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COMPARISION BETWEEN ELECTRICAL AND ELECTROMAGNETICAL METHODS FOR MAPPING SALT WATER INTRUSION

MAHJOUB HIMI, LUIS BAGAN, J, M, CARMONA, & ALBERT CASAS.

GPPG
FACULTAD DE GEOLOGIA
UNIVARSIDAD DE BARCELONA

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INTRODUCTION:

DC- resistivity and electromagnetic techniques are widely used to measure the electrical resistivity / conductivity of materials in the subsurface. Variations in these two parameters are caused by changers in soil characteristics- structures, clay content, degree of water saturation and conductivity of this water. Taking advantage of the relationship between electrical conductivity and chloride ionic concentration, we have attempted to map the saltwater intrusion in the Llobregat delta (Barcelona). For this purpose two different surveys have been conducted: vertical electrical sounding and electromagnetic mapping. This study us to compare the results and to contrast the obtained efficiency using each technique.

GEOLOGY OF THE AREA:

The Llobregat delta, extends over about 100 Km2. Schematically, two detritical Quaternary units can be differentiated. One unit forms a deep confined aquifer composed mainly of gravel, and the other unit constitutes an upper unconfined aquifer made up of gravel and sandy silts. A thick layer of silty clays separates these two aquifers and forms an aquitard. The unconfined aquifer occupies the entire deltaic surface and the aquitard reaches a maximal thickness in the centre of the delta petering out in the margins leaving the two aquifers in communication. Manzano et al. (1990) demonstrated the existence of salt water that was slowly diluted by an upward movement of fresh water from the deeper confined aquifer in the central part of the aquitard near the coast line. This phenomenon has almost reached completion in the inner part of the delta at some distance from the coastline.

METHODOLOGY:

A resistivity survey was done using a Schlumberger arrangement with a maximum separation in AB electrodes of 900 m sufficient to reach the desired investigation depth.
An EM ground conductivity survey was also carried out using the Geonics EM-34-3 instrument. This instrument is a frequency domain electromagnetic method that operates in accordance with the theory of electromagnetic induction (Swift, 1987). Conductivity of soil is determined by measuring, in the ground, the response to an induced magnetic field. Eddy currents generated by EM unit create a secondary electromagnetic field which is shifted from the secondary field. From this secondary field it is possible to measure the quadrature and in phase components.

At the condition of low induction numbers, the depth of investigation of the instrument is determined by the coils spacing and orientation. Intercoil separation of 10 m, 20 m and 40 m can be selected and coils can be oriented either with axes in horizontal (HDM) or a vertical (VDM) position. Although the VMD provides an exploration depth that is twice as large as the HDM, it is more sensitive to lateral conductivity variations and in highly sensitive to coil misalignments (Mc Niel, 1980).

DISCUSION:

EM data was interpreted using the EMIX34Plus modelling package by Interpex Ltd. This code enables us to obtain accurate interpretations of ground conductivity measurements in terms of layered earth models. An example of the models obtained is shown in figure 3.
The increasing conductivity towards the coastline has a direct relationship with the sea water intrusion that affects the delta aquifers. We also noted that along the coast, always at a similar distance from it, the electrical conductivity is quite variable. This can be clearly seen in figure 6 which presents - for the two methods- a similar lateral change in conductivity at two different depths.

In the eastern zone where the aquitard practically does not exist, and consequently there is no difference between the two aquifers, the intrusion was caused by the transformation of irrigation croplands into industrial areas. Moreover, the excavation of the new port resulted in pumping area being closer to the coastline, which increased the intrusion effect.

CONCLUSION:

Both geophysical techniques improve the efficiency of mapping sea water intrusion, which is denoted by different anomalous areas in conductivity. We also noted that electrical conductivity varies largely with depth, which may be attributed to differences in the nature and permeability of subsurface materials.
A comparison between the two methods shows that:

As a rule, in “in-phase” operation the exploration depth of the EM-34 instrument is about 1.5 times the separation distance between the coils, whereas with vertical electrical sounding this depth is nearby 0.3 times the AB/2 distance electrodes. Thus, in studying a big depth in the Llobregat delta-where many obstacles are present-, the EM method is more useful than VES given that it needs a smaller distance between the receiver and transmitter.
The cost-benefit of EM-34-3 is 5 times greater than VES, given that as a mean the number of measured points was 30 per days in the EM method while was only 5 in the VES.

REFERENCES:

- Manzano M., Custodio E. and Carrera J. (1992). Fresh and saltwater in the Llobregat delta aquifer: Application of the chromatography theory to the field data. In E. Custodio and A. Galofré (Eds.) Study and modelling of saltwater intrusion into aquifers. CIMNE- UPC, Barcelona, pp. 207-228.
- Mc Neil, J. D. (1980) Electromagnetic Terrain Conductivity measurments at low induction numbers. Geonics Ltd., Mississauga, Canada. TN-6, pp.15.
- Swift, C.M. (1988) Fundamentals of the electromagnetic method. In: Nabighian, M.N. (ed.), Electromagnetic Methods in Appleid Geophysics, Soc. of Expl. Geoph, (1): 5-10.

Edited by M. Himi