 2010)

 

Study of hydrodynamic and geohydrochemical functioning

The first monitoring studies of point flows in the LSBB demonstrated the spatial and temporal variability of their behavior. This variability, and the location of the sampling points, raise questions about the organization of flows within the unsaturated zone, especially in relation to their depth and the local structure of the rock (lithology, fracturing, etc.) vs. their hydrodynamics and their chemical properties.

The five flows monitored between 2003 and 2007 were the subject of hydrodynamic, hydrochemical, and isotopic monitoring (pH, electrical conductivity, temperature, major-element chemistry, carbon 13 in total dissolved carbon 13CCMTD, DOC – Dissolved Organic Carbon, and silica), at weekly intervals over the hydrological cycles 2003-2004 and 2004-2005, and daily during periods of high water levels. The combination of information provided by tracers such as magnesium, organic matter (TOC), and silica on travel times of water within the host rocks has led to a qualitative classification of the flows within the Unsaturated Zone, by major type of function (Garry et al., 2006). Thus, even if in reality every intermediate case could be envisaged, the Unsaturated Zone was broken down into just three assemblages: a capacitive, a transmissive, and lastly an intermediate assemblage having a hybrid function between capacitive and transmissive (Garry, 2007; Garry et al., 2008). At the same time, the use of 13CCMTD for discriminating between flows has enabled the development of a traditionally acceptable conceptual model. In fact, contrary to received ideas, reservoirs of a capacitive nature feeding the flows may be considered to be sub-systems that are closed with respect to biogenic CO2, unlike flows of a transmissive nature, which are open to biogenic CO2 (Garry, 2006).

 

Figure 3: Quantification of residence time based on HIX index (after Blondel, 2008)

 

 

Moreover, the use of excitation-emission matrices in natural fluorescence has enabled the definition of several types of Dissolved Organic Matter (DOM) and allowed their changes to be monitored both in the ground over the seasons (seasonal variations in indices of fluorescence in soils), and also throughout their long journey through the system (time-lags of fluorescence maxima in the massifs). This method of characterization has proven effective for marking and differentiating a particular mass of water. Accordingly, it can be used to identify the origin (spatial tracing) or time of entry into the system (temporal tracing) of DOMs (Blondel et al., submitted; Pépin Donat et al., submitted). The goal is to develop a tracer for continuous application to a wide variety of karstic flows. The decrease over time in the amount of fluorescent molecules would immediately provide an estimate of the residence time. For this reason humification indices, based on the method for calculating HIX (Humification IndeX) have been selected in order to develop a quantitative tracer for time of residence. A study of the changes in their average values over two hydrologic cycles has established a significant relationship between the HIX of the massifs and the flows’ average times of residence (Blondel et al., 2010).

 

Figure 4: Changes in the number of flows in the LSBB as a function of pluviometry (after Périneau et al., submitted).

 

Since 2008, a change in the pluviometric conditions has led to the appearance of new, non-perennial flows. Their number (61) is now more representative of the variability of hydrodynamic and chemical behaviors in the unsaturated zone, and has enabled a more detailed typology to be established. Early studies have demonstrated an organization of the flows according to depth and fracturing (Périneau et al., submitted).

 

 

Figure 5: Location of flow points in the LSBB by depth and fracturing (after Périneau et al., submitted)

 

 

4. Joint programs and participating researchers

The LSBB is a collaborative platform which allows researchers and disciplines that normally have few occasions to interact - because each field is so extensive in itself - to develop research programs for investigating the dynamics of complex systems. These systems demand a knowledge of the environment and its physical phenomena and parameters, and bring together a number of research themes which define an interdisciplinary space for the development of LSBB’s research and R&D programs:

  • Carbonate reservoirs, and the dynamic behavior of porous, fractured media associated with mass transport, injections, and seismic tremors;
  • Epikarst and the near-surface and deep unsaturated karst zone, their roles in the dynamics of transferring pressure and mass within the aquifer;
  • Magnetometry and terrestrial magnetism, and magnetic phenomenology and coupling;
  • Interactions of seismic waves with porous fractured media, and temporal monitoring of hydraulic disturbances of the environment;
  • Characterization of the natural radiative environment, and radiation-matter interaction, e.g., effect of ionizing particles on electronics, and muon-flux densitometry;
  • Metrological developments and instrumental tests driven by research programs and by industrial R&D (in particular: tiltmetry, low-temperature and neutron magnetometry) which successfully take advantage of the environment’s nature and properties and the infrastructure’s special equipment for their success, e.g., clean room, boreholes, shielded capsule, fiber optics, low-Tc SQUID magnetometer, broad-band 3-D seismometric station, hydrogeochemical monitoring, meteorological stations, GPS.

Thus in addition to the SOERE H+, a number of laboratories of the universities of Avignon, Nice, and Marseilles as well as INSU are directly associated with LSBB, which also has strong ties with, for example, the universities of Savoie, Pau, and Vancouver, CEA-DASE, IRSN, and ANDRA. LSBB distributes its data over the ORFEUS and IRIS networks, and in particular is directly involved in three ANR projects (LINES, MAXWELL, and HPPPCO2) and PACA regional projects (PHYDROMED and PETROPRO).

5. Application of the data acquired

  • The climatic, geologic, geophysical (surface, borehole, and directly in underground tunnels) and hydrogeologic characterization of the site provide data that are of value to all the teams working on the site.
  • The climatic data acquired since 2008 by the two meteorological stations are available in an online database.
  • The geologic data include a geologic and geotechnical survey carried out during the excavation of the 3,700 m of tunnels, a DTM at 5-meter scale, and surface geologic mapping. Within the laboratory site, five 20-m-long cored boreholes drilled at a depth of 280 m outline a block for which a detailed structural model is being prepared.
  • The geophysical data acquired and still being acquired are designed to provide multi-parameter imagery of the temporal changes in the properties of this fractured, porous reservoir due to transfers of pressure and mass resulting from rainfall, and also to the effects of dynamic injections. All of these methods have been implemented at various scales, from the sub-meter to meter scale of the block to the kilometer scale of LSBB itself (53 hectares), and including intermediate scales. By way of example, the dynamics of THMBC coupling in a fractured, porous medium, studied under a joint program with ANR HPPCO2 (with GSRC, GEOAZUR, LGIT + SITES, PETROMETALIC, and INERIS) cover scales from tens of centimeters (horizontal boreholes) to tens of meters (vertical holes). A compilation study of all the data acquired is currently in progress (thesis by S. Carrière) aimed at making use of all the possible combinations and supplementing existing measurements by surface campaigns. All of the geophysical work performed at LSBB will ultimately be used to provide a 4-D geomodel of the system.
  • Geodesic data (tiltmetry and gravimetry) are also being acquired under a joint program with Géosciences Montpellier and represent a particularly interesting set of data which could soon be incorporated into the project.
  • Direct monitoring of flows within the aquifer’s unsaturated zone has been going on since 2003 for four perennial flows and one temporary flow. Following a change in the pluviometry in 2008, 56 new temporary flows appeared. The discharges, electrical conductivity, temperature, and concentrations of major ions and Total Organic Carbon (TOC) are measured on a semi-monthly basis. For certain periods, isotopic measurements (18O, 13C total dissolved mineral carbon, 3H, and 14C) and natural fluorescence are also available. Two points with perennial flows are currently being monitored on an hourly basis for discharge, temperature, and conductivity. In the near future (2011) the natural fluorescence will be monitored on an hourly basis at the two points already equipped for sampling and one to three additional points will be equipped for “continuous” sampling. All of these data will eventually be accessible in a database.

 

6. Short- and medium-term programs

Hydrogeophysics and hydrogeodesy:

The first stage of the project involves (i) applying a wide range of geophysical methods (electrical, electromagnetic, gravimetric, RMP, and seismic) to characterize the geophysical properties within and above the LSBB tunnel; (ii) correlating the results with the structural (geology, presence and degree of fracturing) and hydrogeologic (hydrochemistry, hydrodynamics) information; and (iii) proposing, based on geostatistical studies and geophysical measurements repeated over time (monitoring), a conceptual model of the operation of this portion of the unsaturated zone of the karstic system. The multidisciplinary methodology developed is particularly designed to define the limits of resolution, sensitivity, and complementarity of the geophysical measurements with respect to hydrogeologic measurements in the characterization of hydrodynamic parameters in 4-D, in a complex environment. A better understanding of the effects of the spatial heterogeneity and depth of the environment and of temporal variations in its hydrogeologic properties will be sought, in order to develop an integrated hydrogeophysical approach. As regards equipment available onsite, the priorities are (a) obtaining a data logger for the existing EM-34 conductivity meter (depth of investigation 10-40 m); (b) acquisition of an EM-31 conductivity meter; and (c) installation of a network of SP (Self Potential) electrodes in the tunnels surrounding water-infiltration points.

 

Hydrodynamic and Hydrochemical Monitoring:

A semi-monthly manual monitoring is currently being carried out; the priority is to perpetuate this monitoring and to automate the data acquisition at representative points. Three types of flow points must be distinguished: (i) points where the flow is perennial (4 currently known); (ii) points where flow is not perennial but frequent (a certain number of days per year, and reliably during rains, i.e., “predictable” - about 15 points so far); (iii) points whose flow is occasional, under specific meteorologic conditions (about 40 points so far). All four perennial-flow points must be equipped with a system for the continuous acquisition of temperature, electrical conductivity, discharge, natural fluorescence, and turbidity. A selection of points of type (ii) representing different flow typologies should be equipped with the same system. As a first stage, a set of five systems seems appropriate, each one being capable of being moved to carry out different acquisition campaigns. Eventually, the number of points so equipped should be increased. Lastly, points of type (iii) do not need to be equipped for continuous acquisition, except during specially targeted campaigns. For this purpose, one complete system must be available for installation at a point as soon as flow begins, or when the system is stressed. On the other hand, all the points must be equipped with a flow-detection device (system under development). These systems will (a) provide a binary picture of the duration of flows at non-perennial points, and (b) enable the optimization of sampling and analysis campaigns. The water from each point is regularly sampled and analyzed. The base frequency is semi-monthly; it is stepped up during heavy rains. The following items are quantitatively measured: temperature, conductivity and pH during sampling, major ions, total organic carbon, and 18O and 13C on selected samples./p>