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Opening pores There are few reliable methods for determining insitu permeability of soils and rocks. Max Soudain looks at a new borehole logging system which detects tiny pressure-dependent electrical

Marlborough-based Ground- Flow launched its electrokinetic logging tool (EKL) at the beginning of this year after extensive testing throughout 1997. The technique is based on the company's surface operated electrokinetic survey (EKS) method in which a small sonde, just 70mm in diameter and 720mm long, measures permeability of the ground by sending sonic pulses into the material and measuring the electrical echoes that result from the pressure wave.

Conventional radiometric logging such as natural gamma and neutron gives bulk properties such as bulk density. Total porosity can be obtained indirectly from the results, but there is no indication of permeability or pore fluid resistivity.

A more accurate method, used in the oil industry for a few years, is nuclear magnetic resonance. But Groundflow's John Millar says this can prove difficult to use in practice and can be quite costly. EKL provides a cheaper and more general purpose alternative, he says.

When pressure waves pass through saturated rock or soil, electric fields are set up. This electrokinetic effect is caused by water movement induced by the pressure wave forcing the water through the pores. As the water moves, positive ions remain on the rock or soil while negative ions move further and more freely, creating a electrical potential proportional to the applied pressure.

This potential is typically 30mV-60mV per atmosphere of applied pressure in a wide range of rock types, and amplitude is controlled by changes in the pore fluid resistivity. The rise time of the electrical response is a direct measure of how quickly flow is generated by a given pressure or the permeability of the material. The shape of the electrical pulse becomes steadily more rounded as permeability falls.

By comparing the responses at different frequencies, the char- acteristics of the electrokinetic response in the formation can be measured. Where permeability is high, good response occurs with high frequency, but less permeable material responds only below a critical frequency. Darcy's law and Poiseuille flow in capillaries are assumed, allowing estimates of average pore diameter. Fluid resistivity and permeability is then calculated from the signal amplitude and frequency.

The EKS method was tested in Europe, Canada and Africa. Profiles showing details of conductivity down to 300m along survey lines of 1km to 2km long are now routine and match well with previous calibration data from boreholes and pumping tests.

The new EKL method works on the same principle, but is more sensitive. It continuously sends out several frequencies using a detector tuned to them and analyses the rise time of the electrical responses to the pressure wave it sends out.

The resulting permeability log, with around 200mm vertical resolution at normal logging speed (around 2m/min) identifies all the permeable zones in a borehole capable of having groundwater flow.

EKL can survey material up to 3m away from the borehole, going beyond the surface damaged by drilling. It rapidly builds up a precise image of the subsurface permeability around and between boreholes. The sonde can be used in uncased boreholes or with slotted plastic liner (with or without sand/gravel packs). It is capable of logging up to 100m depth. It does not have to be in contact with the wall or be central in the hole.

A variety of output is available. Pore fluid resistivity, permeability and effective porosity (a measure of the volume of moveable water) can all be shown against depth.

The electrical resistivity of fluids can also be measured in the pores of the formation rock, which can reveal the degree of salinity or chemical pollution of the groundwater. Conventional logging can only give good information about bulk rock properties rather than the fluid.

GroundFlow recently carried out an investigation in Sussex to determine the salinity of a chalk aquifer. Coastal aquifers frequently contain salty layers which may be fossil deep basin water or seawater brought in by over-abstraction. EKL located a thin layer of salt water which Millar claims would not have been identified by conventional method.

The ability to pinpoint polluted layers, he adds, also makes EKL a useful tool in the investigation of contaminated land. It can evaluate flowpaths out of potential landfill sites, help plan extensions to existing sites and investigate leakage and leachate plumes, adding considerable information to geochemical sampling of borehole fluids.

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