Frequency-Dependent Streaming Potential of Porous Media—Part 1: Experimental Approaches and Apparatus Design

Table 2

Summary of the main characteristics and limitations of each of the drive methods. Please see the appendix for details.

Drive method	Maximum practical frequency range (Hz)	Comments
Hydraulic	0–5	Design limited by
		(i) Maximum frequency of hydraulic valves.
Connecting pin (scotch yoke)	0–33	Design limited by
		(i) Lack of a motor with a combination of sufficient torque and speed.
		(ii) Lack of a spring with sufficient stiffness per mass.
Cam	0–33	Design limited by
		(i) Lack of a motor with a combination of sufficient torque and speed.
		(ii) Lack of a spring with sufficient stiffness per mass.
		Advantages:
		(i) Has a lower friction than the connecting pin approach, which reduces the torque that the motor must provide.
		(ii) The profile of the imposed fluid pulse can be varied by changing the shape of the cam.
Pneumatic	0–100	Design limited by
		(i) Speed of servovalves.
		Advantage:
		(i) Does not require a spring for smooth operation because of the compressibility of the gas.
Electromagnetic	2–6500	Design limited by
		(i) The specification of the electromagnetic shaker used (here a VTS-100).
		(ii) Electromagnetic noise is a problem to be overcome with shielding and signal preamplification.
		Advantages:
		(i) Does not require a spring for stable operation due to EM damping.
		(ii) Covers the range of frequencies most useful in the characterisation of sands, rocks, soils and gravels.
		(iii) Control of piston displacement allows the fluid pressure to be well controlled.
Piezoelectric	200–10000	Design limited by
		(i) Lack of availability of transducers for frequencies less than about 200 Hz.
		(ii) It is not possible to drive a piezoelectric system at a wide range of frequencies.
		(iii) The generated displacements are very small, and it is not certain that the electrokinetic effect is fully developed with such small displacements.
		(iv) It is unclear whether check valves can be made to operate effectively at such small swept volumes and high frequencies.
		Advantages:
		(i) Frequencies above 10 kHz may be attained.
		(ii) Covers the range of frequencies most useful in the study of seismoelectric exploration.
		(iii) Forces far greater than that possible with an electromagnetic shaker are possible with a stack of piezoelectric transducers, allowing the study of low porosity, low permeability rocks at specific frequencies.