125
2013). Usually depths of about 50 m are examined, but greater depth penetrations
have also been reported, especially in Polar ice sheets (Arcone et al. 1995;
Dharwadkar et al. 2013; Swain and Goswami 2014; Swain 2018).
This work was carried out by attaching GPR instruments with different antenna
including 100 MHz, 200 MHz and 16–80 MHz ground-coupled antennas to the
snow vehicles (Fig. 3). These antennas were attached to a 16-inch survey wheel
(Model 620) (417 ticks per meter to denote distance). Necessary fabrication and
modifications were carried out to obtain power directly from the snow vehicles for
the instruments to function. Sometimes a 10.8 V lithium ion rechargeable battery,
which gave up to 3 h of survey time at ambient temperature, was also used when the
snow vehicles were not used. This survey was conducted by employing GPR systems with GSSI-subsurface interface radar (SIR-20) that consists of a transmitter
and a receiver. The transmitter generates a short, high voltage pulse and transmits it
into the antenna, which emits electromagnetic waves of a specific frequency into the
surrounding area. After receiving the signal from the control unit, the antenna transmits a series of mono pulses, each one wavelength long, which have the form of a
small negative, large positive and then small negative amplitude. The receiver collects the incoming signals, which are digital representations of the amplitude and
phase of the signal in a certain unit of time. GPR systems transmit a bandwidth of
frequencies equal to their centre frequency, i.e. the frequency where most energy is
emitted. The pulse length is inversely proportional to the centre frequency (Davis
and Annan 1989).
The radar parameters were set up in the field using the control unit of the GSSI
SIR-20 GPR system. Based on the Geophysical Survey Systems Inc. manual (GSSI
2003), the acquisition settings for the depth profiling were set to 64 scans per second, 512 samples per scan, 16 bits per sample resolution, 500 ns range, 100 kHz
transmission rate and five gain points. An infinite impulse response filter was used
(vertical high-pass filter 40 MHz, vertical low-pass filter 400 MHz) to reduce the
amount of external interference, thus increasing the actual signal and decreasing the
amount of background noise. Before starting the GPR survey, the survey wheel was
calibrated by laying a 10 m measured line on the glacier surface and pulling the
wheel to the predetermined distance. Data were acquired using RADAN 6.6 proprietary software and saved in RADAN’s DZT (.dzt) format.
A common mid-point survey was carried out on the Polar ice sheet south of the
Schirmacher Oasis using the 16 MHz frequency of the GSSI multiple low-frequency
antennas. The lower frequency was used to obtain deeper penetration. The common
mid-point data acquisition was carried out by placing the transmitter and receiver in
contact, followed by an incremental length of 1 m in each successive data recording
point to estimate the velocity, which was required to calculate the dielectric constant
of the glacier ice (Swain and Goswami 2014). Although the various subsurface layers showed different velocities, resulting in slightly different dielectric constants,
the radio wave velocity was 0.146 m ns
−1
over the whole depth of the glacier in this
region and the bulk dielectric constant was estimated to be 4.2 using the equation.
Glacier Stress Pattern as an Indicator for Climate Change
2013). Usually depths of about 50 m are examined, but greater depth penetrations
have also been reported, especially in Polar ice sheets (Arcone et al. 1995;
Dharwadkar et al. 2013; Swain and Goswami 2014; Swain 2018).
This work was carried out by attaching GPR instruments with different antenna
including 100 MHz, 200 MHz and 16–80 MHz ground-coupled antennas to the
snow vehicles (Fig. 3). These antennas were attached to a 16-inch survey wheel
(Model 620) (417 ticks per meter to denote distance). Necessary fabrication and
modifications were carried out to obtain power directly from the snow vehicles for
the instruments to function. Sometimes a 10.8 V lithium ion rechargeable battery,
which gave up to 3 h of survey time at ambient temperature, was also used when the
snow vehicles were not used. This survey was conducted by employing GPR systems with GSSI-subsurface interface radar (SIR-20) that consists of a transmitter
and a receiver. The transmitter generates a short, high voltage pulse and transmits it
into the antenna, which emits electromagnetic waves of a specific frequency into the
surrounding area. After receiving the signal from the control unit, the antenna transmits a series of mono pulses, each one wavelength long, which have the form of a
small negative, large positive and then small negative amplitude. The receiver collects the incoming signals, which are digital representations of the amplitude and
phase of the signal in a certain unit of time. GPR systems transmit a bandwidth of
frequencies equal to their centre frequency, i.e. the frequency where most energy is
emitted. The pulse length is inversely proportional to the centre frequency (Davis
and Annan 1989).
The radar parameters were set up in the field using the control unit of the GSSI
SIR-20 GPR system. Based on the Geophysical Survey Systems Inc. manual (GSSI
2003), the acquisition settings for the depth profiling were set to 64 scans per second, 512 samples per scan, 16 bits per sample resolution, 500 ns range, 100 kHz
transmission rate and five gain points. An infinite impulse response filter was used
(vertical high-pass filter 40 MHz, vertical low-pass filter 400 MHz) to reduce the
amount of external interference, thus increasing the actual signal and decreasing the
amount of background noise. Before starting the GPR survey, the survey wheel was
calibrated by laying a 10 m measured line on the glacier surface and pulling the
wheel to the predetermined distance. Data were acquired using RADAN 6.6 proprietary software and saved in RADAN’s DZT (.dzt) format.
A common mid-point survey was carried out on the Polar ice sheet south of the
Schirmacher Oasis using the 16 MHz frequency of the GSSI multiple low-frequency
antennas. The lower frequency was used to obtain deeper penetration. The common
mid-point data acquisition was carried out by placing the transmitter and receiver in
contact, followed by an incremental length of 1 m in each successive data recording
point to estimate the velocity, which was required to calculate the dielectric constant
of the glacier ice (Swain and Goswami 2014). Although the various subsurface layers showed different velocities, resulting in slightly different dielectric constants,
the radio wave velocity was 0.146 m ns
−1
over the whole depth of the glacier in this
region and the bulk dielectric constant was estimated to be 4.2 using the equation.
Glacier Stress Pattern as an Indicator for Climate Change