Michel Dabas

Soil resistivity: a non-invasive tool to map soil structure horizonation

Abstract Since geophysical methods are non-invasive, they can be of great help in soil studies because they disturb neither the structure nor the dynamics of the soil. Moreover, data are acquired with reliable spatial sampling. The usual ways of investigation, like augering and excavation, disturb the soil and are totally incompatible with a spatially dense sampling strategy, which would destroy the object of the study. Both approaches are complementary when excavations have a limited extent and are distributed according to the information conveyed by the geophysical investigation. A basic principle of applied geophysics is to measure different physical parameters without direct access to the studied volume. Horizontal and/or vertical variations of the parameter(s) can be recorded. Possible soil parameters should be restricted to measurements which do not alter the medium (reversible effect). To be significant, the variations of the parameter(s) should exhibit a wide dynamic range over different soil types and should be correlated in some way to soil parameters such as particle size or hydraulic conductivity. After summarising the soil properties, two examples are shown whereby electrical resistivity was used. The first example is a specific soil so-called hardpan (sandy soil in arid area) in Lagadge, North Cameroon. Using resistivity surveys the three dimensional extension of a very coherent horizon was mapped. This horizon is delineated by low resistivities 10 mS/m) because of the disposition of clay particles around the quartz grains. In a second example, a “homogeneous” area ought to be found delimit the extent of a surface where a pesticide transfer experiment is to take place. Accurate mapping of soil horizons was not feasible by augering. Resistivity data have clearly shown the three-dimensional extension of clayey horizons in the complex delta context.

Recent developments in shallow‐depth electrical and electrostatic prospecting using mobile arrays

Application of mobile electrical and electrostatic quadripoles during the past ten years has allowed a considerable increase in the size of the surveyed areas, together with keeping a high spatial resolution and a reduction of the total cost of a survey. Two new developments of towed arrays are illustrated here: (1) a pole‐pole array pulled by the operator provides a lightweight solution for mapping large surfaces at a unique given depth of investigation, as shown by the prospection of the Roman‐British city of Wroxeter; and (2) a multipole, multidepth system allows a 3-D investigation of the ground resistivity, as illustrated by the experiments undertaken on the test site of Garchy and on the archaeological site of Montbaron (Indre, France).

Détection par méthode radar de niveaux anthropiques du Néolithique final sous le niveau actuel du lac de Chalain (France)

Abstract Several GPR profiles arc carried out on Lake Chalain (Jura) in order to detect several anthropic layers dating from Middle Neolithic times. The geometry of the sedimentary deposits is imaged by several GPR reflectors. Several auger borings allow the GPR reflectors to be correlated with anthropic levels (strata interleafed within lacustrine chalk). In particular, the oldest and finest (5 cm) level (of Horgen culture from the 32nd century BC) is detected. This experiment proves that it is possible to characterize the 3-dimensional geometry of the deposits from Lake Chalain built up in Middle Neolithic times.

Aerial features removing from ground penetrating radar profiles

The Ground Penetrating Radar (GPR) has demonstrated its particular ability for many sub-surface investigations. Nevertheless, even in the case of an optimal response from the ground, the image is sometimes disturbed by unwanted features (Sun and Young, 1995). Data processing is then a mandatory way to enhance the useful information inside the radargram. Aerial features are one of the noises encountered using non-shielded antennas.