How Synthetic Aperture Radar Doppler Tomography scans deep underground

The Khafre team are not using conventional SAR imaging in the way critics assume. They are using a patented form of SAR Doppler Tomography, pioneered by Prof. Filippo Biondi. This is not simple surface imaging. It is a phase-coherent interferometric method that detects subtle Doppler frequency shifts caused by internal micro-vibrations within dense structures.

Instead of trying to penetrate rock, it “listens” to tiny seismic vibrations in the stone.

Biondi’s trick is to capture micro-motions. Tiny seismic or structural tremors slightly shift the radar’s frequency (Doppler effect). By analyzing these Doppler shifts across multiple SAR images, they can reconstruct a 3D tomographic image of what’s inside, like a CT-scan from space.

Prof. Filippop Biondi’s patent (PCT/EP2023/064345) explicitly describes processing “coherent vibrational Doppler information” in SAR to allow penetrating 3D imaging “over a depth of several kilometers”. In other words, it effectively turns the radar into a spaceborne sonar, using Earth’s natural vibrations to “sound” the subsurface, something ordinary SAR can’t do.

A peer-reviewed Remote Sensing paper describes using COSMO-SkyMed SAR data to map new shafts and chambers inside Khufu . This case study in a scientific journal shows the technique in action (with high-res 3D results!).

Beyond pyramids, the technique has practical uses. For bridges and infrastructure, Biondi’s SAR Doppler method can extract a structure’s “vibration profile” from orbit. That profile highlights cracks or damage. In one study the team applied it to Italy’s Morandi Bridge before it collapsed, SAR-based vibration maps showed unusual energy spikes right at the failing pylon.

They even imaged deep tunnels. The HarmonicSAR site reports they “detected for the first time the Gran-Sasso Physics Laboratory at 1.4 km below the Earth using SAR”. In other words, their tomography saw a known underground lab 1400 m under Italy! They’ve also done scans of mountain tunnels (San Gottardo).

Biondi was co-author on a 2016 Scientific Reports paper tracking Iraq’s Mosul Dam instability via SAR. That study used spaceborne radar to measure tiny ground motions around the dam over time. It shows that SAR micro-motion techniques can monitor slow structural shifts on a large engineering project.

In short, SAR Doppler Tomography isn’t ordinary radar, it’s like using satellites and the Earth’s own background hum to “see” underground. Think of it as applying a CT-scan or ultrasound-like method from orbit. It’s unconventional, but it’s patent-backed and has some peer-reviewed results.

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