3rd EEGS Meeting

Increasing problems with water quality have made ground water a very important issue in Denmark. In 1995 the Minister of Environment decided a 10-subjects scheme to improve ground water protection. One of the major issues is that all pesticides that can pollute the ground water should be forbidden. Another major issue is that counties in Denmark by the end of 1997 should be classified into 3 types of recharge areas. The classification includes: - Recharge areas of great drinking water interest, - Recharge areas of drinking water interest and - Recharge areas of limited drinking water interest The clasciffication is based on a comprehensive evaluation of the size and quality of the ground water resource. The new ground water resource plan provides guide-lines for the protection of the ground water in the county.

Comprehensive wireline logging investigation is necessary for providing the information needed for the evaluation of the possibility for relief of a deterioration in the quality of the water from a well, and for the choice and design of a remedial solution. Two different techniques are available as remedial solutions both aiming to improve the quality of the water produced from the well. One is a reconstruction of the well in order to close off the inflow of water with unwanted quality.

The tedious and repetative nature of groundwater tracer experiment monitoring using geophysics can result in an incomplete dataset for subsequent interpretation. A fundamental cause is due to the inherent unpredictability of the rate and direction of movement of, and initial geometry of the tracer slug after injection. This problem is inherent to the experiment since if these parameters were accurately known, there would be no need to perform the tracer study. The incompleteness of the geophysical dataset is also directly affected by the balance between collecting enough data to solve the flow problem, and not wanting to collect redundant data due to the high cost of field data acquisition. This paper presents the results of interpreting incomplete geophysical datasets which were obtained to monitor the migration of a conductive saline groundwater tracer through a deltaic sand aquifer in Gray, Maine. In addition to providing a non-intrusive means for monitoring the temporal movement of the conductive tracer, interpretation of these geophysical data has also yielded a more detailed picture of the geologie and hydrogeologic setting explaining observed anisotropy in flow.

In connection with construction of ground water models an extensive geophysical survey of the area is often made. In Denmark the model area is usually mapped by means of dense geoelectrical surface measurements. The geophysical surface measurements give an area coverage and are relatively inexpensive.

Environmental applications of the induced polarization (IP) and the spectral induced polarization (spectral IP or SIP) methods, i.e., the possibility to detect organic and inorganic contaminants in glacial soils, has been studied the Geological Survey of Finland (GTK) for some years. This presentation deals with the IP of natural and contaminated soils (the membrane polarization), and gives two field examples; an area with an organic (hydrocarbon) contamination (Fig. 1), and an area with an inorganic (heavy metal) contarnination.

The nature of coal tar deposits presents considerable difficulties for traditional site investigation metliods such as boreholes and cone penelrometer tests. These intrusive methods normally require access by heavy vehicles and would be at risk on a tar surface. However, the low viscosity of tar (in winter) means site characterisation may be undertaken by a geophysicist using lightweight instrumentation.

A 3D multi-fold GPR investigation was carried out in the archaeological park of Aquileia in the framework of a survey proposed by the local superintendency of cultural heritage. Primary objective of the work was the identification of targets of potential archaeological interest in a polygon of approximately 2800 sqm in area, which borders the town cemetery to the north. The comparison of single-fold and multi-fold test profiles encouraged the design of a complete multi-fold grid. The 3-D option was chosen to correlate and map targets of limited dimensions. Targets of possible archaeological interest were identified at estimated depths not exceeding 200 cm in the part of the dataset processed to date.

Realistic modeling of electromagnetic wave propagation in the radar frequency band requires a full solution of Maxwell's equations as well as a complete description of the material properties. We present a two-dimensional (2-D) flnite-difference time-domain solution of Maxwell's equations that allows to account for the frequency dependence of the dielectric permittivity and electrical conductivity typical of many near-surface materials. In close analogy with the viscoacoustic case, the governing equations are obtained by assuming dielectric and conducting relaxation functions. The finite-difference solution is second-order accurate in time and fourth-order accurate in space, conditionally stable, and computationally only marginally more expensive than its standard equivalent without frequency-dependent material properties. The algorithm is applied to a realistic problem illustrating the respective damping effects of conductivity and frequency-dependent dielectricity.

The Borth salt deposit (FRG) lies in the Zechstein basin, which covers a large part of Central Europe. The mine is located north-east of Duisburg. Although the deposit is over 150 metres thick, only the lower part is mined here. Mining activity (app. 800 metres below surface) is restricted to a 20 m thick layer of extremely pure white salt.

BRGM and the french Projet National Microtunnel funded a research project where Ground Penetrating Radar (GPR) was used to characterize the test-site of the Laboratoire Central des Ponts et Chaussées (France). The objective of this project was to find a method to better interpret the radar signatures by combining data processing to forward modeling, especially in the case of urban heterogeneities. We focused a large part of our work to adapt a sufficient quick and accurate algorithm to be efficient in industrial applications. Modeling algorithm, adapted from radar waves propagation theory in 2D heterogeneous dispersive media, is based on the upward extrapolation of a downgoing field in the frequency-wavenumber domain. Like for other modeling techniques such as ray-tracing (Goodman, 1994; Cai and McMechan, 1995; Powers, 1995), FDTD (Moghaddams et aL, 1991; Roberts, 1994) or pseudospectral methods (Carcione, 1996; Casper and Kung, 1996) some compromises have been introduced and must be kept in mind. In the following, we will first recall the modeling method, and after an example description from the LCPC test site, where real processed data are compared to synthetics, we will insist on the advantages and the weakness of the method.