Time-Bombs

Of a Different Type

One would not think it looking at the juicy green meadow. But beneath this a time-bomb is ticking away. 80 years ago a limestone quarry was shut down here. Then, 50 years ago, the old quarry suffered the fate that befell many other such places at that time, namely that it was filled up over many years with refuse, namely domestic refuse, bulky refuse, industrial wastes, construction rubble and somewhere along the line highly poisonous industrial sludges.

The time-bomb: The geological frame, namely the limestone,  is fissured and karstified and not very far away there are drinking water extraction works. The authorities know about the danger and started investigations a long time ago at the end of which the steps for remediation will be planned. Bore holes have been sunk but on this large area these are no more than just pinpricks. Where e xactly are the edges of the landfill, down to what depth is there waste, does particular material lie in  particular parts of the landfill body, has contaminated water already started to infiltrate into the limestone?

The environmental offices are well aware of plenty of such cases. For two or three decades it has become common practice to deal with historic waste problems with the aid of geophysical measurements. Above all modern digital measurement techniques and miniaturization of the equipment have opened up completely new areas of application in the field of geophysics, a development that has been largely ignored in the field of the locating of bombs, mines and other unexploded military ordnance. However EBINGER, a firm which readers of this magazine will know much rather as a producer of „sophisticated equipment“ for locating explosive ordnance, has not closed its mind to this development. This is appropriate particularly since there are many areas of overlapping in these two areas of reconnaissance/ investigation with very often the same geophysical measuring systems being used. Here we permit the reader to cast a look at these very interesting links as we show – using as example the afore-mentioned historic landfill – how information can be obtained today with the aid of the most modern measuring equipment on the substrate in the area of historic wastes.

Indispensable in most cases with historic waste investigations are passive measurements of the magnetic field (Fig. 1) as well as active induction methods with the aid of frequency electromagnetism (FEM Fig. 2) and transient (pulsed) electromagnetism techniques (TEM, Fig. 3). A special feature with the measurements with magnetism and pulsed electromagnetism is the fact that both data fields are identically structured when the measurements are appropriately recorded and can be evaluated synoptically directly with one another on the computer. In this way and due to the fact that the two methods react to different parameters differentiation can be carried out more easily between the responses from different materials and different objects (ferrous metals, non-ferrous metals, magnetic rock, ceramics, industrial sludges etc.).

Measurements of the magnetic field react in a clarifying manner to the ferrous metal components which almost always are to be found in domestic refuse, bulky refuse and construction rubble and hence in landfills. This means that the extent and lateral limits of a landfill can generally be traced in the image from a magnetic field survey. (Fig. 4). It can be seen that the landfill under consideration here is no exception and that it – following special processing of the data – stands out clearly from the normal geological framework. Low-pass filtering is used here as a type of smoothing process for the anomalies field, which is magnetically very disturbed. On the other hand the field of the horizontal gradients as calculated describes the maximum horizontal measured value change at each grid point whereby this can be regarded as a form of high-pass filter. Above all in the fields that have been processed one can recognize the not-unexpected inhomogeneous structure of the landfill.

The active electromagnetic methods also react to ferrous metal objects in the body of the landfill. Here the electrical conductivity of the material is the parameter which makes possible the formation of an eddy current. For this reason electromagnetic methods – in contrast to magnetic field measurement methods – are also able to „see“ non-ferrous metal objects. This is an advantage that is also becoming recognized to an increasing extent in the field of explosive ordnance disposal when the problem representing serious danger is that of the so-called „non-magnetic“ bombs (or better: bombs the magnetism of which cannot be measured). Such situations can occur when – as a result of magnetic rock, stronger than usual remanent magnetism and/or the fact that an unexploded ordnance device is located unfavourably – the latter cannot be detected by purely magnetic detection methods so that active electromagnetic probing represents an appropriate alternative.

These interrelationships are also made clear taking as an example again the survey of the landfill. Fig. 5 shows on the left the result of the frequency electromagnetism survey with the twin coil frequency electromagnetism device shown in Fig. 2 over a central part of the landfill and – on the right – the data from the magnetic field survey of the same area. It is easy to see that

numerous FEM-detected anomalies correspond with magnetically-detected anomalies and thus are likely to represent ferrous metal bodies. However there are also significant differences and the electromagnetic signature is overall clearer than the magnetic one. This results from the very different measurement parameters (magnetization and electrical conductivity) and could be interpreted in the sense of differentiation between materials.

The perhaps most impressive images from the geophysical survey of the landfill are those obtained with the transient or pulsed electromagnetic method (TEM, Fig. 6). However this method is also the most expensive one in terms of equipment. For better understanding but expressed in very simplified terms one can say that the electromagnetic signal transmitted with the pulses wanders downwards (and also laterally) with time so that it gives its „answer“ in the form of a signal to the reception coil in a continuous manner. Thus in a certain way the substrate is imaged in respect of a time period of the order of microseconds and milliseconds. The measurements at two different distances above the surface of the ground mean that surface factors of influence and deeper areas can be addressed and the differentiation between the signals from the upper and lower loops (similar to the differentiation with the Fluxgate gradiometers) comes close to the so-called vertical gradient which is characterized by a higher resolution. The selection shown here for just three channels (Fig. 6) makes very clear how much the field structure changes in successive time frames.

Time-bombs: The problem of historic wastes, that is becoming ever more urgent, forced geophysicists 20 – 30 years ago to adjust their measurement systems, which up to this time had been used primarily to look down to much greater depths, to the new situations requiring investigation. Explosive ordnance disposal methods have followed this trend only in a very dilatory manner or not at all. This article is intended to indicate that different procedures are also possible.

Author: Prof. Dr. Dr. habil. Kord Ernstson

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