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In my WIP, I have an astropalaeontologist who uses a special device to scan the underground for any macro-size, i.e., visible with a naked eye, fossil remains. This scanner is self-driving and carries out the pre-programmed task autonomously; while slowly driving over the ground of the place of examination, it sends signals down through the ground and translates the reflected signal into a 3D picture, which is then relayed to the researcher's computer.

My question is: what kind of signal would this scanner use, and how deep would it penetrate the ground?

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    $\begingroup$ How deep would it penetrate the ground is highly-dependent of the power of the signal, which itself is more often than not dependent of its size. To answer this part, what size are you aiming for? Handheld device, fridge-sized, truck-sized? $\endgroup$ Commented Jul 19, 2022 at 8:05
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    $\begingroup$ @Tortliena I don't think that's necessary. It's Krišjānis' world. He's looking only for the mechanism to rationalize an action. What we might need today is something the size of a truck - but on his world, it could just as easily be a handheld device. The difference is reflected in the difference between the science-based and hard-science tags. $\endgroup$
    – JBH
    Commented Jul 19, 2022 at 10:28
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    $\begingroup$ Does "WIP" mean "Work In Progress"? $\endgroup$
    – Nat
    Commented Jul 20, 2022 at 17:58
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    $\begingroup$ How is that not describing ground-penetrating radar? If you can't work with that, why not make up an earth-infusing-beam? $\endgroup$ Commented Jul 21, 2022 at 0:57
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    $\begingroup$ @JBH Looking at the tag description now shown above my description of a question, I think probably "hard science" is the wrong tag, "science-based" would fit better. $\endgroup$ Commented Jul 22, 2022 at 8:32

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https://en.wikipedia.org/wiki/Ground-penetrating_radar#Archaeology

Radar has ability to scan for archaeological artifacts, and as been used to detect burial sites. Your scanner is radio transmitter and antenna, and would penetrate the ground depending on the material, from thousands of meters, to less than fifty meters.

It is rare but possible to detect fossils this way: https://www.sciencedirect.com/science/article/pii/S1631068312000796

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    $\begingroup$ Ah... rare for us! Not necessarily rare in the OP's world. But the link is wonderful for a proof-of-concept. $\endgroup$
    – JBH
    Commented Jul 19, 2022 at 10:29
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    $\begingroup$ Krišjānis, it's worth noting that while this answer is great from the perspective that there is a real-world device that does what you're looking for, a more precise answer to your question is "low frequency sound." Exactly what frequency depends, among other things, on the material you're trying to penetrate. In all other aspects, the mechanism is identical to RADAR or SONAR. $\endgroup$
    – JBH
    Commented Jul 19, 2022 at 10:32
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    $\begingroup$ "GPR uses high-frequency (usually polarized) radio waves, usually in the range 10 MHz to 2.6 GHz." But penetrate is a misnomer. If you want a signal to travel through the Earth, it needs to be ELF at "3 to 30 Hz", or ULF at "0.3 to 3 kHz" (Extremely and Ultra Low Frequency, respectively) - "thousands of meters" ? That's one big antenna, a massive power supply, or both (and for GPR Hz, possibly not even feasible). "ground penetrating radar can reach depths of up to 100 feet" $\endgroup$
    – Mazura
    Commented Jul 20, 2022 at 7:05
  • $\begingroup$ @Mazura, scientists in my story are only interested in the layer of the planet's surface where life or remains of life could be found. $\endgroup$ Commented Jul 27, 2022 at 6:28
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One of the best ways of mapping out what's underground is by using acoustic waves. They'll travel through pretty much anything and reflect off buried objects or interfaces between layers of rocks. All you need is a sensor sensitive enough to pick up the rather faint return signals.

A technology you could use here is distributed acoustic sensing. This involves laying a fiber optic cable along the ground and firing a laser through it. Details in the way the light scatters back up the fiber can be used to detect even the most faint, microscopic movement in the cable (including the tiny vibrations of an acoustic signal). Researchers have used this technology with unused, underground telecom fibers where it's proven sensitive enough to "hear" pedestrians walking through several meters of earth, and can even hear echoes from the seismic waves of earthquakes thousands of kilometers away. Geophysicists use this to perform vertical seismic profiling, where they bury a fiber vertically and use it to build detailed maps of underground formations and structures.

Your character is on the move so burying a fiber optic line isn't practical. Instead, have your automated survey vehicle lay out the fiber horizontally across the ground. It could then use something like a drop hammer to generate a seismic wave and listen for the results. Repeat this process from several different points and you should be able to build a detailed 3D map of everything that's underground. You won't get a very good signal without burying the fiber so your range will be somewhat limited, but you should still be able to measure far, far deeper than the deepest depths that your archaeologist could reasonably excavate.

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What kind of signal would the scanner use depends how far you want to penetrate into the earth and what kind of methods you are using to reconstruct the signal. Generally a pulse, or a tone, or a chirped pulse where the frequency changes during the pulse would be used.

The ground penetrating radar is sending an electromagnetic (low frequency radio wave) into the ground, and the amount of reflection you get back depends on the how conductive the material (dielectric constant) of the material. So it is looking for differences. Your fossil would be a slightly different material than the surrounding rock. It would also be sensitive to voids, or groundwater underground since it changes the nature of the material. The resolution is usually not that great, but good for archeology trying to find where buildings used to be, rather than small objects.

If you want to use acoustic methods, it would be more difficult and you would probably have the scanner put stakes into the ground to act as emitters and detectors. This is to match the impedance so the sound energy goes into the earth rather than reflecting off the surface (like putting jelly on an ultrasound probe). Again the resolution and depth would depend on the wavelength (frequency) of the sound. A shorter wavelength you can get higher resolution images, like the ultrasound used to monitor pregnancy, but the high frequencies don't go that far. But it might be a story element to be able to hear the scanner sending pings into the earth. When a submarine pings something underwater you can hear it and depending on the type of sonar may hear something like a short ping and echos, or something like a slide whistle. In your case, the signal might sound like a bunch of clicks, or as the scanner varies the depth it is looking at the pitch of the clicks or pings might be higher when looking at the surface and lower when looking deeper underground. For very large objects underground like salt domes or oil fields geologist will set off explosions and try to map out the structures.

For both the electromagnetic and acoustic methods to get really good images it takes a lot of computation. Usually image quality goes up with the number of detectors, or number of scans that get taken.

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  • $\begingroup$ For acoustic methods, is it important to have an atmosphere with a certain level of pressure (volume of gas)? Probably I should have mentioned that this happens on Mars? $\endgroup$ Commented Aug 3, 2022 at 8:14
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In archaeology we tend to use three different remote-sensing technologies:

  1. Ground Penetrating Radar (GPR)
  2. Resistivity
  3. Magnetometry

GPR will effectively detect the difference in density between two objects, and can detect multiple objects on top of each other. The depth is typically several metres and the horizontal resolution is tens of centimetres. Although the more money you pay the better the resolution. From these signals you can create a "pseudosection" which is a 2D slice or a 3D map of the subsurface as if you had dug it. Depending on soil conditions you might be able to spot something of a couple of handspans in size. So you could perhaps find a velociraptor fossil but not anything much smaller. Normally with GPR we look for things like grave cuts and coffins rather than the bones themselves (fossils are usually a bit too old for archaeologists - we leave that to the palaeontologists).

Magnetometry (well, fluxgate gradiometry which is what I have used) uses the difference in magnetic fields at the two ends of a tube, measured in 10ths of a nanotesla. This is good for detecting metallic things and burned things, but also stones that have been placed deliberately like a wall. The depth of penetration depends entirely on how strong the magnetic field is relative to the top of the tube. Solar weather can affect results, and once we even detected a low-flying fighter jet that flew over us!

Resistivity puts an electric current through the ground and measures the resistivity (which is resistance over distance). Typically this penetrates half the depth of the distance between the two probes, but that's more like it's focal point, it will detect changes in soil moisture at all depths but with a square falloff with distance from the focal point. This detects differences in water content and is great for finding filled-in ditches and postholes, wall foundations, grave cuts, that sort of thing.

There are other technologies used, but they tend to be of lower resolution and fairly specialised in what they are designed to detect. We generally use all three of the ones I've mentioned in concert, overlaying them and squinting at the results to find general features. The we combine that with our expertise to decide the most suitable places to dig to get results. But we tend to deal with the human, so we're looking for walls, ditches, and buildings rather than fossils which by their nature are made of the same stuff as the surrounding stone.

Searching for "archaeological prospection" and "archaeological remote sensing" will yield a lot of pictures of the current state of the art. And Historic England has a page describing these techniques.

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