2D inversion of Radiomagnetotelluric data for mapping waste disposal sites, an example from The Netherlands



The Radio Magnetotelluric (RMT) method is one of the most widely used electromagnetic (EM) methods which employs artificial time varying electric and magnetic fields at the surface of the earth for imaging conductivity. The Radiomagnetotelluric method proposed by Goldstein and Strangway (1975) is based on measuring one set of horizontal electrical and perpendicular magnetic components. After that, many authors for example Sandberg and Hohmann (1982), Bartel and Jacobson (1987), Hughes and Carlson (1987), have studied this method in detail. The electromagnetic signals are emitted by powerful transmitters. The RMT method has a very broad spectrum that is defined here in the range al 10-250 kHz. With regard to this high frequency band, exploration depth is low and is used in shallow engineering studies. In Iran, the application of geophysical methods for the study of waste sites has become increasingly important because of industrial development.
In Autumn 1998, the RMT study was carried out to recognize the pollution zone of a waste site in Collendoorn, in Netherlands. The measurements in the Netherlands are selected to show the RMT application of the EnviroMT system. Collendoorn is a small town situated in the north of the Overijssel province, in the Netherlands. Close to the town, in the middle of a flat area, covered by marine Pliocene sediments, lies the former waste disposal site that was used as a public dumping ground from 1949. Waste disposal dumps occurred in (wet) pits that had been dug for winning sand. In 1960 the site was declared as an official waste disposal site and from then the waste was dumped in dry areas of the site. The waste disposal has been discontinued since 1988 and the area has been used for recreational purposes. Pollution has been detected in the area near the waste disposal site and the leakage of polluted water has moved to regions outside the site. Samples taken from bore-holes in the area show that the polluted water contains iron and chloride ions causing the pollution plume to be electrically highly conductive.
A very long geophysical profile taken by the Netherlands Institute of Applied Geoscience (TNO) shows that the Tegelen formation which mainly consists of clay with a thickness of 2-7 m lies at a depth of approximately 35 m below the surface in the vicinity of Collendoorn. The formations above Tegelen are alternating sand and clay beds. Models of the RMT data across profiles, considerably express conductivity of the layer at 25-30 meters and with regards the data of bore-hole, this layer is estimated as a polluted layer.
The main geophysical objective was to detect and map the vertical and lateral extensions of the pollution plume. Based on the information provided by TNO four RMT survey lines situated at the eastern part of the dumpsite were planned for the survey. Lines 1, 2, and 3 were west-eastward and line 4 was south-northward.
Each line contains stations with 10 m spacing. We already know that the Collendoorn RMT data has a one-dimensional character, meaning that a 1D interpretation may work quite satisfactorily. The EnviroMT database software delivers two 1D inversion routines for on line data interpretation. The Least Singular Values Inversion (LSVI) program developed by Pedersen (1999) was employed for the interpretation of the Collendoorn data. The results of the inversion are presented as resistivity-depth sections. Each section shows a compilation of independent models at stations along a survey line. In order to show the similarities and differences in both directions of the induced currents, namely XY and YX directions, the results are represented in separate sections along a survey line. Resistivity-depth sections of the determinant data are also illustrated for comparison with the other two.
The first test field campaign, carried out in the Collendoorn dumpsite in the Netherlands revealed that the EnviroMT system operates satisfactorily. In spite of some minor hardware problems caused by heavy rainfall, the hardware functionality of the system is stable. The hardware-software and software-software interfacing work and the measured RMT data are correctly processed and properly stored. The data is reliable in that the estimated resistivities correlate with the true values directly measured in the bore-holes close the survey lines, indicating that the system is properly calibrated. The RMT data contains sufficient information to resolve four layers in the upper 25 m with a resistiveconductive- resistive-conductive sequence. The resistivity-depth sections from 1D inversion of the RMT data in the Collendoorn depicted the vertical boundaries and lateral extensions of the pollution plume and indicated that the pollution plume is extended more in the north at the eastern parts of the dumpsite. The resistivity of the pollution plume is so low that the RMT responses become insensitive to the conductivity variations below 35 meters.