**3. Cavity detection in anisotropic regions: a field example**

Slates mining in Valongo (41<sup>o</sup> 11'N, 8o 30'W, 30 km North of Oporto, North Portugal) started in the nineteenth century. Since then many underground works were abandoned and no records are left about their whereabouts and extension. Slates of economic value occur in steeply dipping Ordovician schists where strong orientation effects of physical properties and anisotropy occur.

Increasing urban development pressure requires information about the position and extension of mining chambers but, documents about older works are scarce and field interventions demand the use of fast and non-invasive methods that is geophysics.

Electrical tomography techniques (ERTs) using the Wenner-Schlumberger array were used to investigate the dimensions of an old mining chamber. Today, the only evidence of the chamber at the surface is the shaft and the possible lateral limits of the underground works, depicted by the dashed line in **Figure 6**.

**Figure 6.** Mine chamber and ERT location in Valongo.

**Figure 6** also shows the mine entrance (central gray rectangle), the position of two ERTs, AB and CD, and the geological strike. Inspection of the shaft revealed that the mine is flooded and the groundwater level is very close to the surface. Thus, a large contrast between the cavity (conductive) and the surrounding rock (resistive) is expected. So a conductive anomaly is expected.

In the field, resistivity values can vary largely with the orientation θ of the line of electrodes,

The influence of anisotropy on resistivity measurements has been investigated [14, 15] and, herein, a case study on the location of old mining cavities in anisotropic media will be pre-

the nineteenth century. Since then many underground works were abandoned and no records are left about their whereabouts and extension. Slates of economic value occur in steeply dipping Ordovician schists where strong orientation effects of physical properties and anisot-

Increasing urban development pressure requires information about the position and extension of mining chambers but, documents about older works are scarce and field interventions

Electrical tomography techniques (ERTs) using the Wenner-Schlumberger array were used to investigate the dimensions of an old mining chamber. Today, the only evidence of the chamber at the surface is the shaft and the possible lateral limits of the underground works,

30'W, 30 km North of Oporto, North Portugal) started in

**Figure 5**, and the coefficient of anisotropy can reach values larger than 2 [9, 13].

**3. Cavity detection in anisotropic regions: a field example**

11'N, 8o

demand the use of fast and non-invasive methods that is geophysics.

sented [16].

122 Cave Investigation

ropy occur.

Slates mining in Valongo (41<sup>o</sup>

depicted by the dashed line in **Figure 6**.

**Figure 6.** Mine chamber and ERT location in Valongo.

Because of access difficulties, a first ERT, AB in **Figure 6**, was carried out parallel to the geological strike and, in these conditions, values corresponding to *ρ*<sup>l</sup> should have been measured. Data were inverted using the software RES2Dinv [12], and the model is shown in the left of **Figure 7**.

The model shows a resistive layer overlaying a conductive formation. The boundary, shown by the dashed line in the left of **Figure 7**, is at the same depth of the groundwater in the chamber entrance.

As the ERT orientation is parallel to the strike, the modelled resistivity should correspond to the longitudinal resistivity, *ρ*<sup>l</sup> . Therefore, longitudinal resistivity values must be higher above the groundwater level. However, they must decrease sharply below that level as a response to the water in the chamber and in the schists' foliation. Hence, this boundary is interpreted as the groundwater level.

Below this boundary, no more relevant information is obtained from the ERT model as only a conductive medium is shown.

Thus, tomography AB does not give evidence of the chamber and, if only this ERT was carried out, the location of the chamber would have been completely missed.

Therefore, it was decided to conduct a second ERT, CD in **Figure 6**. Because of access difficulties, CD orientation was at approximate 45° to the geological strike. The same field technique was used and model results are depicted in the right of **Figure 7**. In this case, a conductive body, bounded by the dashed lines, is clearly shown and stands out from the resistive surrounding rocks.

At shallow depths, a resistive layer is depicted with the same thickness in both ERTs. Bearing in mind the previous discussion, this layer corresponds to the resistive ground above the groundwater level.

**Figure 7.** ERT models in Valongo: left, AB (dashed line, groundwater level); right, CD (dashed line, interpreted cavity dimensions).

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

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

Thus, a further section was constructed depicting the square root of the ratio between the resistivities modelled in the two ERTs.

**Figure 8** shows ratio values varying from 1 to more than 3.4. Thus, although this is not the anisotropy coefficient, field conditions are highly anisotropic.

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 medium [13, 17].

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 correspond to data in the conductive chamber region.

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.

case in the field example as the contact between the conductive chamber and the highly resistive surrounding rock corresponds to an interface separating two media with a very high resistivity contrast.

Away from the chamber position, ratio values increase as it should be expected. Therefore, one direction—AB—corresponds to the resistivity in the foliation planes (lower values), but the other direction—CD—is closer to the transverse resistivity (higher values).

There are many examples of the use of conventional ERT techniques in the location of mining cavities with evident success. However, the influence of formation anisotropy is seldom addressed and can mask ERT response. In this case, different field ERT orientations must be used to avoid misleading interpretations.

Often field space is restricted and only one ERT can be carried out. In this case, ERT orientation must be rather different from the strike of the geological formations and, in optimal conditions, ERT orientation should be perpendicular to the strike.

It is also possible to use electrode arrays that take into account orientation effects and anisotropy. Therefore, sets of linear arrays covering a wide range of directions can be used [14]. Alternatively, arrays of different geometry such as the square array have also been proposed [13].

However, the use of alternative arrays demands space and easy field access conditions and this requirement can be difficult to meet in urban areas.
