Dr James Jena and the innovative optical fibre sensor at work.
The optical fibre sensor offers a solution to the dangers of geo-hazards and is the result of extensive research over three years at Nelson Mandela University in the Eastern Cape.
The team, led by physics lecturer Dr James Jena, a postdoctoral research fellow at the University’s Centre for Broadband Communication (CBC), included Dr Gaathier Mahed, Dr Tichakunda Chabata, Professor Moctar Doucoure and Professor Timothy Gibbon.
Their journal article titled “Monitoring and early warning detection of collapse and subsidence sinkholes using an optical fibre seismic sensor”, was published in Cogent Engineering in January 2024.
The device is able to detect changes in vibrational states and related instability before the actual collapse of sinkholes, as well as other geo-hazards, says Dr Jena, whose field of expertise includes physics, optic fibre communication and sensing.
“Geo-hazards are geological processes that pose a threat to the environment, human health and safety, such as earthquakes, landslides, tsunamis and so on. Apart from loss or injury to life, they cause financial loss through damaged buildings and homes, disruption of business and livelihoods, infrastructure repair, reconstruction or remediation, evacuation and relocation costs.
“The device eliminates the need to rely on unreliable visible signs on the ground, and gives an early warning for … prevention of structural collapse of buildings or land, or evacuation (notices) for population safety.”
Going to ground
Ground instability is associated with certain vibrational states, whose properties signify the degree of instability present, and this helps authorities to distinguish the applicable warning levels, from mitigation to evacuation, explains Dr Jena.
The research was conducted at the CBC’s fibre optics lab within the Department of Physics, assisted by Department of Geoscience Senior Lecturer Dr Mahed, who gave expert insight into Earth Science-related matters.
In essence, the study involved detecting changes in vibrational frequencies and sounds, which indicate the presence and evolution of developing sinkholes.
“Sinkholes result when caves forming underneath the ground surface collapse,” explains Dr Jena. “As a cave grows bigger, the sound coming from it differs in pitch and loudness: ‘emptier vessels make the loudest noise’.
“These different sound intensities and frequencies are picked up by the optic fibre buried in the ground, which measures the changes in the subsurface caves. The optic fibre is sensitive to vibrations, the effects of which are seen in the light that’s carried through the optic fibre cable, making every point through which the light passes, a series of distributed sensing locations.
“The expanding underground caves generate different sound waves, which are transferred to the light signal carried inside the optical fibre sensor through vibrations. The frequency information of the detected sound is extracted from the received optical signals and processed to trigger alarms when the shift, range and peak values increase beyond set instability thresholds.”
Certain frequencies are only seen when the underground soil structure becomes unstable, and this was detected when the monitored hidden cave grew bigger, leading to weakening of the ground surface.
Being able to monitor structural surface integrity in this way, by picking up changes in the earth’s substructure – a metre to a few tens of metres below the ground – is an exceptionally valuable process, he says.
A major cause of increased sinkhole prevalence is urban development, as it accelerates the processes leading to collapse by increasing the load on ground surfaces bearing structures, says Dr Jena.
“Secondly, human activities alter aquifer levels and ground water movement. This causes additional vibrations, (which further affect) the ground.”
Cash consequences
In South Africa, 2500 sinkholes and subsidence events were recorded by 2011 and approximately 98% of them occurred in Gauteng, he says, with more events reported since then.
Within the same year, the estimated cost of damage caused by sinkholes was in excess of USD86.7-million (R1.6-billion), with further projected costs totalling USD600-million (R11-billion) to safely relocate around 30 000 households from a danger zone west of Johannesburg.
“Sinkholes have also caused extensive damages globally. In 1995, a 35m-wide sinkhole in central Italy resulted in damage costs totalling USD3.3-million (R55-million).
“Sinkhole prevalence in the past decade has by far superseded figures from the preceding decades, due to the expansive growth of businesses and urbanisation. The increased risk and cost have been a result of increased triggers from anthropogenic activities linked to residential, industrial and commercial development.”
More infrastructure is being constructed on top of a type of land called active karstic landscape, which is underlain by dolomite rock, composed mainly of carbonates of calcium and magnesium, which dissolve and wash away slowly over time, creating caves underground.
Sensor science
The device was built from a laser source, which sent light to the sensing fibre via a beam splitting interferometer and a directional coupler device which sends and receives light in one fibre then redirects it into another, says Dr Jena.
“The reflected light from the sensor returning through the interferometer and directional coupler output was received by a polarisation measuring instrument to track how the sound signals changed with time.
“These signals ultimately contained frequency information. The field is a unique transdisciplinary interlink between the Physics of Optics and Vibrations, and Geoscience.”
While the research focused on sinkholes, the device can easily be co-opted for earthquake and landslide monitoring, which makes it a valuable asset in detecting critical failure conditions before collapse, he says.
Currently undergoing further development for industrial field application, the device project was in 2022 awarded the inaugural Nelson Mandela University Graduate Pitch Competition to convert the laboratory design into a miniature prototype at higher technology readiness level.