[1] The scientific literature is almost devoid of frequency-dependent electrokinetic measurements on geological materials. An apparatus that allows the measurement of the streaming potential coupling coefficient of unconsolidated and disaggregated materials such as sands, gravels, and soils has been designed, constructed, and tested. The apparatus, which uses an electromagnetic drive, operates in the range 1 Hz to 1 kHz and has a 25.4 mm diameter sample chamber for samples up to 150 mm long. We have made streaming potential coupling coefficient measurements on samples of Ottawa sand as a function of frequency. The results have been analyzed using critically and variably damped second-order vibrational mechanics models as well as the theoretical models of Packard for capillary tubes and Pride for porous media. The best fit was provided by an underdamped second-order model with a damping factor of 0.8561 (R 2 = 0.993). Transition frequencies were derived from the two vibrational models and the Pride model either by fitting the model to the data or directly from the model, giving 230, 273, and 256.58 Hz, respectively. These values are in good agreement with the transition frequency expected for a sand with an independently obtained effective pore radius of 6.76 × 10 −5 m from laser diffraction grain size measurements. The Packard model also agrees extremely well with the experimental data (R 2 = 0.987) directly providing a value of the equivalent capillary radius of 6.75 × 10 −5 m that coincides within experimental errors with the independently obtained effective pore radius measurements.

Frequency-dependent streaming potential coefficient measurements have been made upon Ottawa sand and glass bead packs using a new apparatus that is based on an electromagnetic drive. The apparatus operates in the range 1 Hz to 1 kHz with samples of 25.4 mm diameter up to 150 mm long. The results have been analysed using theoretical models that are either (i) based upon vibrational mechanics, (ii) treat the geological material as a bundle of capillary tubes, or (iii) treat the material as a porous medium. The best fit was provided by the Pride model and its simplification, which is satisfying as this model was conceived for porous media rather than capillary tube bundles. Values for the transition frequency were derived from each of the models for each sample and were found to be in good agreement with those expected from the independently measured effective pore radius of each material. The fit to the Pride model for all four samples was also found to be consistent with the independently measured steady-state permeability, while the value of the streaming potential coefficient in the low-frequency limit was found to be in good agreement with other steady-state streaming potential coefficient data.

Electrokinetic phenomena link fluid flow and electrical flow in porous and fractured media such that a hydraulic flow will generate an electrical current andvice versa. Such a link is likely to be extremely useful, especially in the development of the electroseismic method. However, surprisingly few experimental measurements have been carried out, particularly as a function of frequency because of their difficulty. Here we have considered six different approaches to make laboratory determinations of the frequency-dependent streaming potential coefficient. In each case, we have analyzed the mechanical, electrical, and other technical difficulties involved in each method. We conclude that the electromagnetic drive is currently the only approach that is practicable, while the piezoelectric drive may be useful for low permeability samples and at specified high frequencies. We have used the electro-magnetic drive approach to design, build, and test an apparatus for measuring the streaming potential coefficient of unconsolidated and disaggregated samples such as sands, gravels, and soils with a diameter of 25.4 mm and lengths between 50 mm and 300 mm.