CLIAMS:1. A device for monitoring seismic vibrations comprising:
a Fiber Bragg Grating (FBG) sensor;
a cantilever operatively coupled with said FBG sensor with a dead weight at its free end; and
an interrogator configured to interpret signals received from said FBG sensor, wherein said signals are generated based on strain variations transduced from seismic vibrations detected by said cantilever.
2. The device of claim 1, wherein said detection of said seismic vibrations by said cantilever causes generation of said strain vibrations, which are received by said FBG sensor to generate said signals for said interrogator.
3. The device of claim 1, wherein said cantilever is supported on a portable base.
4. An apparatus for characterizing seismic vibrations comprising one or more seismic vibration sensing devices, each device comprising:
a Fiber Bragg Grating (FBG) sensor;
a cantilever operatively coupled with said FBG sensor with a dead weight at its free end; and
an interrogator configured to interpret signals received from said FBG sensor, wherein said signals are generated based
5. The apparatus of claim 4, wherein said apparatus is configured to detect direction of arrival and determine velocity of seismic wave propagation.
6. The apparatus of claim 5, wherein said detection of direction of arrival of said seismic wave propagation is based on identifying devices from said one or more devices that are first and second to receive said seismic wave.
7. The apparatus of claim 5, wherein said determination of velocity of seismic wave propagation is based on delay in reception of said seismic wave between two devices of said one or more devices and further based on distance between said two devices
8. The apparatus of claim 4, wherein each of said one or more devices is configured at a different location coordinate.
9. The apparatus of claim 4, wherein said one or more devices are configured in a rectangular arrangement.
,TagSPECI:TECHNICAL FIELD
[0001] The present disclosure generally relates to field of seismology. In particular, the present disclosure pertains to instrumentation for monitoring of seismic vibrations using Fiber Bragg Grating (FBG) based seismic sensor.

BACKGROUND
[0002] Background description includes information that may be useful in understanding the present invention. It is not an admission that any of the information provided herein is prior art or relevant to the presently claimed invention, or that any publication specifically or implicitly referenced is prior art.
[0003] Environmental disasters, in particular geodynamic events such earthquakes, volcanic eruptions and landslides and their resultant events such as tsunami are natural phenomena that can occur anytime and cause catastrophes, especially in densely populated areas. Early detection of geodynamic events and quick prediction of resultant catastrophic events can forewarn authorities to take precautionary measures such as evacuation of population from potential disaster area and save lives and property.
[0004] Geodynamic events can be detected by reading and analysis of seismic waves created/radiated by them. Seismic waves are detected by seismic sensors, commonly known as seismographs or seismometers, which are typically placed at near and far locations of potential sites. Having sufficient number of the explicit sensors placed at strategic locations is essential for monitoring and accurate & timely detection of these events.
[0005] There exist several detection/sensing techniques for detection of seismic waves and prediction of resultant occurrences. However accuracy, infrastructure cost, complexity of implementation/installation varies and are major concern for seismologists involved with detection of such occurrences in priori.
[0006] Seismic waves are waves of energy that travel through the earth's layers, and can be result of an earthquake, explosion, or a volcano that imparts low-frequency acoustic energy. Many other natural and anthropogenic sources create low amplitude waves commonly referred to as ambient vibrations. Seismic wave fields are recorded by a seismometer, geophone, or accelerometer. The regions that are known to hold potential for such events are often monitored using a wide network of sensors that typically record velocity, displacement or acceleration of ground by recording and analyzing the seismic waves.
[0007] Seismometers/seismographs are instruments that measure motion/vibration of ground including those due to seismic waves generated by earthquakes, volcanic eruptions, and other seismic sources. A simple seismometer that is sensitive to up-down motions of the earth can be understood by visualizing a weight hanging on a spring. The spring and weight are suspended from a frame that moves along with earth’s surface. Any movement of the ground moves the frame. The mass tends not to move because of its inertia, and by measuring the movement between the frame and the mass, quantum of motion of ground can be determined. If a recording system is installed, such as a rotating drum attached to the frame, and a pen attached to the mass, this relative motion between the weight and earth can be recorded to produce a history of ground motion, called a seismogram.
[0008] Seismometers/seismographs are also used in other applications such as vertical seismic profiling (VSP), which is a method of determining acoustic wave characteristics of rock layers. The method includes lowering one or more sensors into a wellbore to a pre-selected depth. Typically several sensors are spaced apart to allow coverage over a preselected depth interval. A seismic signal is generated at or near the surface of the earth that propagates through earth and received by sensors. These sensors convert acoustic energy to sensing signals, which are transmitted to the surface for recording and processing.
[0009] In another application, high sensitivity seismic sensors are used to detect and analyse low intensity vibration of earth such as those created by movement of heavy vehicles or movement of military troops and thus have application in security and military establishments for use in border areas or/and protected zones.
[0010] Known seismometers/seismographs based on their sensing principle, can be generally classified into two groups, where first group works on the principle of inertia, and second group works on the principle of displacement or strain. The inertial systems are more sensitive to ground vibrations and are widely used in comparison with strain based meters.
[0011] As stated earlier, for detection of geodynamic events such as earthquakes, volcanic eruption and landslides etc., an array of seismic sensors is placed at different distance from the potential sites/zones that read and record vibrations and that are analyzed to predict resultant unwarranted events. However, seismographs/seismometers especially those based on inertial system may not be quite compact or easy to deploy in remote areas or logistically difficult situations such as volcanoes.
[0012] Therefore, there exists need for improved seismic sensors that are compact, easy to deploy even in hostile environments, and can record low intensity vibrations for vertical seismic profiling of earth, and at the same time is accurate and reliable.

OBJECTS OF THE INVENTION
[0013] An object of the present disclosure is to resolve problems and disadvantages of conventional technologies as described above.
[0014] An object of the present disclosure is to provide a FBG based seismic sensor that is easy to install, use, and configure.
[0015] Another object of the present disclosure is to provide a device for reading and analyzing seismic vibrations of earth/rock using FBG sensor.
[0016] It is an object of the present disclosure to characterize propagating seismic waves.
[0017] It is an object of the present disclosure to provide a system that can detect direction of seismic wave propagation.
[0018] It is an object of the preset disclosure to provide a system that can calculate velocity of seismic wave propagation.
[0019] It is an object of the present disclosure to provide a high sensitivity seismic data reading means that can be used to detect movement of heavy vehicle or troops.

SUMMARY
[0020] Aspects of the present disclosure relate to displacement/strain based device and system for monitoring seismic vibrations that use Fiber Bragg Grating (FBG) sensor. Signal from FBG sensor can be used by a programmable FBG interrogator to detect geodynamic events including but not limited to earthquakes, volcano and landslides, among other like events. In another aspect, Fiber Bragg Grating seismic sensor (FBGSS) of the present disclosure is highly sensitive, simple, compact, easily and widely deployable even in logistically difficult environment. Another aspect the present disclosure provides a method for deployment of multiple FBGSS’s in specific patterns to characterize seismic source and its constituent wave propagation
[0021] In an aspect, present disclosure provides a system and method for detecting direction of seismic wave propagation, wherein the system includes one or more FBG sensor-based seismic vibration reading devices, placed in a rectangular arrangement, and a programmable FBG interrogator that receives data from the one or more FBG sensor-based seismic vibration reading devices, wherein the programmable FBG interrogator compares vibration reading of one or more FBG sensor based seismic vibration reading devices to determine the direction of seismic wave propagation. In an aspect of the present disclosure, FBG sensor-based seismic vibration reading devices can interchangeably be referred to as FBG devices or FBG sensors hereinafter.
[0022] In another aspect, the present disclosure provides a system and method for measuring speed of seismic wave propagation, wherein the system can include one or more FBG sensors configured at different locations, and a programmable FBG interrogator that receives data from the one or more FBG sensors, wherein the programmable FBG interrogator compares vibration reading of one or more FBG sensors to determine the speed of seismic wave propagation.
[0023] In yet another aspect, the present disclosure provides a system for detecting movement of heavy vehicle or troops in a particular area, wherein the system can include one or more FBG sensors.
[0024] Various objects, features, aspects and advantages of the inventive subject matter will become more apparent from the following detailed description of preferred embodiments, along with the accompanying drawing figures in which like numerals represent like components.

BRIEF DESCRIPTION OF THE DRAWINGS
[0025] The accompanying drawings are included to provide a further understanding of the present disclosure, and are incorporated in and constitute a part of this specification. The drawings illustrate exemplary embodiments of the present disclosure and, together with the description, serve to explain the principles of the present disclosure.
[0026] FIG. 1 illustrates an exemplary configuration of FBG Seismic Sensor (FBGSS) system in accordance with an embodiment of the present disclosure.
[0027] FIG. 2 illustrates an exemplary FBGSS fabricated in accordance with embodiments of the present disclosure.
[0028] FIG. 3 illustrates an exemplary experimental set up for validation and calibration of the FBGSS, in accordance with an embodiment of the present disclosure.
[0029] FIG. 4 illustrates an exemplary recording of responses of FBGSS and a commercial seismometer to simulated excitations in accordance with an embodiment of the present disclosure.
[0030] FIG. 5 illustrates plot of response of FBGSS to different excitations against those of commercial seismometer for the same excitations in accordance with an embodiment of the present disclosure.
[0031] FIG. 6 illustrates an exemplary response of FBGSS and a commercial seismometer to different levels of excitations simulated by dropping a ball from different heights in accordance with an embodiment of the present disclosure.
[0032] FIG. 7 illustrates an exemplary plot of response of FBGSS to different levels of excitations simulated by dropping a ball from different heights against those of commercial seismometer for same excitations in accordance with an embodiment of the present disclosure.
[0033] FIG. 8 illustrates an exemplary experimental set up for field tests for validation of FBGSS in accordance with an embodiment of the present disclosure.
[0034] FIG. 9 illustrates an exemplary peak to peak response of FBGSS and a commercial seismometer to different seismic excitations simulated by sledge hammer at different distances from seismometers in accordance with an embodiment of the present disclosure.
[0035] FIG. 10 illustrates an exemplary graph of recorded responses of FBGSS and a commercial seismometer against distance of excitation in accordance with an embodiment of the present disclosure.
[0036] FIG. 11 illustrates an exemplary response of FBGSS recorded during different seismic excitations simulated by sledge hammer at different distances against those of commercial seismometer 304 for the same excitations in accordance with an embodiment of present disclosure.
[0037] FIG. 12 illustrates an exemplary deployment pattern for detecting direction of arrival and speed of seismic wave propagation in accordance with an embodiment of the present disclosure.
[0038] FIG. 13 illustrates an exemplary experimental set up for validation of system for detection of direction of arrival and speed of seismic wave propagation in accordance with an embodiment of the present disclosure.
[0039] FIG. 14 (a) illustrates exemplary seismograms generated by FBGSSs deployed for detection of direction of arrival and speed of seismic wave propagation in accordance with an embodiment of the present disclosure.
[0040] FIG. 14 (b) illustrates exemplary focused seismograms generated by FBGSSs deployed for detection of direction of arrival and speed of seismic wave propagation in accordance with an embodiment of the present disclosure.

DETAILED DESCRIPTION
[0041] Unless the context requires otherwise, throughout the specification and claims which follow, the word “comprise” and variations thereof, such as, “comprises” and “comprising” are to be construed in an open, inclusive sense that is as “including, but not limited to.”
[0042] Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure or characteristic described in connection with the embodiment is included in at least one embodiment. Thus, the appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments.
[0043] As used in this specification and the appended claims, the singular forms “a,” “an,” and “the” include plural referents unless the content clearly dictates otherwise. It should also be noted that the term “or” is generally employed in its sense including “and/or” unless the content clearly dictates otherwise.
[0044] The headings and abstract of the disclosure provided herein are for convenience only and do not interpret the scope or meaning of the embodiments.
[0045] Reference will now be made in detail to the exemplary embodiments of the present disclosure, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers are used in the drawings and the description to refer to the same or like parts.
[0046] The following discussion provides many example embodiments of the inventive subject matter. Although each embodiment represents a single combination of inventive elements, the inventive subject matter is considered to include all possible combinations of the disclosed elements. Thus if one embodiment comprises elements A, B, and C, and a second embodiment comprises elements B and D, then the inventive subject matter is also considered to include other remaining combinations of A, B, C, or D, even if not explicitly disclosed.
[0047] Embodiments of present disclosure described herein relate to a device and system for real time dynamic monitoring of seismic vibrations that is based on Fiber Bragg Grating (FBG) sensor. FBG sensor is configured to transduce seismic vibrations into strain and generate corresponding signal that are interpreted and recorded by a FBG interrogator for further analysis.
[0048] In an embodiment, a displacement/strain based device for real-time dynamic seismic vibration detection is provided. The device can include a cantilever fixed on a portable base/stand, and a mass fixed on its other end, and thus configured to transduce seismic vibrations to strain variations. A FBG sensor bonded to the cantilever detects these strain variations and generates corresponding signal. FBG sensor of the device can be operationally coupled to a FBG interrogator that receives these signals for interpretation.
[0049] In an embodiment, present disclosure provides a system for detecting direction of seismic wave propagation, wherein the system can include one or more FBG Seismic Sensors (FBGSS) configured/placed/positioned in a rectangular arrangement, and a programmable FBG interrogator that receives data from one or more FBGSS, wherein the programmable FBG interrogator compares vibration reading of one or more FBGSS to determine the direction of seismic wave propagation.
[0050] In an embodiment, the present disclosure provides a system for measuring speed of seismic wave propagation, wherein system includes one or more FBGSS placed at different locations and a programmable FBG interrogator that receives data from one or more FBGSS, wherein the programmable FBG interrogator compares vibration reading of one or more FBGSS to determine the speed of seismic wave propagation.
[0051] In an embodiment, the present disclosure provides a system for detecting movement of heavy vehicle or troops in the secure area that uses one or more FBGSS.
[0052] Fiber Bragg Grating (FBG) sensors, that have advantages such as low fatigue, high sensitivity, and ultra-fast response, can be advantageously utilized for detecting and recording seismic vibrations. FBG is a periodic modulation of refractive index of a core of a single-mode photosensitive optical fiber along its axis. In implementation, when a broadband light is launched into an FBG, a single wavelength that satisfies the Bragg condition can be reflected back while the rest of the spectrum is transmitted. This reflected Bragg wavelength (?B) of the FBG can be given by
?B=2neff ?
where, ? is periodicity of grating, and neff is effective refractive index of fiber core.
[0053] Any external perturbation such as strain, temperature, etc. at the grating site of the FBG sensor can alter periodicity of grating and in turn change the reflected Bragg wavelength. By interrogating shift in Bragg wavelength, parametric external perturbation can be quantified. For example, the strain effect on an FBG sensor is expressed as,

where, P11 and P12 are components of the strain-optic tensor, ? is the Poisson’s ratio and E is the axial strain change [13]. The strain sensitivity of a FBG inscribed in a germania-doped silica fiber, is approximately 1.20 pm/µE.
[0054] FIG. 1 illustrates an exemplary FBG Seismic Sensor (FBGSS) system 100 in accordance with embodiments of present disclosure. The system 100 can include a flat, circular ring 102 made of moderately strong material such as perspex sheet. The ring 102 can have exemplary inner and outer diameters of 40mm and 50mm respectively and can be configured with a plurality of supports such as three legs 110-1, 110-2 and 110-3 (collectively referred to as legs 110 hereinafter) of say equal length for example 50mm so that the ring 102 is generally horizontal when the legs 110 are placed on a flat horizontal surface. Legs 110 can be made of same material as ring 102, and can be fixed to ring 102 by a suitable process compatible with material of ring 102 and legs 110. One end of a cantilever beam 104 can be made of thin sheet of thickness such as 0.3mm and of elastic material such as stainless steel with uniform width for example 8mm and length such as 35mm can be fixed rigidly to the ring 102. A dead weight 108 can be attached to other end of the cantilever beam 104. In the exemplary embodiment a metallic weight weighing 55g, can be fixed to the cantilever beam 104.
[0055] In an embodiment, the cantilever beam 104 with its one end fixed and other end having a dead weight 108 can effectively form means to transduce seismic vibrations to strain variations. For example, if above configuration of cantilever beam 104, ring 102, legs 110 and dead weight 108 is placed on a surface 112 experiencing vertical vibrations, ring 102 and legs 110 shall move vertically in sympathy with ground vibrations forcing fixed end of cantilever beam 104 to to move with them. However, dead weight 108 fixed to other end of cantilever beam 104 shall on account of inertia, tend to retain its position resulting in relative displacement between two ends. Relative displacement between two ends of cantilever beam 104 shall cause strain in cantilever beam 104, which can be picked up for recording the vibrations.
[0056] In another embodiment of the present disclosure, an FBG sensor can be used to pick up strain variations from cantilever beam 104. For this purpose, cantilever beam 104 can have a FBG sensor 106 mounted/placed over it such that it faithfully acquires dynamic strain variations on the cantilever beam 104. The dynamic strain sensed by the FBG sensor 106 can be correlated to the corresponding surface vibrations.
[0057] In another embodiment, quantum of dead weight 108 can alter sensitivity of the FBGSS. In the exemplary configuration employing cantilever beam 104 of exemplary size and material, a dead weight of 55g was found to be optimal as a compromise between the higher sensitivity and lower damping time.
[0058] FBG sensor 106 can be operationally connected a FBG interrogator 114 that can interpret the signal received from FBG sensor to determine dynamic vibration. Signal from FBG sensor 106 can be received by the FBG interrogator 114, which can interpret received data to get dynamic vibration reading. The FBG interrogator 114 receives data from FBG sensor 106, by pumping a band of wavelengths 116 and continuously receiving the Bragg wavelength 118 which indicates the strain variation in turn correlating to the surface vibrations 112.
[0059] FIG. 2 illustrates an exemplary experimental FBGSS 200 fabricated in accordance with embodiments of present disclosure. Depicted therein are ring 202, cantilever beam 204, FBG sensor 206, dead weight 208, and three legs 210-1, 210-2 & 210-3.
[0060] In another embodiment FBGSS system 100 configured in accordance with above embodiment needs calibration for correct interpretation of interrogated signal from FBG sensor 106. Calibration can require controlled environment and can be done using commercial seismometer such as GURALP CMG-6T1 that has response spectrum of 1–100 Hz, along with a suitable digital recorder such as KELUNJI.
[0061] FIG. 3 illustrates an exemplary set up for validation and calibration of the FBGSS system 100 in accordance with an embodiment of the present disclosure. Depicted therein are commercial seismometer 304, digital recorder 302, and FBGSS 100. Commercial seismometer 304 can typically record ground movements in three mutually perpendicular directions, essentially one vertical and two horizontal directions. However, since FBGSS 100 in accordance with the present disclosure can detect only vertical ground vibrations, only the vertical component of detected vibrations from commercial seismometer 304 can be used. Calibration can be done by placing both FBGSS 100 and commercial seismometer 304 alongside each other and dropping an iron ball to act as a seismic excitation source. The ball can be of fixed mass say 500 g and can be dropped from a height of say 1 m at a point approximately at a distance of 1 m from the two sensors.
[0062] FIG. 4 illustrates an exemplary response of FBGSS system 100 and commercial seismometer 304 in accordance with an exemplary embodiment of the present disclosure. The two graphs 402 and 404 depicted in FIG. 4 pertain to response of commercially seismometer 304 and FBGSS 100 respectively to ground vibrations generated by dropped ball of above exemplary calibration procedure. In the illustrated exemplary calibration experiment, each of the graphs 402 and 404 records five distinct seismic vibrations corresponding to five droppings of iron ball. Data from both sensors selected for computation is the peak to peak amplitude for individual excitation and not the absolute positive or negative response, since the FBGSS is a secondary sensor of the primary seismic vibration generated. In exemplary experiment, multiple trials are conducted by dropping the ball from the same height at the same distance from the two sensors to show the repeatability of the recorded signals.
[0063] Table 1 below shows the comparison of recorded peak to peak amplitude response from FBGSS 100 and commercial seismometer 304.

Seismic Excitation FBGSS (µ?) Seismometer(mm/s)
1 76.2 2.38
2 75.8 2.36
3 74.3 2.32
4 74.6 2.34
5 74 2.31
Table 1: Peak-to-peak amplitude response obtained from commercial
Seismometer and FBGSS

[0064] It can be seen from the Table 1 above that FBGSS system 100 is comparable in its performance with that of commercially seismometer 304. Moreover, FBGSS sensitivity can be changed by changing dead weight 108 as required for different applications and that’s an added advantage.
[0065] FIG. 5 illustrates plot 500 of peak to peak amplitudes recorded by FBGSS system 100 during different excitations against those of commercial seismometer 304 for the same excitations. The plot 500 clearly brings out the linear relationship between the two results. Further, slope of plot 500 represents sensitivity of FBGSS and determined as 32µ? strain variation per vibration velocity of 1mm/s. Also, the interrogator such as Micron Optics Interrogator (SM 130- 700), used to record response of FBGSS, has resolution of 1pm which translates to 0.83 µ?, resolution of the exemplary FBGSS based on above sensitivity works out as 0.038 mm/s.
[0066] In another embodiment, response of both FBGSS 100 and commercial seismometer 304 can also be checked by dropping ball from different heights to simulate different levels of excitations. In an exemplary experiment, ball was dropped from heights of 1.75m, 1.25m and 0.75m at a distance of approximately 1mt.
[0067] FIG. 6 illustrates peak-to-peak response 600 of two seismometers to different levels of excitations simulated by dropping ball from different heights. The two graphs 602 and 604 depicted in FIG. 6 pertain to response of commercially seismometer 304 and FBGSS 100 respectively to ground vibrations generated by dropping of ball from different heights.
[0068] Table 2 below shows the comparison of recorded peak to peak amplitude response from FBGSS system 100 and commercial seismometer 304 to simulated excitations by dropping of ball from different heights and again corroborates linear relationship between the two recordings.
Seismic Excitation FBGSS (µ?) Seismometer (mm/s)
1 112.72 3.46
2 86.76 2.68
3 65.78 1.99

Table- 2. The peak-to-peak amplitude response obtained from commercial
seismometer and FBGSS
[0069] FIG. 7 illustrates plot 700 of peak to peak amplitudes recorded by FBGSS 100 during different excitations simulated by dropping ball from different heights against those of commercial seismometer 304 for the same excitations. Slope of plot 700 is again found to be 32, and validates earlier found sensitivity of FBGSS as 32 µ? for 1 mm/s vibration velocity and resolution of FBGSS 100 as 0.038 mm/s.
[0070] FIG. 8 illustrates an exemplary experimental set up 800 for field tests for validation of FBGSS 100 against commercial seismometer 304. During the exemplary experiment, FBGSS 100 and commercial seismometer 304 can be placed alongside each other and source of seismic disturbance can be shifted towards/away from the two in steps of 1m and responses of the two can be noted. A 5kg sledge-hammer can be employed as seismic excitation source by striking it on the ground.
[0071] FIG. 9 illustrates exemplary peak-to-peak responses of the two seismometers to different seismic excitations simulated by sledge hammer at different distances from seismometers. Two graphs 902 and 904 depicted in FIG. 6 pertain to response of commercially seismometer 304 and FBGSS 100 respectively. As expected, amplitude of recorded seismic vibrations decreases as source of seismic excitation is moved away from the two seismometers. This progressive decrease in amplitude with increasing distance from seismic excitation source is a clear indication of the sensitivity of FBGSS. It can be seen that the FBGSS almost mimics the seismometer in both laboratory and field conditions tested.
[0072] Table 3 below shows the comparison of recorded peak to peak amplitude response from FBGSS 100 and commercial seismometer 304 to simulated excitations by sledge hammer at different distances from two seismometers.
Seismic Excitation Distance from the sensors
(m) FBGSS
(µ?) Seismometer
(mm/s)
1 1 132.87 4.22
2 2 58.38 1.83
3 3 44.61 1.44
4 4 37.12 1.24
5 5 28.68 0.85
6 6 22.41 0.65
7 7 17.17 0.49
8 8 11.72 0.35
9 9 9.63 0.29
10 10 7.46 0.22

Table. 3. Peak-to-peak amplitude data acquired from FBGSS and commercial seismometer during field trial

[0073] FIG. 10 illustrates exemplary graphs 1002 and 1004 of recorded response from commercial seismometer 304 and FBGSS 100 against distance of excitation. It can be seen that peak-to-peak amplitude responses from the two seismometers go hand-in-hand, along the distance of seismic disturbance.
[0074] FIG. 11 illustrates an exemplary plot 1100 of peak to peak amplitudes recorded by FBGSS 100 during different excitations simulated by sledge hammer at different distances against those of commercial seismometer 304 for the same excitations. Performance of both seismometers is comparable and is in good conformance. The slope of this curve is found to be 31.17, which validates earlier computed sensitivity of FBGSS as 32 µ? for 1 mm/s vibration velocity and the resolution of the FBGSS as 0.038 mm/s.
[0075] In an embodiment, plurality of FBGSS 100 can be placed in a set pattern to detect direction of arrival (DOA) and speed of propagation of seismic wave. Time at which different FBGSS 100 detect a seismic vibration can be compared to detect the DOA and velocity of seismic wave propagation.
[0076] FIG. 12 illustrates an exemplary deployment pattern 1200 for detecting DOA and speed of seismic wave propagation in accordance with an embodiment of the present disclosure. The system includes one or more FBGSS 100 such as FBGSS 1 1204-1, FBGSS 2 1204-2, FBGSS 3 1204-3 and FBGSS 4 1204-4, collectively referred hereinafter as FGBSS 1204, placed in a rectangular arrangement. Distance between two FBGSS 1204 along the side of square can be say 5m. The encircled angle of 3600 around the four FBGSS 1204 can be divided into 8 octants of 450 each. An FBG interrogator can be configured to use exemplary Table 4 provided below to determine direction of arrival and velocity of seismic wave propagation. For example, if the first FBGSS that detects the seismic wave is FBGSS 1 and second is FBGSS 2, divided octant 1 can be determined to be the direction of arrival. Likewise if first FBGSS that detects vibration is FBGSS 2 and second is FBGSS 1, using exemplary Table-4, the FBG interrogator can determine that the octant of direction of arrival is octant 2.

Detection of Seismic Wave Octant of Direction of Arrival
First Sensor Second Sensor
FBGSS 1 FBGSS 2 1
FBGSS 2 FBGSS 1 2
FBGSS 2 FBGSS 3 3
FBGSS 3 FBGSS 2 4
FBGSS 3 FBGSS 4 5
FBGSS 4 FBGSS 3 6
FBGSS 4 FBGSS 1 7
FBGSS 1 FBGSS 4 8
Table 4: Exemplary table for determining direction of arrival

[0077] It is to be understood that pattern of deployment of FBGSS 100 for determining DOA of a seismic wave is only exemplary and those skilled in art can configure other patterns and all of them are well within the scope of present disclosure. For example, it is possible to have an octagonal pattern employing eight FBGSS 100 and divide encircled angle of 3600 around these eight FBGSS 100 into 16 sectors, each of 22.50 to get more precise DOA. In another example, distance between two FBGSS 100 along the sides of polygon can be increased to get better differentiation of delay in arrival of seismic wave or vibrations at two corresponding points. All such variations are well within the scope of present disclosure. In another embodiment, two or more such patterns can be deployed at geographically spread out places to assess location of seismic disturbance from point of intersection of two or more DOAs recorded at these locations.
[0078] FIG. 13 illustrates an exemplary experimental set up 1300 for validation of system for detection of DOA and speed of seismic wave propagation in accordance with above described embodiment of the present disclosure. As illustrated, there are four FBGSS 1302-1, 1302-2, 1302-3 and 1302-4 (collectively referred to as FBGSS 1302) all in operational communication with a FBG interrogator 1306 that collects data from one or more FBGSS 1302 and interpret it to get the direction and speed of seismic wave propagation. 5kg sledge hammer can be used to generate seismic excitation. In an embodiment there may be an external computer system 1304 such as a laptop connected with the FBG interrogator 1306. External computer system 1304 can get the raw data/ vibration reading from FBG interrogator 1306 and can use it for further analysis. In an exemplary embodiment, it is possible for the FBG interrogator 1306 to generate some alert signals and forward it to connected computer system 1304 that may be located locally close by or at remote site such as a monitoring room. In a typical installation there can be several such systems arranged at different places over a wide area that needs to be monitored and analyzed. To cover a wider area, a grid arrangement of FBGSS 1302 can be formed.
[0079] FIG. 14 (a) illustrates an exemplary seismogram 1400 generated by FBGSS 1302, in accordance with an embodiment of the present disclosure. It illustrates four plots such as 1402-1, 1402-2, 1402-3 and 1402-4 each pertaining to a corresponding FBGSS 1302. As it can be observed, all the four FBGSS 1302 detect an abnormal activity as highlighted by 1404 during time interval 14.80 to 15 sec. FIG. 14 (b) illustrates exemplary focused seismograms 1452-1, 1452-2, 1452-3 and 1452-4 of four FBGSS 1302 in accordance with an embodiment of the present disclosure. A closure study and comparison of these seismographs can be used to determine direction of arrival and speed of seismic wave propagation. As it can be seen, FBGSS 1302-1 detects an abnormal activity at time T2-14.809 sec., second graph 1452-2 of FBGSS 1302-2 depicts an abnormal activity at T1-14.805 sec., third graph 1452-3 of FBGSS 1302-3 detects an abnormal activity at T3-14.827 sec. and fourth graph 1452-4 of FBGSS 1302-4 detects an abnormal activity at T4-14.835 sec. By analyzing these seismographs, DOA can be easily determined. In the above exemplary data, generated seismic wave arrives first to FBGSS 1302-2 at T1 being 14.805sec and thereafter to FBGSS 1202-1 at T2 being 14.809sec. This implies that the DOA of the detected seismic wave is in Octant 2.
[0080] In another embodiment of the present disclosure, difference in arrival time (DIT) of seismic wave between two FBGSSs can be used to assess velocity of seismic wave propagation. In the exemplary data collected during the above experimentation, time difference in arrival of seismic wave at FBGSS 1302-2 (T1-14.805) and FBGSS 1302-3 (T3= 14.827) is (14.827 - 14.805) 0.022 sec. The wave takes this time to travel from FBGSS 1302-2 to FBGSS 1302-3, a distance of 5m based on time taken and distance travelled, propagation velocity can be calculated as 227.27m/s. Similarly DIT between FBGSS 1302-2 and FBGSS 1302- 4 (T4= 14.835s) is observed to be 0.03sec for travelling a distance of 7.07m, from which speed of propagation of wave works out as 235.66m/s. From these two values an approximate speed of propagation of 231m/s can be deduced for the generated seismic wave.
[0081] In another embodiment, a single FBGSS 100 can also provide DOA, propagation velocity and location of seismic source by obtaining the delay in arrival time between the P wave (primary wave) and S wave (surface wave) generated by the seismic source.
[0082] In another embodiment of present disclosure, dead weight 108 can changed to configure FBGSS 100 to make it highly sensitive to detect vibrations caused by movement of troops, or heavy vehicles. Such detection can be useful for surveillance of border areas or other such protection areas and can have applications in military and security establishments. In an embodiment of application, plurality of such highly sensitive FBGSS 100 can be deployed in patterns as disclosed in embodiment depicted vide FIG 12 to assess direction in which such movements are taking place.
[0083] The above description represents merely an exemplary embodiment of the present invention, without any intention to limit the scope of the present invention thereto. Various equivalent changes, alterations or modification based on the present invention are all consequently viewed as being embraced by the scope of the present invention.
[0084] As used herein, and unless the context dictates otherwise, the term "coupled to" is intended to include both direct coupling (in which two elements that are coupled to each other contact each other) and indirect coupling (in which at least one additional element is located between the two elements). Therefore, the terms "coupled to" and "coupled with" are used synonymously. Within the context of this document terms "coupled to" and "coupled with" are also used euphemistically to mean “communicatively coupled with” over a network, where two or more devices are able to exchange data with each other over the network, possibly via one or more intermediary device.
[0085] It should be apparent to those skilled in the art that many more modifications besides those already described are possible without departing from the inventive concepts herein. The inventive subject matter, therefore, is not to be restricted except in the spirit of the appended claims. Moreover, in interpreting both the specification and the claims, all terms should be interpreted in the broadest possible manner consistent with the context. In particular, the terms “comprises” and “comprising” should be interpreted as referring to elements, components, or steps in a non-exclusive manner, indicating that the referenced elements, components, or steps may be present, or utilized, or combined with other elements, components, or steps that are not expressly referenced. Where the specification claims refers to at least one of something selected from the group consisting of A, B, C …. and N, the text should be interpreted as requiring only one element from the group, not A plus N, or B plus N, etc.
ADVANTAGES OF THE INVENTION
[0086] The present disclosure provides a FBG based seismic sensor that is easy to install, use and configure.
[0087] The present disclosure provides a stand-alone, compact and light weight FBGSS which has the potential to detect seismic vibrations.
[0088] The present disclosure provides a device for reading and analyzing seismic vibration of earth/rock using FBG sensor.
[0089] The present disclosure provides arrangement for reading the seismic waves using FBG for detection of geodynamic events such as earthquakes, volcanic eruptions and landslides.
[0090] The present disclosure can characterize the propagating seismic waves using one or more FBG seismic sensor.
[0091] The present disclosure provides a system that can detect the direction of seismic wave propagation using one or more FBG sensors.
[0092] The present disclosure provides a system that can calculate the velocity of seismic wave propagation using one or more FBG seismic sensors.
[0093] The present disclosure provides a high sensitivity seismic data reading means that can be used to detect the movement of heavy vehicle or troops using one or more FBG sensors(s).