GeoPASS: a decision support system for selecting the optimal geophysical technique for shallow surveys

Abstract

In this paper we present a decision support system for selecting the optimal geophysical technique to detect shallow land targets (< 80 m-bs, i.e. less than 80 m below the surface). The system is freely available on the Internet. The aim of the decision support system is to stimulate geophysicists and end-users to assess the applicability of geophysical techniques prior to a survey in order to increase the chance of a satisfactory outcome. In the long term this will hopefully lead to enhanced recognition of the value of geophysics. The use of geophysical techniques for investigating environmental problems is less then it could be. In our view an important reason for this is that, in the Netherlands at least, geophysics in quite a number of cases does not supply the information expected by the end-user. The information is often vaguer or more ambiguous than environmental managers are used to, and/or the performance of the geophysical technique is poor due to the characteristics of the site and/or soil. The poor performance may also be caused by the application of a less suitable technique or the use of a non-optimal field-design. Applying geophysical methods without satisfactory results for end-users is, in the long term, bad for geophysics in general. Often these unsatisfactory results are due to a lack of assessment of the applicability of the geophysical techniques at specific sites prior to the survey. Such assessments hardly cost any time, especially when site characteristics are known and are backed by a geophysical decision support system. As geophysicists, we think that geophysical measurements can, if used properly, become more and more important for subsurface investigations for environmental and infrastructure projects. The well-known advantages of geophysical techniques are that a lateral continuous image of the subsurface is obtained and that geophysics is generally less destructive for the subsoil than, for example, drilling. To overcome the problem of applying non-optimal techniques, a decision support system was designed within the framework of the European HYGEIA Project (Hybrid Geophysical technology for the Evaluation of Insidious contaminated Areas). The system is called GeoPASS. In the past, several organizations developed tools for selecting a geophysical technique for a specific problem. Crosstables indicating which technique(s) could be applied to certain problems have been prepared by several organizations and companies. These tables are mostly general and do not indicate whether the technique will be successful for a specific site. A good example can be found at the website of the Swiss Geophysical Commission (www.aug.geophys.ethz.ch/gpmapping). One other geophysical decision support system asking the user more detailed information about the site characteristics is known to the authors. This system, called GAES, was specifically designed for detecting pollutants (Geophysics Advisor Expert System, Gary R. Aloft, USGS Open-File Report 92- 526). However, the system is not very user-friendly, rather old (1992), and the outcome is a set of rather general remarks about the applicability of geophysical techniques. GeoPASS is designed to provide the user with an estimate of the chance of success of several geophysical techniques. The target may be either a pollutant or a relevant subsurface feature (e.g. sealing clay layer, fracture). The outcome of GeoPASS may even be that none of the assessed geophysical techniques can detect the target given the site conditions. The aims of GeoPASS are to: ■ Provide an estimate of the success chance of geophysical techniques at a specific site and therefore avoid the application of geophysical techniques that are not appropriate ■ Make the end-user aware of the chance of success expected of the geophysical technique(s) to be applied and the factors that influence that chance ■ Give an initial estimation of the costs of a geophysical survey on the site GeoPASS also serves as a checklist. If no information about the site and target parameters is available, it is difficult to predict whether a geophysical technique is capable of supplying the required results. This situation is prone to mis-use by some enthusiastic geophysicists and geophysical companies, often resulting in disappointed end-users. Hopefully the use of GeoPASS will lead to more 'happy faces' among the end-users of shallow geophysical techniques and therefore enhance recognition of the value of geophysical techniques in general.