absolute gravity (AG) and water table meas- ... 20â30 nanometers per second squared, within ... A magnitude 7.7 earthquake off the coast of .... of 981,011,438 nanometers per second squared (equivalent to the acceleration due to grav-.
Eos, Vol. 87, No. 30, 25 July 2006
Recent Tsunami Highlights Need for Warning System PAGE 294 A magnitude 7.7 earthquake off the coast of Java, Indonesia, on 17 July triggered a twometer high tsunami that killed at least 339 people, injured 600, and destroyed many structures, according to the U.S. Geological Survey. This destruction highlights the fact that, more than 18 months after the 26 December 2004 Indian Ocean tsunami, the region still lacks a comprehensive warning system. However, by 2008 systems could be operational for Indonesia and the rest of the Indian Ocean region. UNESCO’s Intergovernmental Oceanographic Commission (IOC) began coordinating the formation of the Indian Ocean Tsunami Warning System (IOTWS) in March 2005. IOTWS will be comprised of an international system to detect, identify, and predict earthquakes and tsunamis—the core of which is already in place—and national warning and mitigation systems. Already, 26 national tsunami information centers have been set up in countries in the region, 25 new stations have been added to the regional seismographic network, three
Deep-ocean Assessment and Reporting of Tsunamis (DART) sensors have been deployed, and the Commission of the Nuclear-Test Ban Treaty Organization has begun contributing information to the seismographic network. The national warning and mitigation systems are more difficult to implement because they are the responsibility of the member countries, said Peter Koltermann, head of the Tsunami Coordination Unit for UNESCO/IOC. These national programs “[have] to incorporate many levels of government, and thus people, to get, [as] in the most recent case of Indonesia, the warning from the one focal point to the fisherman at the beach in less than an hour,” he said. The lack of such a system in Indonesia played a role in the destruction on 17 July. The U.S. Pacific Tsunami Warning Center in Hawaii issued a tsunami alert for southern Java, which was transmitted to the Indonesian government, but that government lacked ways to transmit these alerts to the public, according to news reports. Within Indonesia, a joint German and Indonesian project—the German-Indonesian Tsunami Warning System (GITEWS)—is being
SECTION news Karst Aquifer Investigation Using Absolute Gravity PAGE 298 Karst aquifers, formed by the dissolution of carbonate rocks such as limestone or dolomite, supply drinking water to 25% of the global population. Their highly variable sizes and heterogeneous hydrogeological characteristics are difficult to characterize and present challenges for modeling of storage capacities. Little is known about the surface and groundwater interconnection, about the connection between the porous formations and the draining cave and conduits, and about the variability of groundwater volume within the system. Usually, an aquifer is considered as a black box, where water fluxes are monitored as input and output. However, water inflow and outflow are highly variable and cannot be measured directly. A recent project, which began this year, sought to constrain the water budget in a Belgian karst aquifer and to assess the porosity and water dynamics, combining absolute gravity (AG) and water table measurements, the latter giving measurements of pressure head in a conduit. The advantage of gravity measurements is that they integrate all the subsystems in the karst system. This is not the case with traditional geophysical tools like boring or monitoring wells, which
are soundings affected by their near environment and its heterogeneity.
Karst Aquifers and the Rochefort Caves In karst aquifers, the void spaces consist of caves and conduits, smaller fissures, and a porous matrix. From a drainage point of view, they are structured in two types of subsystems: the drains with low retention capacity
and high permeability, and the annex systems with high capacity but weak connectivity to the drain [Mangin, 1994]. The degree of interconnection or the permeability of the different subsystems can vary over several orders of magnitude. Because of the heterogeneity of such aquifers, as expected based on data from speleological investigations, it is not possible to know the spatial distribution of the different subsystems. When the system is flooded, the rise of the water table depends on the characteristics of the subsystems. The drains are distin-
implemented and will be handed over to the Indonesian government in 2008. Six seismological stations already have been deployed, and three global positioning system (GPS) land stations and two tide gauges will be installed in August. Two marine test systems consisting of a GPS buoy and an ocean bottom unit (pressure sensor and ocean bottom seismometer) are being reconfigured after their initial test last year and will be redeployed in November 2006, according to Alexander Rudloff, who is part of the project management team at GeoForschungsZentrum Potsdam, the coordinating agency. GITEWS also will address the problem of disseminating tsunami warnings, according to Rudloff.“This is a long-term process and needs [the] intensive involvement of Indonesian authorities,” he said.“The generic problem of these warning systems for any government is that they are ‘sleeping systems.’ If nothing happens, nobody cares, and one day a treasury [official] will just decide to cut costs. If, however, something happens and it did not perform, or was not in place, governments have a hard time to explain why.” —SARAH ZIELINSKI, Staff Writer
guished from the annex subsystems by very short residence time of water. This dynamic is essentially nonlinear and does not permit reliable predictions. The Rochefort cave results from the meander cutoff system of the Lomme River and belongs to one of the most famous and extensive cave systems in Belgium. The main inputs are swallow holes of the Lomme River crossing the limestone massif. The river is canalized and the karst system is partly disconnected from the hydraulic system (Figure 1). However, in strong flood conditions, the water spills over the dyke and sinks into the most important swallow hole, the Nou Maulin, connecting to the cave network. Therefore, the man-made canalization prevents progressive recharge of the karst conduits, suddenly filled up during flash floods. This induces very fast rises of the water table in the caves, potentially causing fast gravitational effects that can be investigated using precise gravimetric methods. In 2005, two probes for monitoring the water table were installed in the cave, and the absolute gravimeter FG5-202 was installed on the ground surface above the cave, from 20 December 2005 to 20 March 2006.
Absolute Gravity Measurements Ballistic absolute gravimeters are able to detect temporal gravity change of about 20–30 nanometers per second squared, within a few hours. These instruments are quite appropriate to monitor gravitational effects of aquifer storage change. Because of mechanical wear, absolute gravimeters are not well-suited
Eos, Vol. 87, No. 30, 25 July 2006
Fig. 2. February–March 2006. Level of the Lomme River in Rochefort (blue), piezometric surfaces in the caves (green and red), rainfall (purple), and absolute gravity measurements (each set is represented by a black dot). The precision of one set is about 20 nanometers per second, and this includes the experimental standard deviation of the mean of one set and the instrumental setup noise [Van Camp et al., 2005]. Tidal, atmospheric, and polar motion effects were removed. Water level probes were installed 60 meters below the surface. For legibility, the average value of 981,011,438 nanometers per second squared (equivalent to the acceleration due to gravity at the Earth’s surface in the Rochefort Station) was removed. An increase in gravity can be observed when the caves were flooded.
Fig. 1. (a) The Rochefort cave is located under the left slope of the Lomme River, near Rochefort, Belgium, and belongs to the karstic meander cutoff network of the Lomme. (b) The canalized Lomme River. In strong flood conditions, the water spills over the dyke and sinks into the Nou Maulin swallow hole, (c) flooding the cave. for continuous measurements lasting longer than a few days. In Rochefort, as the instrument ran for several weeks, only one gravity set was recorded every four hours. The value of a set is the average of 100 drops of the test mass performed during 17 minutes. Although the instrument was installed for three months, measurements were available sporadically, due to repeated power supply failures in the field laboratory. Fortunately, the instrument was working properly when the Lomme River flooded the Nou Maulin swallow hole in February and March of 2006.
Observations and Preliminary Interpretation On 15 February and 8 March, the Lomme River spilled over its dyke and sank into the Nou Maulin (blue line in Figure 2). This resulted in dramatic and nearly instantaneous increases in the piezometric (pressure head) levels in the cave, reaching up to 13 meters on 9 March (green and red lines). Meanwhile, gravity increased by 50 and 90 nanometers per second squared in February and March, respectively. A first conclusion is that during
these sudden floods, the pores and fine fissures were poorly connected with the enlarged fractures, cave, and conduits. With a rise of 13 meters in the water level and a 5% porosity, a gravity change of 250 nanometers per second squared should have been expected. This moderate gravity variation suggests either a weaker porosity or that only the filling of the cave and conduits was responsible for the observed gravitational effect. In February, gravity change was later than water rise, while it was synchronous in March. This could be due to different saturation conditions in the capacitive porous subsystem and the epikarst (weathered shallow bedrock) one, or due to the flow dynamics between the two subsystems or inside the same subsystem.
Future Work When more data are available, it will be possible to investigate recharge and discharge processes as a function of the degree of saturation in the karst. The use of artificial tracers, as well as monitoring discharge and natural tracers at the outlet of the whole system (located at the spring of Eprave, four kilometers away), also are being considered to assess the dynamics. In particular, the influence of soil moisture variation in the epikarst, as well as the vertical heterogeneity of the saturated zone when the water table is close to the probes, also is questionable. New gravity measurements, possibly using two absolute gravimeters to better investigate flow dynam-
ics, are foreseen during winter 2007. This study also will benefit from measurements of environmental probes and extensometers installed in the cave to monitor deformations induced by tectonics and hydrology.
Acknowledgments We thank the ASBL (association sans but lucratif; non-profit organization) Grotte de Lorette–Rochefort for logistical support; S. Castelein, G. Evrard, and Clément for their assistance; the Ministry of the Walloon Region (Belgium), Water Division, for providing surface stream levels; and European Geophysical Services for assistance in installing level probes.
References Mangin, A. (1994), Karst hydrogeology, in Groundwater Ecology, edited by J. Gilbert et al., pp. 43–67, Elsevier, New York. Van Camp, M., S. D. P. Williams, and O. Francis (2005), Uncertainty of absolute gravity measurements, J. Geophys. Res., 110, B05406, doi:10.1029/2004JB003497.
—M ICHEL VAN CAMP, Royal Observatory of Belgium, Brussels; E-mail: mvc@oma.be; PHILIPPE MEUS, DGRNE, Division de l’Eau, Direction des Eaux souterraines, Jambes, Belgium; YVES QUINIF and OLIVIER KAUFMANN, Faculté Polytechnique de Mons, Belgium; MICHEL VAN RUYMBEKE, Royal Observatory of Belgium; MARC VANDIEPENBEECK, Royal Meteorological Institute, Brussels; and THIERRY CAMELBEECK, Royal Observatory of Belgium.
