Electrochemical Seismometers of Linear and Angular Motion

Traditionally seismic sensors use a mechanical system with moving solid-state inertial mass in combination with either velocity or displacement transducers. The latter convert mechanical motion of the inertial mass into the electrical signal. An alternative approach based on employment of liquid inertial mass and four-electrode electrochemical transducer with potential difference applied between the electrodes as a converter of the liquid motion into electrical signal was developed in recent years (Huang et al. 2013; Agafonov et al. 2014). Such devices are known asMET (molecular electronic transfer) or electrochemical sensors. Basically, the operation of the METsensors uses the sensitivity of an interelectrode current in the electrochemical cell to the movement of a liquid electrolyte relative to the electrodes fixed on the sensor housing. Along with the solid-state electronics (charge transfer by electrons and holes in a solid conductor or semiconductor) and vacuum electronics (charge transfer by free electrons in an ionized gas or vacuum), theMETsensors could be considered as the third class of fundamental electronic devices, characterized by charge transfer via ions in solution – hence the older name of the sensing technology “Solion.” Solion technology was first implemented into a practical device in the 1950s by a US Navy-sponsored project. The early applications of Solion devices were to detect low-frequency acoustic waves, either in the form of an infrasonic microphone or limited-band seismometer (Hurd and Lane 1957; Wittenborn 1958; Collins et al. 1964). The first theoretical model describing the physical principles of operation of such devices was developed by C. W. Larcam in 1965 (Larcam 1965) who studied a simple one-dimensional model of four electrodes (two anodes and two cathodes) placed along a channel filled with an electrolyte moving under the action of external inertial forces. Significant work on Solion motion detectors was continued in Russia, where the term “molecular electronic transfer” was firstly introduced to describe such a device (Lidorenko et al. 1984). Inspired by the exceptionally high rate of mechanical signal conversion to electric current inMET involving mass and charge transport, pioneer studies of MET (Kozlov et al. 1991; Kozlov and Safonov 2003; Panferov and Kharlamov 2001; Reznikova et al. 2001; Agafonov and Krishtop 2004; Volgin et al. 2003) provided an alternative paradigm in the development of seismic sensors. The advantages of MET motion sensors include small size, lack of fragile moving parts, high shock tolerance, wide operating frequency range, high sensitivity, and low noise, as well as low dependence or complete independence of the response on installation angle. These features are widely used in practice especially in field and ocean-bottom