**4. Conclusions**

As the chamber is filled with water, the conductive anomaly should correspond to the mining chamber and, in fact, the proposed lateral boundaries match the expected limits of the dashed line in **Figure 6**. The higher resistivity values recorded away from the conductive anomaly

Because of the orientation used, modelled resistivity values in CD do not correspond to the transverse resistivity of the schists. Therefore, it is not possible to calculate the anisotropy coefficient, but an indication of field anisotropy values can still be computed by calculating

Thus, a further section was constructed depicting the square root of the ratio between the

**Figure 8** shows ratio values varying from 1 to more than 3.4. Thus, although this is not the

The high resistive shallow layer, above the groundwater level, should correspond to ratio values close to 1. In this case, resistivity values modelled for AB and CD refer to a 'dry' or non-saturated area and, thus, should be similar. Hence, the dashed red line in **Figure 8** must correspond to the groundwater level as shown in **Figure 7** and, above this boundary, ratio values vary from 1 to 1.4. These values are considered to correspond to a low anisotropic

The central area of the section displays resistivity ratio values near 1 (or less than 1). This area corresponds to the position of the chamber depicted in the right of **Figure 7**. As the chamber is filled with water, resistivity values modelled for AB and CD ERTs should be similar as they

It must be noted that ratio values less than 1 are depicted in **Figure 8**. This behaviour has been registered where structural anisotropy prevails [13, 17], such as in the vicinity of interfaces separating media with high resistivity contrast. In these circumstances, there is a rotation of the anisotropy axis and the so-called oblate anisotropy is observed [13]. This should be the

**Figure 8.** Ratio between modelled resistivities in Valongo; dashed line, proposed cavity limits.

refer to the schists' resistivity as measured with this ERT orientation.

anisotropy coefficient, field conditions are highly anisotropic.

correspond to data in the conductive chamber region.

resistivities modelled in the two ERTs.

medium [13, 17].

124 Cave Investigation

the square root of the ratio between the modelled resistivities for CD and AB.

Geophysical methods are a powerful tool for cavity location. They provide fast, economic, automated, non-invasive techniques that offer relevant information to restrict areas of interest and, thus, to guide more expensive direct exploration methods. As non-invasive methods, they do not require excavation or drilling and thus can be adapted to operate in urban areas.

These methods are indirect techniques as they measure the difference in physical properties between the cavities and surrounding media and not the properties of the cavities themselves. Fortunately, most of the times, there is a contrast between the physical properties of the cavities and those of the surrounding rocks. However, the size and depth of the cavities can be a limiting factor for the use of geophysical methods.

The complexity of the geology formations where the cavities are installed can be another limitation factor as demonstrated in this case study. Therefore, orientational effects and formation anisotropy can mask cavity response inducing ambiguity and uncertainty in the interpretation. In this case, field survey design must be carefully planned to overcome or reduce orientational effects in the final interpretation, to avoid misleading interpretations and the use of alternative non-linear arrays must be considered.
