Combined use of seismic, isotopic and piezometrics data to reduce uncertainty in models

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Combined use of seismic, isotopic and piezometrics data to reduce uncertainty in models

Marc Saltel, Alain Dupuy, Carine Grelaud, Michel Franceschi

To cite this version:

Marc Saltel, Alain Dupuy, Carine Grelaud, Michel Franceschi. Combined use of seismic, isotopic and piezometrics data to reduce uncertainty in models. International Interdisciplinary conference on predictions for hydrology, ecology and water resources management using data and models to benefit society (Hydro’Predict 2008), Sep 2008, Prague, Czech Republic. �hal-01330143�

Combined use of seismic, isotopic and piezometrics data to reduce uncertainty

in models

M. Saltel, A. Dupuy, Grelaud. C, M. Franceschi

EA GHYMAC Institut EGID Université Bordeaux 3, 1 allée

Daguin 33 607 Pessac France saltel@egid.u-bordeaux3.fr

Seismic data

A set of 2D seismic reflection lines was used to reveal and to delineate major structural elements having a significant impact on the regional hydrogeology (various erosional gaps, faults, …). A total of 15 seismic lines have been used, representing a total length of 248 km.

Isotopic data

Isotopic analysis (

O,

H,

H,

C,

C), allowed us to study the recharge, to confirm the influence of the structure in this process and to constrain residence time in the model.

This important part of the study involves the interpretation of analysis coming from 4 different aquifers, sampled in 21 wells located on the structure and its near vicinity.

Predictions for Hydrology, Ecology Predictions for Hydrology, Ecology And Water Resources Management And Water Resources Management

Using Data and Models to Benefit Society Using Data and Models to Benefit Society

Prague, Czech Republic, 15 –18 September 2008

General Framework

To improve modelling of complex multi-aquifer systems, a better understanding of the reservoirs geometry and an improvement of calibration are required. Multidisciplinary studies can provide a better comprehension of those systems [1]. Combining seismic data, isotopic analysis and piezometric measurements is a way to improve flow simulation.

In Gironde (France), many hydrodynamic models have been developed to guide choices in groundwaters exploitation and to assist management. They all underline a lack of knowledge on the Villagrains-Landiras structure corresponding to a potential recharge area for the aquifers of the region. In order to correct these shortcomings, a detailed study of this area was undertaken to measure its potential for exploitation.

Piezometric data

The interpretation of the various seismic profiles, tied with well data, allowed us to improve the geometry of the model on the dual anticline structure.

In the previous version of the model, zones of gaps were represented by inactive cells. A more accurate geometry allows us to simulate subcrops using appropriate layers that are reduced to a minimum thickness with fictitious parameters where the hydrostratigraphic unit is, physically, not present.

References

[1] Anker Y., Rosenthal E., Shulman H. et Flexer A., 2008. Multi-disciplinary modeling, in stratigraphy and groundwater stratigraphy of the jordan river basin.

Environmental Geology, Published online.

[2] Huneau F., Blavoux B. & Bellion Y., 2001. Differences between hydraulic and radiometric velocities of groundwaters in a deep aquifer : example of the Valréas Miocene aquifer (southeastern France), Comptes Rendus de l’Academie des Sciences - Series IIA - Earth and Planetary Science, 333(3), 163–170.

Geological interpretation of seismic profile 94-B2 and 94-M2.

Hydrogeological interpretation of cross-section through wells. Simplified schematic diagram of hydrostratigraphic unit representation in MODFLOW grid.

Conclusion

The isotopic composition in

O and

H of samples taken in the local aquifers shows a meteoric origin of waters. The values align themselves with the world meteoric water line.

The most recent waters are the most enriched in

18

O and

H; Some of them show tritium traces.

Deuterium versus oxygen-18 diagram

Waters taken from the Villagrains-Landiras structure correspond to recent waters. This area can be interpreted as a recharge zone.

Assuming that, as an approximation, we consider that the radiometric velocities are equivalent to the hydraulic velocities, it is possible to estimate a range of conductivity and to compare these values to those implemented in the model [2].

1 2

14

t l t V

= ∆ −

e H

K n i V = ×

i n K = V

×

With V_14C the hydraulic velocity, V_H the hydraulic velocity, ∆^l the distance between two wells, t the residence time, i the hydraulic gradient, n_e the effective porosity and K the conductivity .

K

0 +2 5 +1

0 +1 +5

Schematic representation of the calculation method.

This multidisciplinary approach, based on the interpretation of seismic, isotopic and piezometric data, helped us to improve our knowledge of the structural architecture of the aquifers and our understanding of the hydrodynamic processes to simulate more efficiently flows in the model.

The realisation of water level maps allowed us to study the distribution of hydraulic heads in comparison to the new structure borders and to measure its potential influence on the recharge of groundwaters.

The analysis of the piezometric fluctuations underlined many similar variations that characterize local behaviours. The extensive exploitation of the Eocene aquifer of the past 5 decades led to the formation of a large piezometric depression. The analysis of the piezometric fluctuations, within this aquifer, brings to light various types of behaviours giving indications on the hydrodynamic of groundwaters.

These observations give information on the transient functioning of this aquifer and show the existing interactions with the studied structure.

Classification of groundwater behaviours and its hydrogeological implications.

In the North of the structure, hydraulic heads are stable. In the East, a continuous decline of the heads is observed.

It seems that groundwaters from the Eocene aquifer are put under pressure on the northern side of the Villagrains-Landiras

structure.

Cross section through the model

. To simulate efficiently flows in the model, the aquifers layers of the Cretaceous, corresponding to the heart of the dual anticline structure, were completely redrawn.

This cross section shows the importance of the structure in recharge of the regional aquifers.

This method can be useful to give an order of magnitude when very few data are available.

Example of comparison of the estimated conductivity to the conductivity of the model.