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New sediment layers accumulating on the floors of rivers, canals, lakes, and harbours in industrialised countries contain varying amounts of contarninants, which have been discharged from a range of activities, both legal and illegal. These contaminated sediments are known collectively as sludge and have been accumulating for decades. The identification of contamination in waterways has led to the realisation that there is a need to carry out surveys of the pollution, in many cases followed by remediation. Contaminated water floors, though probably created in the past, continue to disperse their pollutants in the water, thus creating a potential effect on the quality of drinking water and recreational water. Remediation work can only be carried out in a cost effective way if it is preceded by a site survey, which provides high resolution data. 95% of current site surveys are done by collecting hand drilled cores for examination and analysis. These point sources of data are then used to try to give an impression of the sludge layers, but the resolution is extremely poor. Contamination may be found, but the volumes cannot be exactly quantified.

During the operation of a refrigerator factory in the 50’s, 60’s and 70’s drummed hazardous wastes were dumped and covered with soil illegally in sand pits located near to the factory. About 20 years later the during an overall environmental remediation project these illegal waste disposal sites were excavated and restored. The locations of these dumping sites were determined by aerial photographs, and using local people recalls. As work progressed new reports were made to local government and factory management about waste dumping sites, all of which were examined by means of excavating, visual inspection, waste (if found), soil, and sometimes groundwater sampling. Originally 7 waste disposal sites were assigned for examination, the final number of the examined sites was fourteen. No. 14 site (WDS-14) was reported by an excavator operator of a sand mine, who actually was ordered to dig a trench, later in wich he witnessed dumped drums. Because of subsequent mining operations, and land use change the operator could not identify the location of a reportedly 40 x 2 meters trench, only approximately within a 100 x 100 meter area. The site was first excavated at several locations based upon the operator’s memory, but nothing was found, not even soil disturbance. As the operator insisted on the existence of the dump a geomagnetic survey was chosen to scan the area for geomagnetic anomalies, which could indicate burried metal objects. The site was prepared by creating a 20 x 20 m grid oriented to magnetic north, and handed over to the geomegnetic surveyors. Using fluxgate gradiometer one considerable anomaly and several disturbances were found within the area of interest. However, the proof of the existece of buried drums demanded systematic survey. A 120 x 120 meter area was systematically surveyed with a proton procession magnetometer. Measurements were taken at a 1 x 1 m grid, which means 12000 measurement points. The sensitivity of the instrument was 0,1 nT, and the probe was held at a 0,7 m level from ground surface. Data processing started with diurnal correction. This was followed by noise reduction as such reduction to the pole (resulted the elimination of anomalies that had permanent magnetic effect, since it could be presumed, that the burried objects have only induced magnetization). Geological effects (low frequency noises, e.g. facies changes, clay lenses etc.) were removed by band pass filtering and high frequency noises were compensated by optimum smoothing filtering. The picture of the burried objects was sharpened by downward continuation, and the result was visualized as a colour picture.