High resolution geophysics provides optimal results on inland waterways

Tamás Tóth of Hungarian survey company Geomega provides some convincing evidence for why a combination of geophysical technologies will provide the best result when surveying shallow water sites for environmental and engineering purposes. Detailed modelling of the shallow subsurface is often a must for engineering and environmental studies. High-resolution geophysics combined with shallow boreholes can provide a cost effective solution for these investigations. In many cases a significant step forward can be achieved simply by extending the 1D information provided by the boreholes into 2D, or still better, into 3D using geophysical techniques. Obviously this is a critical step at locations where the subsurface shows high variability in vertical and/or horizontal directions. We are all aware that high variability is often characteristic of the near surface region, and mapping the shallow subsurface in 3D using geophysical techniques can be a challenging task. This is especially true for those areas which are covered by water. Natural or artificial waterways, lakes, rivers and canals often present an obstacle for mapping simply due to the difficulties accessing the area. A lucky contra version is that, in many cases, performing a geophysical survey on water is simpler, quicker and therefore cheaper than a similar survey on land. Sometimes the resolution of the survey performed on water is actually superior to a land survey. This is typically the case for seismic surveys for example. Surveying water covered areas presents difficulties, but some of these can be overcome by applying new technologies. A good example is positioning, which used to be a challenging task on water a few decades ago, but became relatively simple nowadays using high precision GPS (DGPS and RTK GPS) technologies. Accuracy of these positioning techniques became adequate for high-resolution geophysical surveys, in the process making even detailed 3D surveys on waterways feasible. Every geophysical technique has its limitations and there is no ‘magic tool’ in the hands of geophysicists. Seismic, electric, electromagnetic or magnetic techniques can be applied separately or combined depending on the actual problem. We are, however, convinced that in many cases application and joint evaluation of more than one geophysical technique can provide an added value. An example for this is the combined application of seismics and GPR which enables a direct calculation of physical parameters like density, acoustic and electromagnetic velocities using the geophysical data only. In this paper examples of water borne geophysical surveys will be presented with special emphasis on the importance of 3D imaging. An example of combined seismic and GPR surveys with an environmental focus will be discussed as well.