Should the "Lascaux Simulator" be improved? Comments on the programs proposed by the "subterranean climate-modeling" study group |  |
| On
December 14, 2011 the LIST spokesman, Michel Goldberg, attended a
session of the Lascaux Scientific Council (see "Latest News" entitled News from Lascaux: a translucid or transparent scientific council ?).
During this session Mrs Valérie Plagnes, who coordinates the Council's
study group on "Subterranean Climatology - Modeling" reported on the
progress of their studies. V. Plagnes indicated that this group works
in collaboration with members of the GMO (Groupement Maître d'œuvre -
the executive body for the preservation of Lascaux) to ensure a better
flux of information between the two groups and ensure that questions
that are important for the GMO are dealt with by the Scientific Council. She reported that after several meetings - some of which were held as video-conferences - the group had come to the following proposals, whose aim were : - to improve the « Lascaux Simulator » - to get a better understanding of the various fluxes (heat, water, gases, organic substances, microbes,…) that reach the cave. It is noteworthy that the major aim of the proposal, which was initialy to improve the simulator, evolved with time towards gaining a more precise knowledge of the various fluxes. The program proposed by the group deals with three aspects : 1- topography of the limestone cover of the cave and of its main structural discontinuities. These studies would be made by electric resistivity and possibly by means of a geological radar (as studies recently reported in a thesis by B. Lopez from Bordeaux suggest that this technique may bring good data except where clay is the main rock component). This would be followed by drillings to extract rock samples for characterization and determination of their parameters usefull for the simulator (heat capacity). The group has identified possible actors of these studies and obtained from them a time schedule and a cost evaluation for this part of the project. 2- geomorphology This study aims at extending the characterization of the cave’s cover to a wider area. It would include studies on the morphology and ancient formations of the surface and subsurface of the region, the hill and the cave, as remote discontinuities may lead for instance to infiltrations in the cavity originating far away from the hill. The group has identified possible actors for this study (one academic group and a private company). 3- pedology of the cover This study aims at establishing a map of the soil and at characterizing the main sources of organic and mineral substances as well as their mobility. Very few details were provided on this part of the project. The discussion that followed this presentation showed a consensus of the council on the main lines of the project. Valérie Plagnes was congratulated for the work she and the group have done. The importance of a detailed knowledge of the morphology, heterogeneity, discontinuities, … of the karst was emphasized. It was indeed considered that this study would enable one to determine which are the « sensitive » areas of the cave (points where vermiculations, or microbial growth for instance are more likely to appear), and thus to permit early detection and intervention. However, besides these general statements, no detailed discussion took place. Neither the adequacy of the methods used, nor a precise timing, nor the impact of human presence in the cavity needed for the studies, nor the precise budgets were discussed. The single noteworthy input by members of the council was the mention by Jean-Jacques Delannoy of a new method recently developped to detect subterranean clay. This could be a usefull complement to the geological radar approach proposed for the topography study (see above). In spite of what seemed to M. Goldberg a rather superficial discussion, the Scientific Council approved the program proposed by the study group without further discussion (see the summary of conclusions available on the Ministry of Culture's website) Valérie Plagnes was asked whether or not a study of the vegetal cover had been envisaged. This question was raised in the GMO by M. Rieu. V. Plagnes replied that this problem should be dealt with by an other study group (led by Y. Perrette). Some informations were provided by Jean-Michel Geneste about the composition of the vegetation above the cave. Several members of the council considered that this study should come at a later stage, when more would be known about the geomorphology and pedology of the hill. Muriel Mauriac disagreed with this view and pointed out that the forest is aged, needs replacement, and might be a real threat in case of a strong storm like that of December 1999. By the end of this discussion, Yves Coppens asked M. Goldberg for questions and comments. M. Goldberg agreed with the need to include the results of this study in the model used in the simulator, but only in case one would find it usefull to improve the simulator, an option that will be discussed below. Valérie Plagnes then moved to other subjects discussed in her group. She indicated that discussions had taken place related to the possibility of implementing « cold points » in the cavity, but that the person who was asked to present a project had not done it yet. There was therefore no point in discussing it in the council. The group also discussed the need for pumping out the CO2. V. Plagnes showed a few graphs, compiled by Mallaurent, describing the CO2 partial pressure at different locations in the cavity as a function of time and rose the question of whether or not the pumping of CO2 should be stopped. Without much controversy, a consensus arose that one should change as little as possible the conditions in the cave, and therefore should go on pumping. Valérie Plagnes indicated that Mallaurent is working on a protocol for deciding when the climatic assistance machinery should be turned on (or off ?). Comparing the climatic paramaters in the cave and the times of turnig on along the years, he failed to find a simple correlation. He therefore goes on making the decision by himself. One of the persons working in the cave will be trained by him to take the relay. The LIST's point of view on some of the issues discussed during the session 1- Should the simulator be improved: Let us first discuss the present limitations of the Simulator and the ways to improve it. The shape of the surface above the cavity, the morphology of the hill, the rock heterogeneity, the faults, cracks and cavities in the karst, … cannot but have an influence on the heat fluxes between the outside and interior of the cave, and hence on the shape of the thermal isotherms within the karst. They should therefore be taken into account during the first phase of the simulations by the "Lascaux Simulator". Indeed, this phase consists in trying to predict the temperature and humidity at a given time in each point of the cave based on the temperatures observed in the air above the Lascaux cave during the preceding months. The result of this first phase of computation is called by the authors "the initial conditions". These initial conditions - the precision of which has never been stated - are used as the starting point for the second phase of computation by the Simulator. It is supposed to lead to a "model" of the internal climate of the cave. One should therefore keep in mind that the quality of the modeling relies heavily on the accuracy of the initial conditions. It is therefore obvious that, if one wants the simulator to be able to produce reliable climatic models based only on the outer temperatures, the predictions of the "initial conditions" must approximate with high accuracy the real conditions inside the cave at the desired time. Any feature of the cave and of its environment that may influence the heat fluxes should therefore be quantitatively understood and included in the equations underlying the simulations. As explicited in our critical analysis of the simulator published on our website, and as he pointed out by M. Goldberg to the council during the March 2011 session he attended, the published version of the Simulator relied on several assumptions likely to limit the accuracy of the simulations, or at least its precision. One of them was to assume that the surface above the cavity is flat and horizontal. An improved version of the Simulator has been developed that includes the real shape of the ground above Lascaux. We have been told that this does not change the modeled temperatures significantly. However, no data has been published or provided to the Council to support this statement and we wonder to which precision the simulations were not affected. Some of the other improper assumptions - relative to the geomorphology and geostructure of the karst - could be corrected by introducing the results of the research programs proposed by the Scientific Council. In this respect, the LIST can only be satisfied that part of its criticisms have finaly been heard. However, the LIST considers that, for only the purpose of improving the simulator, it would be premature to invest much time and important amounts of money in these programs. Indeed, several studies on the morphology and rock structure of the Lascaux karst have been performed in the past, in particular by Vouvé and his coworkers. These studies were published in Vouvé’s thesis as well as in several papers. We therefore suggest to start by obtaining a few samples of rocks and sediments (based on Vouvé’s description of the Lascaux karst), by estimating their heat capacity, and by using these « preliminary » data to test the impact of the geomorphology and geostructure on the simulator. This would rapidly provide indications on whether or not one should expect significant improvements of the Simulator by taking into account the detailed structure and morphology of the karst. And hence of whether or not one should bother to enter into the Simulator the results of the costly and long detailed studies approved by the Scientific Council. Several other factors that are likely to influence the simulations are not taken into account in the hypotheses underlying the calculations of the Simulator: - Partial protection of the ground by the vegetation from direct sun heating. This might be taken into account by introducing the results ultimately obtained from the "Vegetal Cover" program requested by M. Mauriac. - gas exchanges between the inside and outside. It already has been pointed out that one of the hypothesis in the current version of the Simulator is that there is no such gas exchange during a period of 7 hours, which corresponds to the period covered by each simulation. As we indicated in our critical analysis of the simulator, several observations demonstrate that such exchanges DO exist. Furthermore, we recently became aware that the volume of gas pumped out of the cavity to limit the partial pressure of CO2 is about 90 m3/hour. Thus, during the 7 hours covered by the simulation, a total of 630 m3 of air are pumped out of the cavity, and replaced by fresh air coming from either the outside or the underground. This corresponds to more than 20% of the total volume of the cavity. This is certainly not a negligible fraction of the air initialy contained in the cavity. - important climatic perturbations of internal origin that impact the walls. The computation of the "initial conditions" does not take into account the perturbations artificialy introduced inside the cavity, for instance by human activities. - water infiltrations in the karst. The transmission of heat from the external atmosphere to the cavity takes about six months. During that time, rain water penetrates the karst and perturbs heat exchanges between discrete rock regions. Taking this effect into account requires a precise knowledge of the network of cracks and cavities in the rock, and of the corresponding water fluxes and temperatures. These considerations points to the many factors, far from being completely understood, that are likely to influence the transmission of heat from the external atmosphere to the interior of the cave. We therefore doubt that the effects of all these factors could be modeled with an accuracy sufficient to obtain models of the "initial conditions" inside the cavity approaching the precision of simple temperature measurements. From that we conclude that efforts to improve the Simulator should be concentrated on phase 2 of the calculations, i.e. modeling the internal climate on the basis of "experimental initial conditions" determined by use of the elaborate network of temperature probes recently implemented in the cave. The main improvement of phase 2 we can envisage deals with taking into account the gaseous exchanges. Proceeding along this line would ultimately result in a tool that should reliably predict the effect of human interventions in the cave under the real internal climatic conditions that exist in the cave. Purists of modeling might argue that phase 1 of the calculations (modeling the initial conditions) would in principle allow for a long-term prediction of the internal climate of the cavity based on the atmospheric conditions outside six months earlier. But would these predictions be reliable enough ? Would they be more usefull for preventive actions than those, admitedly on a somehow shorter term, obtained by extrapolation of the very smooth temperature curves recorded, rather than modeled, inside the cave ? How long before the critical climatic change inside the cave should preventive actions be taken ? Would decisions made on the climatic conditions observed (not predicted) come too late ? As long as these questions are not clearly answered, we see no practical use for a long term prediction of the internal climate, and therefore on spending time and money on improving phase 1 of the simulations. 2- Should the geomorphology, geostructure and pedology studies be performed? By all means. While initialy proposed for the main purpose of improving the simulator - which we find at this stage at least premature and probably useless - these studies are interesting in the long term for understanding the gas, liquid and nutrient fluxes that generate the environmental equilibrium in which the paintings, the walls and the microbial flora of the cave have to remain stable. We believe that the "critical" areas of the cavity - where infiltrations, vermiculations, microbial growth, ... are most likely to occur are already known and do not require more detailed knowledge of the karst. Yet, completing the earlier studies (Bouvé and coworkers) with a more detailed analysis of the geomorphology, geostucture and pedology of the Lascaux hill should be considered as a long term goal that will undoubtedly provide a better basis for understanding the future climatic, chemical and biological behaviour of the cave. However, in view of the severe financial restrictions imposed to the scientific council, one wonders whether these studies should be given a top priority. 3- Should cold points be introduced in decorated parts of the cavity ? By no means at this point, for the following reasons. - the idea of introducing cold points in decorated parts of the cavity emerged at a time when it was thought that the convection currents in the cave had disappeared as a result of the "global warming" on Lascaux. This interpretation turned out to be completely erroneous and convection currents spontaneously resumed when the cave was simply left at rest (while there is no sign of regression of the global warming ...). - the number, positions and characteristics of the proposed cold points resulted from simulations performed by the Simulator in its initial version, deemed unreliable as discussed earlier (see critical analysis of the simulator) at least at the precision yet used to conclude on the existence or non existence of convection currents. - cold points inside the decorated parts of the cavity would be fed by pipes with cold liquids running in them. With potential condensation on the pipes and possible leaks, this represents an extremely severe risk for the equilibrium of the cave which, in our view, precludes such cold points. 4- Should the pumping out of CO2 be interrupted ? No ! The LIST shares the Scientific Council's view that CO2 pumping out was performed all along the years when the cave was stable. There is no obvious reason why one should take the risk of changing a "winning" procedure ... 5- What protocol for turning on/off the climatic assistance machine ? The LIST is astonished that this question still seems pending. During their presentation at the December 2010 session of the Scientific Council, Paul-Marie Guyon and Pierre Vidal very clearly and explicitely stated the conditions that the machine had to ensure: a humidity at the vault of the entrance to the Bulls' Hall very close to 99 % at the temperature of the walls. The machine was turned on when the humidity rose above 99%. This was seen from the daily measurement of the temperature and humidity and confirmed by the appearance of traces of dew on the visual probes (polished nail heads) scattered in the cavity. It is suggested that Pierre Vidal, who was in charge of deciding when to turn on/off the machine during all the years when the climate was stable, should be officialy consulted by the Scientific Council on this very precise point. |
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