Wavelength / signal which would pass through concrete and rubble

[Jack_A]

TL;DR Summary

What wavelength signal can pass through concrete and rubble, and be accurately located by a signal reading radar-like device.

Greetings,
Please imagine a huge pile of rubble, meters deep. Somewhere inside, there's a device emitting a certain signal.
This signal can be picked up by a radar-like device which locates the origin of that signal, with the least interference from the pile of rubble, concrete blocks, metals etc.

What would that signal be? Is there wavelength signal can pass through concrete and rubble, and be accurately located by a signal reading radar-like device.
Thanks

 

[osilmag]

Cell phones use microwave as their carrier frequency. Those frequencies are able to transmit and be picked up when I'm inside a concrete building. You would have to look into how the signal strength diminishes as it penetrates the depth of the concrete.

 

[hutchphd]

Muons

 

[Baluncore]

Welcome to PF.

The presence of random metal reinforcing, water pipes and electrical wiring will confuse short wavelength signals by reflecting and changing the phase and polarisation.

The signal radiated must therefore have a much longer wavelength than the depth of the rubble. The VLF transmitter antenna is usually a compact coil wound on a Ferrite rod, tuned with a capacitor.

To locate the source in the rubble, you would need to climb over and around the heap of rubble with a loop antenna, measuring the orientation of the receive antenna null, then computing the intersection of those planes.

Cave and mine radios work to communicate with VLF signals.
https://en.wikipedia.org/wiki/Through-the-earth_mine_communications

 

[TonyStewart]

If you want to actively detect some object by reflections at any wavelength, then radar-like device such as ground-penetrating radar (GPR), which uses a high-frequency electromagnetic pulse to detect and locate objects and structures beneath the surface. GPR is commonly used in construction, archaeology, and geology to locate pipes, cables, voids, and other subsurface features.

Whereas VLF is excellent for long distance and submarine communication but poor for location detection within <100 m as the wavelength is so long in km unless you were locating it from an airplane.

Friis Loss defines the loss in terms of Tx and Rx aperture areas and squares of wavelength and distance. Dry concrete is an insulator at microwave wavelengths unless it is wet and metals are conductors and reflectors. By scanning the area with diffused reflections but sharp gains in signal strength with proximity on the surface, one might be confused with metal and moisture reflections to mask those from interfering reception.

An ideal object has resonant reflection properties at a frequency like an omnidirectional RFID that reflects energy at the same frequency using magnetic coupling. Yet this lacks sensitivity to use the fringe fields of long wavelength magnetic fields. So my best bet is GPR.
https://www.bing.com/images/search?q=ground-penetrating+radar&form=QBIR&first=1

 

[Baluncore]

Whereas VLF is excellent for long distance and submarine communication but poor for location detection within <100 m as the wavelength is so long in km unless you were locating it from an airplane.

You are not considering the null orientation of a loop antenna on the surface. Before nearby excavation, a loop antenna is used to determine the position of VLF-excited buried cables and conductive pipes. That is the best technique available, and works at a range of about 1 metre.

The problem with GPR, at VHF or UHF, is that it is bent and reflected by the typical lengths of conductors found in the rubble.
Buried conductors that are short in wavelengths make inefficient scatterers, so do not reflect or bend the VLF field.

 

[TonyStewart]

You are not considering the null orientation of a loop antenna on the surface. Before nearby excavation, a loop antenna is used to determine the position of VLF-excited buried cables and conductive pipes. That is the best technique available, and works at a range of about 1 metre.

The problem with GPR, at VHF or UHF, is that it is bent and reflected by the typical lengths of conductors found in the rubble.
Buried conductors that are short in wavelengths make inefficient scatterers, so do not reflect or bend the VLF field.

You are correct. I used to design/make bear beacons, arrow beacons and Rocket beacons and VLF nav was my 1st design for global Doppler. I hadn't considered the sender orientation and depth for null tracking. But I did find a representative VLF square loop antenna.

 

[Baluncore]

I hadn't considered the sender orientation and depth for null tracking. But I did find a representative VLF square loop antenna.

That seems to be one appropriate design. The multi-turn loop can be any polygon or circle, but the loop should be in a metal conduit that does NOT close to form a shorted turn. The conduit forms an electrostatic screen, so detection is of the magnetic field, not the electric field.

The orientation of the under-rubble transmitter can be random. It is the orientation of the RX loop that makes 3D VLF position finding possible.

Here is one strategy. First walk around the heap, looking for somewhere safe with a good signal on the surface. Then orient the plane of the RX loop to be vertical, and turn to find the null bearing. You have then identified a vertical plane that contains the TX. Then hold the loop plane horizontal and tilt it along that earlier bearing, to find the dip angle for the null of the TX signal. That gives the intersection of two planes, defining a line, that passes through the TX.
Next, move to another good signal position on the other side of the heap, and repeat the process. That gives you the intersection of two lines, and so an estimate of the TX's 3D position, and an error estimate based on the closest separation of the two lines underground.