THE MOLECULAR ELECTRONIC SEISMOMETERS

The Russian Prince Golicyn developed the first seismometer at the beginning of the 20th century. It was a very simple instrument; it looked like a stylus attached to a mass supported by spring. Later, electro or opto-mechanical converters were introduced which provided significant amplification of signals, and allowed for correction of the frequency response. Despite the enormous advances of science and technology in the 20th century, this traditional scheme of the seismometer hasn’t changed for the most part. Pendulum based oscillating systems are delicate, expensive, precision instruments, which are usually installed and maintained by qualified personnel. These seismometers operate perfectly at high frequencies, but at low and especially at the lowest frequencies the disadvantages of the pendulum based seismometers become extremely visible. The instability of the natural frequency caused by temperature variations, inner convective flows, electromagnetic interactions between different mechanical parts of the seismometer and slight deviations of the angle of installation are lead to a dramatic loses of stability of the device.

Another approach to seismometer design is to simplify the oscillating system by restraining the pendulum and increasing the value of its natural damping. The increase of the damping plays the same role as the feedback mechanism that is usually used for expanding the frequency pass band of any recorder. Simultaneously, it is necessary to increase the conversion factor of the transducer, since the large value of damping leads to an essential decreasing of the sensitivity of oscillating system. This task has been successfully solved using the molecular electronic transducers (MET). From the pure technical point of view, an MET transducer is by itself a combination of an inertial mass, sensitive element, and amplifier of the external acceleration, with a power gain of 10⁶ and large damping. The original scientific achievement is that the output current of an MET transducer is proportional to the velocity of the ground motion, since its transfer function is inversely proportional to the frequency. As for the mechanical system of the sensor, it represents a cylindrical tube filled with an electrolyte and provided with elastic membranes on the top and bottom of the tube (vertical sensor), or a toroid partially filled with electrolyte (horizontal sensor). In both cases the electrolyte (10 – 15 g) plays role of inertial mass and the hydraulic impedance of the transducer ~ (10⁹ N*s/m⁵) provides the necessary value of damping. Thus a three component molecular electronic seismometer is light and compact since it does not require the use of large solid suspended inertial mass; rugged since it contains no moving parts; and inexpensive since no expensive materials or complex manufacturing processes are used.

Of course, an MET needs power, since it works as a vacuum triode. When a voltage source (~300 mV) is connected, an electric current starts flowing between the anode and cathode and in the external bridge circuit. The value of this current depends on the charge concentration and the distance between the anode and cathode. The distance between the anode and cathode for the real transducer usually varies in the limits of 100 – 200 um. The charge concentration may be varied arbitrarily. As a result, the current’s variations are within the limits 10 uA – 1.5 mA. The value of this electric current is critical to the design of the support electronic circuit. The circuit design must balance a low self-noise against the greater cost of components need to achieve it. Several electronic boards have been designed for different applications, and it is possible now to reduce seismometer’s current to 100 uA. That is very important for field, and especially ocean bottom, applications.

The design of MET seismometers makes them insensitive to the angle of installation, up ~ 5 deg. for vertical and 15 deg. for horizontal sensors. This is very important for the borehole version. It is a result of the fact that the liquid inertial mass (water in a cylinder ) has a tendency to maintain a horizontal position.

The PSD of MET seismometer self noise in acceleration units is flat over passband. It reflects the fact that MET transducer has a very high gain output, excluding 1/f noise of electronic components.

The temperature range of operation is limited by freezing point of electrolyte. The low-temperature version of MET seismometer uses electrolyte with some special agents and has freezing point about - 55 °C.

It is very important to note that MET technology based seismometers are extremely suitable for mass production.