A dipole flow field was created between the injection and extraction boreholes in the BK area (boreholes BK 09 and BK08 respectively). Boreholes BK 15, 16 17 and 18 were equipped with geoelectrical arrays (electrodes).

EFP - Location of Boreholes

A steady state dipole field to was maintained the test boreholes by pumping out water at a constant rate. Then a concentrated salt solution (strontium chloride) was then added into the flow line via the bypass system.

The brine increases the conductivity of the injected fluid to around 3000 mS cm-1. A pulse injection of the salt tracer was carried out for four days with subsequent injection of fresh water over five days. The injection equipment can be seen below.

EFP - Injection Equipment in Grimsel

The figures below are time and depth dependant plots of the changes in apparent resistivities during and after the injection of a salt solution. The higher the saline tracer concentration, the greater the electrical conductivity and therefore the lower the resistivity. This creates a strong interface between the sparsely fractured granite and the water conducting features. The figures below show changes to the resistivity with time and depth.

The "troughs" in the graphs represent depths down the boreholes where the salt solution was flowing during the tests (the salt solution increases the conductivity and therefore lowers the resistivity). The more salt solution, the deeper the trough. If the salt solution was only transported as in one fracture, we would expect a narrow trough in the graphs. However, these graphs have relatively wide areas where the resistivity was reduced. This again indicates that the transport of the salt solution is via a series of fractures rather than one discrete fracture.

EFP - Resistivity with time and depth graphs


Effective Field Parameters (EFP) - Hydraulic and In-situ Tracer Tests