Developed by Geostudi Astier s.r.l. in collaboration with the American Multi-Phase Technologies LLC, ERTLab is the software for 3D inversion of resistivity and induced polarization which is an innovative tool in the interpretation of geoelectric measurements.

Thanks to the algorithm of modeling Finite Tetrahedral Elements, ERTLab is able to invert measures acquired with topography.
The powerful graphical environment ERTLab Viewer allows you to view the results of the inversion through a complete set of graphical objects (sections, iso-resistive surfaces, volumes, …).
The ERTLab Sequencer module, included in the package also allows you to create quadrupole sequences for any electrode arrangement (2D and 3D configurations) into the hole, on the surface or mixed.

The development of interpretive tools for the investigation of electrical resistivity tomography in recent years has expanded the range of possibilities provided in the acquisition phase. Besides the usual geometry using 2D profiles, 3D grids of surface electrodes, electrodes in the hole, innovative 3D geometries are now applied in different contexts, such as civil engineering and geotechnical investigations for environmental purposes, and archeology. Among these geometries must be mentioned in particular the surface configurations that make use of electrode arranged in the shape of L, U or annular (loop electrodes).
In Figures 1 and 2 are presented some examples of surface electric 3D tomography inversions for some of the geometries mentioned above, used to investigate the structural characteristics of a building. The electrode array is spread along the perimeter boundaries of the building and the electrodes have variable spacing.

The reasons that lead to the choice of these electrode configurations are many, but the most common is to overcome the constraints that the urban context poses to the survey. Very often, it is necessary to investigate land and structures beneath buildings or artifacts that would not otherwise reach if not using expensive and invasive techniques.

Let’s explore some of the reasons that make this new approach viable to the investigation geoelectrical 3D. In figure 3 are represented the maps obtained from the sensitivity analysis performed with the software ERTLab for a resistive model homogeneous, subjected to investigation with spread of surface electrodes in the shape of L.
In the section a) shows the sensitivity map for a horizontal depth of 1.5 meters below ground level, in section b) of the figure is a vertical section that affects the electrodes of the quadrupole under consideration (measured pole dipole cross cable , that is with the transmitter on a branch of the L and dipole reception on the other branch).
The images sets in evidence that for such a quadrupole there is a great degree of coverage and resolution of the measurement, even in those areas that are not directly employed by the electrodes.

To be able to obtain satisfactory results from this kind of investigation is necessary to refer to a protocol design, data acquisition and reversal of rigorous and well managed in every aspect. The software ERTLab allows you to do so in full, from the generation of the sequence, inversion, to the graphical representation of the results. The critical elements to perform a good investigation can be listed below:

– The generation of the sequence must be carried out with tools to create sets of quadrupole tailored to the specific geometry of the electrodes examined. The tool ERTLab Sequencer allows you to easily build the desired electrode geometry and then makes it available for the generation of advanced tools for the most common quadrupole electrode arrays. In particular, along with the usual measures common cable (or with transmitter and receiver on the same “branch” of the spread) is necessary to generate specific quadrupole cross cable, to ensure that the electrodes on different branches “communicate” with each other. All branches of the electrode array, moreover, must involve both transmitters and receivers. The set of quadrupole generated is then filtered on the basis of the geometric factor K, to remove those measures for which we expect a level signal/noise unsatisfactory. Finally , it is not to overlook the possibility of acquiring an adequate number of reciprocal quadrupole, in pre-processing of the data, a reciprocal check, to get an idea of the noise level of the site investigated.

– The ability to use tools of survey design and sensitivity analysis is another aspect that should be considered as an aid to planning the geophysical survey: it is therefore recommended that users of ERTLab to use them before designing a measurement campaign.

– The inversion procedure, finally, has the task of managing in a flexible manner each single processing element, in particular with regard to the aspects described below.
The procedure of mesh generation must allow to build models that take into account the topography and, due to the possible irregular spacing of the electrodes, to add nodes to the mesh where necessary.
Another important aspect is linked to the presence of any remote poles: given that the urban context often reduces the possibility to move to a distance suitably “infinite”, the inversion algorithm must be able to model the exact position of these electrodes.
The algorithm of direct modeling of the measures quadrupole (forward modeling) must also give the possibility to force the boundary conditions appropriate in correspondence of special situations: for example, are required boundary conditions of Neumann type for those borders where a retaining wall interrupts the continuity of the model.
The control on the main parameters of inversion must be total, in particular for the definition of the levels of environmental noise and error on the data.
It is not to be neglected the possibility of reversal of the part of the algorithm to perform suitably modeling the electric effects of anomalies present in the investigated area (pipes , pipelines, well effects , … ).

– Mathematical solver using least squares with a smoothness constraint.

– Robust inversion.

– Interface for pre-processing of data (pseudomaps, reciprocal check, filtering measures).

– Mesh generator with modeling of topography.

– Starting Model defined by user. Direct modeling of resistivity / chargeability and sensitivity analysis.

– Graphical interface for defining cables and electrodes. 3D visualization of the arrangement of the cables.

– Tool for generating sequences 2D/3D on surface, in hole or mixed (linear and equatorial dipole-dipole, pole-dipole, pole-pole, Wenner, Wenner-Schlumberger).

– Optimization of sequences for multi-channel instruments. Generation of reciprocal quadrupole.

– Several Import / Export formats (Electre II, ERTLab Solver, text).

– Definition xy, yz, xz or generic of sections.

– Extraction of resistive volumes. Define isoresistive surfaces.

– Different colormap (rainbow or red-blue, linear or logarithmic normal or reverse). Export in bitmap format. Save the configuration file.

– Advanced views (transparency, labeling, orthographic view / perspective).