Computer-generated image of an underground bunker released by the Israeli army. IDF/Twitter
Following the raid on Gaza’s al-Shifa hospital by Israeli army units on October 15, the IDF claims to have found evidence of tunnels beneath the hospital. A video released on November 19 showed a tunnel running under the Al-Shifa medical complex at a depth of ten meters, running 55 meters past what IDF sources said was an explosion-proof door.
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The entrance to the tunnel was reportedly exposed when a booby-trapped truck exploded in a garage on the hospital grounds. The IDF claims this is proof that there was a command and control center beneath the hospital complex.
The video has yet to be independently verified. Given the critical importance that such evidence will play in justifying a raid on a premises protected by the rules of war, such verification will be critical.
Claims had also been published just days before the tunnels were reportedly uncovered, citing a 2014 article claiming the complex had been uncovered. built in 1983 when Israel controlled Gaza. Again, this claim has not been investigated. Forensic investigation of the site and any data relating to construction work there will be of great importance.
As forensic investigators, we have extensive experience in underground exploration, including locating mass graves in Colombia, where – for ethical reasons – it is not possible to simply go in and start digging. It is therefore necessary to use a range of technologies above ground to collect sufficient evidence to conduct excavations in a potentially vulnerable area.
Each location is unique, but researchers typically use a phased approach. This may involve initial analysis of satellite and airborne remote sensing data, including photographs, near-infrared image datasets, historical and modern maps, and other available information. Geological maps and subsurface models can be combined with knowledge of the technology and resources available to those excavating tunnels, shafts and chambers.
The terrain beneath the Gaza Strip consists of rocks and sediments that thicken from the eastern Red Sea to the Mediterranean Sea. This makes the area suitable for tunnel construction, although dangers still exist in the form of fractures that displace rock layers and allow water to enter underground spaces (especially close to the sea).
Map of tunnels uncovered by the Israeli army after Operation Protective Edge in 2014.
Hi-tech tunnel hunters
This pattern of surface and deep geology would then identify areas for surface geophysics surveys, where active technology could be used to pulse signals and measure the responses before mechanical drilling or excavation to identify areas or objects of interest. Even relatively deep targets such as mine shafts and sinkholes have been located with this approach.
Specialized seismic and ground penetrating radar (GPR) geophysical surveys have been used to detect tunnels in Gaza. But this becomes increasingly difficult the deeper they are – some in the Gaza Strip are reportedly as deep as 70 metres.
To detect tunnels and bunkers under man-made structures, remote sensing methods often use instruments on satellites, unmanned aerial vehicles, and aircraft to provide data sets for analysis. Once interpreted, these can pick up disturbed versus undisturbed ground and pinpoint areas of high gases such as methane or radiant heat. But these would have to be very sensitive and interpretation is usually very difficult for deeply hidden targets. Entrances would be crucial to settling here – that is apparently the case at al-Shifa Hospital.
Microgravity methods have been used in buildings that could detect missing mass, such as tunnels and bunkers. For example, they have been used to locate burial sites under church floors and Egyptian pyramid tombs, as this technique has no depth limitation. Aerial gravity gradient surveys, using so-called microgravitmetry to detect targets, show potential, but in this case may not have the required resolution to detect bunkers at such depths.
Both the microgravity surveys, data processing and numerical modeling are used to calibrate the results to estimate what is causing the data signal. It is then important to accurately digitally remove the effect of the extra mass of buildings above ground and then look at what is underneath. The costs of this can be significant in terms of time and resources.
In the past, high-resolution GPR investigations have discovered unmarked cellars beneath commercial shop floors and hidden pipe bombs and detonators behind shower tiles in Northern Ireland. But GPR is limited to a depth of several meters in most cases, with metal reinforcements causing problems in the data.
Listen
An alternative approach is perhaps the oldest: listening to sounds from the underground. From listening to tunnels being dug (as they did on the Western Front during World War I), or detecting vibrational energy from sound sources, especially sound produced by tunnels being dug or by people themselves, walking, talking and making noise.
This also entails serious limitations. You’re essentially ‘listening’ to what’s happening beneath sometimes meters of ground – the vibrations – and it’s often difficult to tell the difference between the sound of low-energy seismic movements and man-made sources. It also takes time to process and possibly model the data.
There are various intrusive research techniques. This includes sinking slim borehole systems and inspection cameras on cables. But they are slower digging methods, especially when it comes to drilling through rock and reinforced concrete.
The final stage is visual inspection. To verify the function of the tunnel, shaft or chamber, examination of the equipment contained therein is required. A judgment must also be made as to whether the structure was physically capable of being used for military purposes – whether for the storage of weapons, as a bunker for troops to hide in or for a full-fledged command and control facility. control center with all equipment. , power sources and connectivity required for this.
The only real way to definitively confirm the presence of underground structures in Gaza is through physical investigation, which will take a lot of time and effort, especially given the reported depths below ground level.
This article is republished from The Conversation under a Creative Commons license. Read the original article.
The conversation
Jamie Pringle receives funding from the HLF, the Nuffield Foundation, Royal Society, NERC, EPSRC and EU Horizon2020. He is affiliated with the Geological Society of London. Jamie works for Keele University.
Alastair Ruffell receives funding from RCUK (NERC and EPSRC). He is affiliated with Queen’s University, Belfast, the Geological Society of London and the International Union of Geosciences
