Artificial sources of interference

Artificial sources that clearly affect the EMR measurements are power lines (irrespective of their voltage), communication lines and television cables, water-, and gas pipelines. Their disturbing signals do normally not reach further than 500 m (high voltage power lines), or only few metres (water pipes), and within these distances, it is, not possible to obtain correct geological results.

Fig. 2.19: Typical pattern of linear EMR measurement while passing a power line (near Västervik, Sweden).

The location of the power line (grey arrow) is marked by an increase of intensity relative to the background noise. A similar pattern is produced by gas- and water pipes.

The influence of artificial sources of interference manifests in different ways for the different measuring methods. For linear measurements, the effect of artificial sources is in most cases easy to detect, because typical symmetric patterns occur when crossing such sources (Fig. 2.19). This pattern is characterised by a dramatical increase in intensity towards e.g. a power line or water pipe. Typically, directly above or below a power line (or above water- and gas pipes), a significant decrease in intensity occurs. For linear measurements, this symmetric pattern of signals is a clear evidence of an artificial origin. Walking parallel to power or communication lines results in unusually high intensities that vary less than those typical for presumably geological EMR emissions and form a high, plateau-like pattern.

Furthermore, the intensity of the artificial signals is often significantly higher than the intensity produced by natural sources, such as faults. While natural sources induce intensity increases that commonly exceed the background noise by a factor of 1.5 to 2, the effect of power lines easily reaches a factor 4 and higher. The effect of water- and gas pipes depends on the size of the pipe, but it is generally not as high as the effect of power lines.

The disturbance caused by power lines, water pipes etc. requires careful planning of the location of measurements. While overhead power lines can be easily avoided, underground power lines or telecommunication cables are often located directly under paths or roads and are often only detected during EMR measurements. For this reason, pathways are often not usable to carry out linear measurements.

Other observed disturbances of linear EMR measurements are characterised by distinct changes in intensity when changing the measuring location. This effect exceeds the typical

Fig. 2.20: Two examples of horizontal measurements. A) The contour of the plot is smooth; the intensities show two symmetric peaks with a NE-SW strike.

B) Disturbed horizontal measurement, the contour of the intensity distribution is rougher, and the peaks are asymmetric and point into different directions.

fluctuation of intensities between two successive measurements by about 30%. These intensity changes generally occur in the vicinity of power lines at a distance of a few hundred metres. A possible explanation may be that the human body (operator) that consists of around 90% of water, acts as a lens for electromagnetic waves; therefore, the arrangement of a power line, the human body, and the antenna may influence the density of the electromagnetic field, and therefore the measurements.

During horizontal measurements, artificial interferences are not as easy to recognise as during linear measurements. Main evidence for an uninfluenced measurement is the geometry and the smoothness of measured signals. Typical unaffected intensity diagrams show a nearly mirror symmetric pattern in both directions of maximum intensity (Fig. 2.20).

Another indication for artificial disturbances is the occurrence of significant deviations of the main EMR direction over short distances.

Subsurface gas- and water pipes only seem to have an effect on measurement directly above the pipes. This could be observed during the field studies in the Västervik area in southeast Sweden (see Fig. 2.21). At one point, the measured main radiation direction changed locally (within metres) by about 80°. In this case, neither the symmetry nor the smoothness of plotted measurement showed any irregular pattern; only the intensities were unusually high. By checking the area with a linear measurement, the location of an underground gas pipe could be detected exactly where the horizontal measurement had been made.

Fig. 2.21: Effect of a gas pipe on horizontal EMR measurements, example from the Västervik area, Sweden.

The location of the gas pipe was detected using a linear measurement. Even with a lower amplification, the maximum intensities exceed the usual natural EMR values. The main horizontal EMR direction, measured directly above the gas pipe, coincides with the strike of the gas pipe. Areas shaded in grey show location of sources of disturbance.

In general, horizontal EMR measurements seem to be mainly insensitive to artificial disturbances, as measurements carried out in the direct vicinity of buildings, towns, or main roads showed no dependence on the distance to the object. However, I recommend not to carry out measurements in the direct vicinity of towns or other man-made facilities.

Furthermore, I recommend to turn off electronic devices like mobile phones and cameras during measurements, because the effect of the illumination of LCD-displays and of the actuators from lenses can be significant.

In addition, for all types of EMR measurements carried out with the Cerescope, it is essential to be aware that by using higher amplifications, it becomes more important to keep a large distance from sources of disturbances and that the fluctuations during measurements can increase significantly.

Other artificial sources of interference like transmission masts (e.g. for mobile communication) do not seem to have an effect on the measurements. This is possibly the result of the high frequency spectrum (normally at least several MHz) that is not recognised by the Cerescope. As transmission masts act as a point source and if the transmissions would

lie within the measuring range of the Cerescope, they would be easily identifiable by a radial pattern of measured main EMR directions around them.

Several powerful military and civil VLF transmitters work in the used frequency range and are by their nature receivable everywhere. The signal of the VLF transmitters manifests on the display of the Cerescope as a distinct narrow peak. As these transmitters turned out to be an important interference factor, influences on the EMR method are discussed in a separate chapter (see Chapter 5).