The not useful in spite of their simple operational

The annual global death rate has been
increasing every decade, especially due to natural catastrophes like
earthquakes, tsunamis, cyclones and floods. As per the statistics, the largest
and most massive death tolls are caused from earthquakes, followed by tsunamis.
The initial 72 hours after the disaster referred to as the “Golden 72 Hours”,
if effectively used, can help to decrease the mortality rate to a great extent.
Hence, it is important to detect the presence of life in the trapped victims
during such emergency rescue operations so that maximum lives can be saved in the
limited time available, rather than wasting time in rescuing the dead victims. It
demands for an efficient life detection system to rescue the alive-but-trapped
victims faster and more effectively.

Besides detecting the presence of life
under the debris, other major factors which need to be taken into account for a
rescue system for life detection are its ease of operation, ability to
distinguish the presence of multiple subjects in the affected area and
capability to operate effectively in a noisy environment, accuracy of the
measured data, reliability of the system, issues of motion artifacts and
penetration depth i.e., reachability of the signal in terms of depth and
distance.

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Initial methods for rescue operations were
utilizing dogs, which would waste time since the victims detected may be
already dead. Then optical devices were developed for life detection. They had
the drawbacks of limited degrees of freedom, inability to be used in remote and
inaccessible locations as well as the requirement of experts to operate them.
Acoustic detectors like geophones require noise free operating environment
which is impossible in rescue and surveillance operations, and hence prove to
be not useful in spite of their simple operational model. Later, rescue robots or
radar robots using temperature sensors were developed, which could reach deep
inside the debris to search for trapped or buried victims. But once these
robots go out of range, they could not be tracked.

This paper gives a brief review on the conventional
life detection systems. Also, a new model is proposed using the non-ionizing
Far Infrared Rays (FIR) or terahertz rays. Conventional systems use ionizing
radiation of microwave and infrared regions of the electromagnetic spectrum,
which are hazardous to human health. The current proposal involves application
of FIR rays to detect the presence of alive victims under rubble, along with the
additional benefit that these rays are propitious to human health and
facilitate the subject to overcome illness and feel healthy i.e., there is
hardly any threat to health involved.

Section II of the paper briefs on the
existing life detecting methodologies and their drawbacks. The benefits of
terahertz rays and the design and principle of operation of the proposed system
are detailed in the sections III and IV respectively.

In a microwave Doppler radar sensing
system, the specialized radar transmits a microwave signal beam towards the
desired direction (debris). It then listens for the reflected beam from the
target and analyzes the phase or frequency variation in the received signal due
to the victim’s motion. According to Doppler theory, an object with
quasi-periodic movement modulates the phase of the transmitted signal by the
time varying position of the object and reflects it back. Since this system
detects the presence of life by analyzing the heart beat and respiratory
movements of the victim, the desired target is the victim’s chest. Therefore,
the reflected signal would contain the information about displacement of the
chest due to breathing and heartbeat 1.

Nevertheless, the reflected signal would
contain only the information of chest displacement caused by heartbeat alone,
if the victim holds his breath. In that moment, the heartbeat signal of the
subject could be well detected. However, heartbeats cannot be accurately
extracted when overlapped with breathing. This is because the infinitesimal
chest movement produced by respiration is stronger than the one produced by the
heartbeats, thus making it hard to separate the heartbeat signals when there is
an overlapping of both signals. Doppler sensing systems using signals of the
frequency range 800-2400 MHz as well as 10 GHz were proved in several works to
detect the cardiopulmonary motion of chest for relatively still or isolated
victims.

If there is random motion of the subject
or if other moving objects are present, say, people walking around, it is
almost impossible to extract the life signals accurately, as the amplitude of
the interference signals would be stronger than the minute movements on the
body surface of the subject. In order to overcome the background noise hence
created, and to improve the signal-to-noise ratio (SNR), different approaches
have been tried by researchers over time.

Though the use of high precision
amplifiers would improve the SNR, they fail to restrain the interferences.
Stationary signal processing methods like FFT and high order linear narrowband
filtering could be effective in improving the SNR except in complex situations
2. Another method uses Dual Filtering Algorithm (DFA), wherein two filters
and an algorithm to trace the spectral peaks of interference signals are used
to restrain the interferences; and one dimensional wavelet transform technique
is used to filter out the respiratory signal from the noise. Here again, the
effective extraction of heartbeat signal increases the computational cost 3.

Using multiple antennas for transmission
and reception could partly forbid the interferences, but it is complex and also
the cost of implementation is high 4. A compact rescue radar system is
proposed in 5, where a SAW oscillator generates signals of frequency 10 GHz.
Upon reception, the signal is processed by Independent Component Algorithm
(ICA). ICA extracts the heartbeat and respiratory signal information from the
backscattered data by removing noise and clutter.

An integrated solution based in a
heterogeneous network consisting of a through-the-wall radar sensor for detecting
and tracking moving targets and a vital sign detector (VSD) to determine the
presence of standing and living subjects is illustrated in 6. This method
uses a traditional quadrature demodulation technique along with a Multiple
Signal Classification (MUSIC) algorithm for detection of signs of life behind
walls. A modified version of this method is used in 7, where the phase
modulation caused in a continuous wave signal at 10 GHz is used to detect the
respiratory signals and by applying a suitable spatial smoothing decorrelation
strategy over the MUSIC algorithm, it significantly enhances the SNR.

However, microwaves cannot penetrate
metallic bodies and hence Doppler radar based sensors will yield very poor
results if the debris consists of such materials, for example, if the rescue
site is located in an industrial area. Acoustic sensors can overcome this
limitation.

Since sound waves can penetrate metal
walls, acoustic sensors are preferable to microwave based sensors. In 7, the
author demonstrates a high power mechanical acoustic through-wall sensor to
detect humans through the metallic walls of a cargo container. The drawback in
this method is the frequency produced, which is not sufficient to detect
victims at a faster rate and it can only produce waves of limited frequency
which are unable to penetrate deep.

Inspired from the aforementioned work, an
ultrasonic method for life detection is developed in 8. The mechanical method
is used to produce ultrasound waves which are then transmitted into the debris.
The reflected signals are analyzed for the respiratory movements of the chest
using Doppler signatures. The transmitted frequency is then incremented with
each consecutive meter of depth to scan for the presence of live victims. The
depth of the trapped individual is calculated based on the frequency emitted at
that time stamp. The location coordinates are calculated and plotted using
Google Earth software, making the data globally available. The limitation of
this method is that the piezoelectric crystals are able to generate ultrasonic
sound only up to a certain limit, after which they get damaged and lose their
property. As a result, the effectiveness of this system is limited to just a
few meters.

Infrared (IR) beams can also penetrate
rubble and reach buried humans. Therefore by using high precision long range infrared
distance sensors with powerful LED, the distance from a victim can be precisely
measured and indicated to the rescue operators. The intensity of the reflected
light could be used to estimate the distance from the subject as shown in the
works 10-12. Also the inherently fast response of the IR sensors could be
helpful to enhance the real time response of a rescue robot 13.

A hardware prototype of the IR based life
detection system is proposed in 14, which consists of IR transmitter and
receiver units, life detection circuit and processing unit using ATMega18
microcontroller which is programmed in arduino compiler using C++. The system
operates in (i) IR distance meter mode to measure the depth at which the victim
is present and (ii) life detection mode to detect the heartbeat signal of the
victim. The IR signal gets absorbed into the victim’s body and the blood volume
changes in the tissue varies the intensity of the IR signal that is reflected
back from the victim’s skin. These intensity variations account for variations
in the received
voltage and hence help to determine the heartbeat rate.  

                                                                                                               

The world of terahertz is
an area of recent interest and research, yet very challenging 15. It is the
extreme region of the infrared band (hence called Far infrared rays or FIR rays) which is the “terahertz gap”
with wavelengths that lie between 1 mm to 1 µm. FIR rays are also called T-rays/ T-light/
Invisible light which occupy the frequencies from 300 GHz to 3 THz of the electromagnetic spectrum. Recent studies reveal that the FIR rays play an
important role in the formation and growth of living things and hence called
the “light of life”. The terahertz gap in the far infrared
region of the electromagnetic spectrum is shown in Fig.1.

In spite of the difficulties in the generation, transmission and detection of these tremendously high frequency signals 16, due to its undisputed benefits and hardly any ill-effects
to humans, researches are underway in
the applications that are yet unexplored in the fields of science,
industry and medicine 17. Current applications of
sub millimeter waves are in radio astronomy,
remote sensing, material
testing, medical imaging, dentistry, drug detection, security
scanning, non-destructive
testing and communications.

In this paper, a new model for life detection system is proposed
which uses FIR rays in order to detect the presence of life in the victims
trapped under debris and to locate them, subsequent to disasters like
earthquakes. Former approaches use microwaves and infrared rays which are hazardous
to human health. They induce issues like skin cancer, low and high blood pressure,
ischemic heart disease, slow pulse and even damage to the human immune system and
lead to other fatal health conditions. Moreover, they need to be extremely
sensitive to detect the heartbeat signals of the person, which may not be
optimal. The current proposal involves the application of terahertz rays
or T-rays that helps in detecting whether the entangled victim is alive or not,
besides locating them under the rubble. It also has the additional benefit that
T-rays are propitious to human health and facilitates the subject to overcome
illness and feel healthy. That is, there is hardly any threat to health
involved as FIR rays are non-ionizing radiation. The effects of T-rays on human
body are discussed in the section V.(A) of this paper.

                                                                                                           

      The life detection system proposed in this
paper has two sections; one for transmission of terahertz rays into the rubble,
and the other for reception and detection of the rays received. Fig.2 and Fig.3
show the schematic representations of these respectively.

For the transmission of FIR rays into debris, a
continuous beam of FIR rays need to be generated with a frequency of approximately
1THz using an appropriate terahertz emitter. Terahertz rays are generated in
the range of few milli watts power, and hence require to be amplified before
that energy is radiated by the antenna using an optimal
high power amplifier module.
The amplified beam is then given
to a suitable terahertz antenna in order to focus it on the rubble.

In the receiver section, the
FIR rays which are received back by the receiver
antenna are first
amplified and then fed into the terahertz detector. The
detector analyzes it for the variations in its properties
with respect to the transmitted
beam.

An ideal choice of terahertz source and detector
can be made based on the range of operating frequency of the device, its portability
and cost considerations.

A. Terahertz Source

To generate rays of 1 THz frequency, there are several sources
available. Various technologies used for terahertz generation
and their performances are reviewed in 18.

1) Dielectric Resonator Metamaterials:
A number of techniques for the fabrication and analysis of negative index
materials (NIM) to generate terahertz frequencies are listed in 19. Several
metamaterials were developed in the work, such as a NIM based on yttria
stabilized zirconia spheres, and terahertz metamaterials based on an array of
custom synthesized micro-spheres and an array of lithium tantalite micro-rods. The
size of dielectric resonator materials required at these frequencies will be in
the range of few tens of microns.

2) A Ring of Oscillators:
A ring of oscillators and the circuits coupling the oscillators to set the
frequency at which they will lock in, can be used as
the THz source. The signal emerges along the axis of the ring
and by adjusting the couplers separately they could aim the output, making it
possible to scan large areas with a narrow, high-powered beam. The power could
be increased by adding more oscillators to the ring or by using multiple rings 20. A detailed study on the structure of different types
of ring oscillators and their principle of operation are described in 21.

3) Terahertz MMICs (TMICs):
Monolithic Microwave Integrated Circuits that can operate up to THz frequencies
fully exploit the sub millimeter wave band. To make these THz MMICs, THz
transistors having maximum
oscillation frequencies larger than
1 THz are required. Two indium phosphide double-heterojunction bipolar transistor (DHBT) based TMIC amplifiers with
operating bandwidths in the THz range are reported in 22, both of which use
Teledyne’s 130nm InP DHBT transistors in the common base configuration. Results
of the work demonstrate that 130nm InP DHBT technology can be used to enable
sophisticated TMIC circuits for operation in the terahertz band. The increase
in maximum oscillation frequency of InP High Electron Mobility Transistors
leads to the increase in operating frequency of TMIC to approach 1THz (~850 GHz) which is reported in 23.

4) Schottky diode-based THz Sources: These sources rely on the nonlinearities of the Schottky diode to
produce harmonics of an input signal, thus generating power at integer
multiples of the drive frequency. Schottky diodes are typically paired in
systems with lower-frequency active devices such as Gunn or IMPATT (impact
ionization avalanche transit-time) oscillators, backward wave oscillators and
transistor amplifiers 24. Schottky diode-based components are used to extend
these active components into the THz.

Schottky
diodes integrated with compound planar antenna structures that radiate a frequency multiplied tone from the
received fundamental acts as sub-THz sources. These devices are termed as
multennas and their designs are optimized to frequency multiplication from
0.1-0.3 THz in 25.

Planar Schottky
diode frequency multipliers are one of the most employed devices for local
oscillator power generation at THz frequencies. Up to 2.4 THz can be generated
using these devices which are demonstrated in 26.

      A suitable power amplifier is to be used to amplify the T-rays since the power of the radiated
beam will be in a few milli watt range. Hence, in order to analyze the beam of rays
further in the case of a THz emitter which is not integrated with an amplifier
circuit or an antenna system, they must be amplified and then given to antenna. In the same way, the
received THz rays should also be amplified before analyzing them using the
detector.

C.
Terahertz Antenna

To
transmit the terahertz rays towards the debris as well as to receive the rays
that are reflected back, a properly designed antenna system is required; except
for those terahertz sources that have integrated antenna systems in them, like the
multennas. Owing to the very small wavelength of these waves, the antenna has
to be very small in size.

Reconfigurable
photo-induced Fresnel zone plate based terahertz antenna with two dimensional
beam steering and beam forming capabilities operating at 0.75 THz is designed
and beam steering up to ±120 is demonstrated in 27. Two double
bow-tie slot antennas operating at 0.1-0.3 THz and 0.2-0.6 THz are designed for
wideband millimeter wave applications in 28.

A novel design of MEMS technology based fully on-chip 3D helical antenna
which operates at 4 THz has been fabricated on a silicon substrate in 29. The
antenna has a metallic helix and a microstrip on the substrate for feed. In
addition to the tunable parameters of the helix which can be varied to improve
the performance of the antenna, it has a high gain of 17.6 dBi and wide
operating bandwidth from 2.8-4.4 THz which are the main characteristics of this
antenna.

Helical antenna is one suitable option that is proposed here for
relatively compact antenna geometry. The antenna’s geometry is wavelength
dependent, but is acceptable from several hundred MHz and higher, with the
upper limit being dominated by the high voltage operation. It offers a good
gain factor and can be operated as a narrow band or wide band device; and the
fabrication cost is also low.

Researchers have recently proposed many other antenna
configurations for sub millimeter waves. A terahertz antenna, devised by a
graphene dipole and two parasitic elements, which operates at 1.84 THz is
proposed in 30. By choosing appropriate values of chemical potentials applied
on the graphene parasitic elements, three operation states can be chosen in
order to dynamically control the radiation pattern of the antenna. Flat
dielectric metamaterials lens with synthesized gradient refractive index to
operate at terahertz frequencies is designed in 31. It is demonstrated to
focus an incident terahertz plane wave in its focal plane. For indoor
applications, a short range wideband monopole antenna operating at 0.67 THz with
-10 dB impedance bandwidth of 100 GHz is designed in 32.

D. THz Detector

Terahertz detector analyzes the received
rays and measures its intensity, amplitude, time, frequency and other
properties with respect to the transmitted T-rays. The variations in these parameters can be used to
determine whether the trapped victims are alive or not; and the depth at
which they are enmeshed. Direct detection
mechanisms and principles of room temperature THz detectors are reviewed in 33.
 

Terahertz
Time Domain
Spectrometer: THz-TDS is a spectroscopic technique
in which the properties of a material are examined with short pulses of terahertz radiation. The generation and
detection systems are sensitive to the variations produced by the sample material
on both
the amplitude and the phase of the
terahertz radiation. Novel concepts for portability and miniaturization of
THz-TDS is designed, developed and evaluated in 34.

                                                                                                                                                  

The high directivity beam of T-rays
transmitted by the antenna is directed towards the debris. These rays penetrate
through the
rubble, during which it may come across living beings and materials like stones,
bricks and concrete. Besides penetrating human bodies,
the terahertz radiation can penetrate diverse materials like clothing, paper, cardboard,
wood, masonry,
plastics and ceramics.

A. Effect of FIR rays on human body

Despite the recent technological
applications of Terahertz radiation in biology and biomedicine, very little is
known about its interactions with biological systems. Contrary to
X-rays, terahertz radiation has relatively low photon energy for damaging
tissues and DNA. Terahertz being non-ionizing radiation does not harm the
biological tissues.

Pure FIR rays are
studied to generate therapeutic effects to human health 35. Some frequencies of T-rays can penetrate several millimeters of living
tissue having low water content (e.g., fatty tissue) and then reflect back. These
rays can diagnose the differences in water content and density of the tissue.
Hence these methods could allow for the effective detection of epithelial
cancer by providing a safer and less painful system using imaging.

In the proposed system, the T-rays that are incident
on a live
human body will penetrate deep inside the cells (up to 4-7 cm inside the tissues) and are
readily absorbed by them. This is based on the principle that the living cells
of body and T- rays vibrate at the same resonant frequency. If not alive, then the FIR rays incident on the body will not be absorbed; instead, it will be just reflected back. This is because
dead cells do not resonate.

The optical
properties of skin and water, which are the primary constituents of biological
tissues of all living matter, are well-characterized to absorb terahertz
frequencies 36. As T-rays get absorbed in the cells,
they cause warming effects and raise only the core body temperature due its
thermal reaction with the tissues. The heating is not caused at the skin level.
T-rays facilitate an increase in
the circulation of nutrients and oxygenated blood.
This in turn makes the person feel healthy and the pace of metabolism
increases. The
cells, tissues and the organs get revived. The heated
cells and tissues re-radiate the absorbed terahertz radiation according to the
property of black body radiation. These re-radiated waves will be degraded in
intensity when compared to the intensity of transmitted T-rays. This difference
between these intensities indicate the presence of surviving victims
under the rubble and forms the
principle of operation of this life detection system.

Additionally, the depth at which the
victim is present under the rubble can be determined by analyzing the waveforms
in THz detector. For this, the time period between the first and the second
harmonics of the received rays must be analyzed, which can be used to locate
the victim. It is based on the simple distance-speed relationship given by:

                                                D
= C*T                                           (1)

where,

D = depth at which the victim is present

C = velocity of light

T = time duration between the first and the second harmonics of received
rays

Depending upon the depth calculated, the appropriate rescue method
that needs to be applied to haul and rescue the person can be decided.

B. Effect of FIR rays on non-living matter

As dead cells do not
resonate, they do not absorb the incident T-rays. The incident FIR beam is hence reflected back,
and as a result, there will be no variation in its received intensity (except for some clutter which can be
cancelled or removed). This clearly indicates the
absence of life which helps the rescue operators to move ahead
and search for live victims in other areas of rubble.