**1. Introduction**

The importance of cavities in engineering, mining, environment, archaeology, tourism, etc. has long been acknowledged. Caves have natural origin (lava, karst or other dissolution processes) or can originate from anthropic activities, such as mining, engineering excavations and archaeological features.

Knowledge of cave location and extension is very important during the construction and maintenance of infrastructures (tunnels, highways, railways, sewage systems) and when urban

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areas extend to old mining areas or karst regions. In fact, risks of collapse and pollutants transportation can lead to unwanted and hazardous events. Caves are also used as reservoirs, deposits of gas, hazardous and toxic residues and, therefore, cavity monitoring and fourdimensional (4D) studies must be considered for safety reasons.

Cavity location is often done by surface mapping, documents, local and oral descriptions. Therefore, before engaging in expensive and comprehensive exploration programmes, it is important to review all the relevant available information. However, often local records are difficult to obtain, particularly in old mining areas. It is also possible that no information is available in regions with no evidence of caves at the surface or in cases of fast cavity development, such as sinkholes.

Often invasive exploration methods (excavation, drilling) are used. These invasive methods must be carefully planned to reach the targets at a suitable cost and, furthermore, operations must not interfere and damage the caves [1].

Other approach uses indirect investigation techniques from the surface that is geophysical exploration methods. However, the efficiency of these methods depends, among other factors, on the contrast between the physical properties of the cavities and those of the surrounding media [2].

Fortunately, in most cases there is a contrast between the velocity of seismic wave propagation, density, electrical and magnetic properties of the cavities and those of the rock formations where they are installed [3] and, thus, geophysical exploration methods can be used and adapted to cavity detection and location [4]. However, the relation between the dimensions of the cavities and their depth can also be a major limitation factor for the use of geophysics. In fact, cavities can be too small or too deep to be detected in spite of a large contrast in physical properties [5].

Since these limitations are overcome, the main objective of the use of geophysical methods in cavity location is to provide information and restrict the area to investigate by direct methods, to guide later exploration operations and to diminish costs while preserving the targets [6].

In this contribution, the application of geophysical methods in cavity exploration is focused on the use of 2D resistivity methods—electrical resistivity tomography (ERT).

There many examples of the use of 2D and 3D ERT in cavity location [7, 8]. As in any other geophysical method, the success of cavity detection by resistivity methods depends on factors such as depth, size and contrast between the resistivity of the cavity and that of the surrounding media [5]. Cavities can be more conductive or more resistive then the rocks that surround them. Cavities filled with water are more conductive then the surrounding media. However, when empty they are more resistive as air is not an electricity conductor. Thus, in the first case, cavity response to resistivity methods is a conductive anomaly whilst, in the second case, the response will be a resistive anomaly.

The nature of the surrounding media is another factor to consider. Usually, resistivity fieldwork consists on recording data in one unique direction. Therefore, two-dimensional effects arising from local geology such as contacts, schistosity, etc., can induce orientational variation on surface resistivity data and complicate cavity detection.

In the past, resistivity fieldwork was a slow operation. Nowadays, the development of automated equipment allows the fast acquisition of large field data sets. This enables the production of high-resolution images of the ground and answers the engineers' and planners' needs for fast, non-intrusive and high-resolution methods to detect underground targets such as cavities.

This work will give a brief introduction to the resistivity methods and to the field techniques necessary to carry out an ERT. Then an approach to the resistivity behaviour in anisotropic media is presented. Finally, a case study concerning the location of mining cavities in anisotropic regions is discussed. The ambiguity and uncertainty in the ERT interpretation will be addressed and field strategies to reduce or overcome those limitations are also presented.
