A. Osella

Fresnel reflection coefficients for GPR-AVA analysis and detection of seawater and NAPL contaminants

We obtain the transverse electric (TE) and transverse magnetic (TM) Fresnel reflection coefficients for different interfaces in the subsoil: air/fresh-water, air/seawater, fresh-water/seawater, air/NAPL (non-aqueous phase liquid), NAPL/water and water/NAPL. We consider a range of NAPL saturations, where the complementary fluid is water with 0.65 ppt (parts per thousand) of NaCl. The common feature is that the TM mode (parallel polarization) has a negative anomaly and the TE mode (perpendicular polarization) has a positive anomaly. For the cases studied in this work, pseudo-Brewster angles appear beyond 40° for the air/NAPL and NAPL/water interfaces and at near offsets (below 40°) for the water/NAPL interface. Pseudo-critical angles are present for the water/NAPL interface. Besides the reflection strength, the phase angle can be used to discriminate between low- and high-conductivity NAPL, when the properties of the upper medium are known. A wavenumber–frequency domain method is used to compute the reflection coefficient and phase angle from synthetic radargrams. This method and the curves can be used to interpret the amplitude variations with angle (AVA) of reflection events in radargrams obtained with ground-penetrating radar (GPR).

ERT for Localizing 17th Century Tunnels at a Jesuit Mission in Buenos Aires, Argentina

We performed a geophysical study in a historical site of Buenos Aires, corresponding to a Jesuit Mission established during the 17th and 18th centuries. It consisted of a Church, the Cloisters, a School and a Procurator Office; also several tunnels were built, connecting the Mission with different public buildings of the town. In 19th century the Faculty of Sciences was constructed over one of the site, functioning until its demolition in 1973. At present, this space is a cobbled square. With the aim of preserving and restoring the buried structures, we worked in this square, looking for tunnels and also for remains of the basement of the old building. Taking into account both the conductive features of the soil, mainly formed by clays and silt, and the urban characteristics of the environment, we performed pre-feasibility studies to determine the penetration-depth of different methods. We obtained reliable data surveying with dipole-dipole profiles with apertures of 1.5 and 3 m, provided that the electrodes were buried at least 15 cm among the cobble stones. By correlating the resulting nine ERTs with the original plans, old structures and evidences of remains of tunnels were identified.

Development of a Geoelectric Device of Capacitive Contact for Studying Archaeological Targets in Very Resistive Zones

In studies of highly resistive subsoils, geoelectric method with galvanic contact is not effective due to the difficulty to obtain a good contact and therefore a good current flow. The potential differences generated are very small, of similar magnitude to the noise level. The aim of this work is to develop a technique in order to facilitate the electric contact in such cases. In order to do more efficient the electrical contact, we designed and developed a low power geoelectric equipment using alternating injection current and capacitive coupling which can be used in highly resistive mediums and also on hard surfaces like asphalt or concrete. We have studied the processing and interpretation of data obtained with this equipment. It was studied the conditions under which the equipment can operate similarly to geoelectric galvanic contact so that data can be interpreted under the same patterns of interpretation. The studies made on scale models with the capacitive equipment on simulated soils with very high resistivity media, agree with the results obtained with the usual galvanic contact.

Geoelectric and EMI Methods to Study Soils with Superficial Leakages of Hydrocarbons

We present the results of the geoelectric and electromagnetic induction methods applied to detect and delimit an area contaminated with hydrocarbons. The joint implementation of these methods was performed in order to enhance the resolution of the prospecting, since each method presents different sensitivity and resolution according to the physical properties of the materials. On the basis of the results of this study, stabilization/solidification methods were applied to remedy the contaminated area. Additional surveys were performed immediately after remediation showing that the contaminants were effectively encapsulated.