Samvel G. Gevorgyan

*Center on Superconductivity & Scientific Instrumentation, Chair of Solid State Physics, Faculty of Physics, Yerevan State University; Institute for Physical Research, National Academy of Sciences; Precision Sensors/Instrumentation (PSI) Ltd. Armenia* 

### **1. Introduction**

During a last decade scientists and engineers step-by-step are developing a Single-layer Flat-Coil-Oscillator (**SFCO**)-based *new measurement technology*, and looking for its effective use in a research, and elsewhere. It was introduced in 1997 by our group in Armenia [1-2] and then improved by an integrated research group in Kyushu University, Japan, during next 4 years (1998-2002) [3-4] – allowing to reveal fine physical effects related with the basic properties of high-*T*c superconductors (**HTS**) [5-8]. Starting with 2004 the method passed further development in Armenia, and was then applied for creation of a new *absolute-*position sensor of *nano-*scale resolution [9]. Advantages of the *SFCO* method-based position sensor become more evident when applied to the *quasi-*static Seismometry to study slow movements of ground. Due to these, the *SFCO measurement technology* (in a whole [1-4]), and its first application as a novel seismic detector of slow movements (in particular [9-10]) appeared among the Top six World Security Technologies at the 2008 year's *"Global Security Challenge"* competition details on "**GSC-2008**" forum see in: http://www.globalsecuritychallenge.com. In this Chapter, we discuss principle of operation, and test data of such a new *absolute-*position sensor, installed (for validation) in a well-known seismometer, as an additional pick-up component showing its advantages compared to traditional technique. We discuss also wide potential of this new method, as a real-time measurement technique for early detection of incoming earthquakes, tsunamis and tidal motion. We also outline prosperous future of such a sensor. To sense what are advantages of the flat-coil-based this unique method, let's remember: oscillators are among the most of precise measuring instruments, because the frequency is possible to measure with a very high accuracy. Among them, those at *MHz* frequencies, having volume pick-up coils (mainly, solenoid-shaped), activated by a *low-*power (*backward*) tunnel diodes (**TD**) (see [2, 11-12] and references therein), are of special interest. Replacement of such a standard coil by the unusual, single-layer flat (open-faced) one, as a detecting circuit in a stable-frequency and amplitude *TD-*oscillator, enabled to make coil's filling factor close to the maximal possible value (the *unit*) for flat objects, resulting in strong enhance-

we outline perspective future of such an unprecedented sensor – involving substitution of a normal-conducting pick-up coil by a superconductive one, and replacement of a tunnel diode by the *S/I/S hetero-*structure – as much less-powered active element in a detecting oscillator, compared to the tunnel diode. These may improve stability of oscillators, created by the use of *SFCO* method, and thus, enhance the resolution of seismic devices, and tsunami detectors as well – by at least another 2-3 orders of magnitude. Such improvements may enable to reveal and study *quasi-*static deformations and *low-*order free oscillations of earth's crust, precursor to earthquakes. It may also permit to study features of the tidal motion and tsunami waves. Such a sensor may be also used as a *position*/*vibration* sensing element in *micro-* and *nano-*electronics (in probe microscopy), in

A *Traditional inertial seismometer* converts ground motion into electrical signal, but its properties cannot be described by a single-scale parameter, such as the output volts per millimetre of the ground motion [13] (*as occur in case of the absolute-*position *sensors*). Its response to ground motion depends not only on the amplitude of motion (*how large it is*) but also on its time-scale (*how fast it is*). So, the suspended (*hanging*) seismic mass has to be kept in place by certain restoring force (*electromagnetic*, *mechanical*, *else nature*). But, when ground motion is slow, the mass will move with the body of a seismometer, and the output signal even for a large motion will thus be negligibly smaller. Such a system is so a high-pass filter for ground shifts. This must be taken into account if the ground motion is reconstructed from the recorded signal. So, creation of seismic detectors, which may give large output both for fast and slow motion (*regardless of the rate of motion* – *as absoluteposition sensors behave themselves*), still remains among the prime important problems in

To this end, a prototype of the *SFCO* method-based position sensor has been created and installed by us in a setup of the Russian seismometer of *SM-3* type (Fig.1a). In such a *"hybrid SM-3"* device (Fig.1b) a flat coil serves as a pick-up in a stable *16MHz*–oscillator, driven by a *low-*power Russian tunnel diode of the *AI-402B* model. Actually, 2 similar flatcoil oscillators are mounted in *SM-3*. One is used as a position detector, the other – to detect background at all times (*bottom* and *top* oscillators in Fig.1b respectively). Let-in *SM-3* position sensor is extra to its own *vibro-*sensor one, based on excitation of the electromotive force (**EMF**) in a solenoid coil (Figs. 1a and 1c). In case of the *SFCO-*based sensor, measuring effect is proportional to changes of mutual distance between the coil and metallic plate vibrating parallel to the coil face (*d* in Fig.1c). This results in the changes of

So, new seismic detector converts ground motion into shift of a flat-coil-oscillator frequency *due to ground shaking*. The measuring signal appears as a result of the coil motion (fixed on seismograph's body Figs. 1b-1c and 2) relative to metallic plate (fixed on hanging pendulum (Fig.1c), or membrane (Fig.2)), positioned near the coil. Figures. 1c and 2 schematically illustrate *SFCO* sensor-based novel seismic detectors' possible designs: *F***S** is the shock force, and *d* amplitude of vibration of a pendulum (see Fig.1c) or membrane (Fig.2), caused by it.

security systems, and in medicine as well.

**2.1 Traditional inertial seismometer** 

the Seismology (*and not only*…).

the *test-*oscillator frequency.

**2.2 Principle of operation of new seismic detector** 

ment of the resolution of measurements by 3–4 orders of magnitude (especially, in studies of thin, plate-like *HTS* materials [1, 3-8]). For comparison, typical values of the filling factor for solenoid coils are 104103 for the said samples. Advantages of the *SFCO* technique become more evident at slow movements of the objects, positioned near the coil face. Just therefore, this method has been very soon applied for the creation of a *nano-*scale *absolute-*shift position sensor, which one may successfully use in many areas: for example, for the *quasi-*static (*slow*movement) Seismometry [9-10], in various security systems. Why this problem is so urgent? Basically, there are two types of seismic sensors, acting presently [13]: *inertial seismometers*, which measure ground motion relative to some inertial reference (*suspended inert mass*), and *strain-meters* (or *extensometers*), which detect shift between two points of the ground. Although strain-meters are conceptually simpler than inertial seismometers, their technical realization is much more difficult. Besides, as ground motion relative to the suspended inert mass is usually larger than differential motion within a test tube of reasonable dimensions, inertial seismometers usually are more sensitive to earthquakes. At low (and especially, at *super-*low) frequencies, however, it becomes hard to maintain the hanging reference fixed, and for detection of *quasi-*static deformations and *low-*order free oscillations of the earth's crust, tidal motion (*moon movement*), and for observation of mechanical vibrations of buildings, bridges, etc., the strain-meters may take noticeable lead over inertial seismometers. We describe in this Chapter how to overcome such lack of acting seismographs/accelerometers/vibrometers by the use of the recently offered by us flat-coil-based, super-broadband, nano-scale-resolution position sensor [9-10]. The more so, because further development of such a highly sensitive sensor technology may contribute also to on-time tracking (*prediction*) of potential incoming tsunamis, and monitoring of the state and zone borders as well.

#### **2. Flat coil-based** *absolute***-position sensor for** *nano-***scale resolution,** *super***broadband Seismometry**

And so, a new class *super-*broadband, *nano-*scale resolution position sensor is developed and tested by our group. It can be used, in particular, as an additional sensor in presently acting seismographs. It enables to extend *frequency-*band (theoretically, up to "zero"), and enhance *absolute-*resolution (*sensitivity*) of seismographs available on the market (*by at least an order of magnitude*). It allows transferring of the mechanical vibrations of constructions, buildings, bridges & ground with amplitudes over *1nm* into detectable signal in a *frequency-*range starting practically from the *quasi-*static movements ("zero"!). It is based on detection of position changes of a vibrating normal-metallic plate placed near the coil face being used as a pickup circuit in a stable *TD-*oscillator. Frequency of the oscillator is used as a detecting parameter, and the measuring effect is determined by a distortion of the *MHz-*range testing field configuration near the coil face by a vibrating plate, leading to magnetic inductance changes of the coil, with a resolution *1-10pH* (*depending on operation temperature of a technique*). This results in changes of test oscillator frequency. Below, we discuss work-principle, and test data of such a new position sensor, installed in a known Russian *SM-3* seismometer (for validation) as an additional pick-up element – showing its advantages compared to traditional techniques. We also discuss potentials of this novel *absolute-*position sensor, operating down to liquid–4He temperatures, and in high magnetic fields – as a real-time measurement element for early detection of earthquakes, incoming tsunamis, tidal motion, and for tracking borders. We discuss also possible design of seismic detectors based on this sensor. Besides, we outline perspective future of such an unprecedented sensor – involving substitution of a normal-conducting pick-up coil by a superconductive one, and replacement of a tunnel diode by the *S/I/S hetero-*structure – as much less-powered active element in a detecting oscillator, compared to the tunnel diode. These may improve stability of oscillators, created by the use of *SFCO* method, and thus, enhance the resolution of seismic devices, and tsunami detectors as well – by at least another 2-3 orders of magnitude. Such improvements may enable to reveal and study *quasi-*static deformations and *low-*order free oscillations of earth's crust, precursor to earthquakes. It may also permit to study features of the tidal motion and tsunami waves. Such a sensor may be also used as a *position*/*vibration* sensing element in *micro-* and *nano-*electronics (in probe microscopy), in security systems, and in medicine as well.
