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Let's take what's ostensibly a solid brick wall. It looks solid from the front and the back. It's typically at least two widths of brick thick, plus some. What techniques could we use to non-destructively establish the location and dimensions of any voids (larger than, say, $10 cm^3$) within the brick wall?

So, the subject of scanning is:

  • brick & mortar
  • 9 - 50cm thick
  • with a surface area of several square metres

Options could include:

  • instrumented hammer & sound measurements
  • x-rays
  • ultrasound
  • ground-penetrating radar
  • other

The objective is to find these things out about the voids larger than $10 cm^3:

  • dimensions
  • locations

Indications as to the resolution (accuracy / precision / confidence intervals) of the measurements, appropriate frequencies, and any constraints re penetrating depth would be very welcome

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Did You ever rip down a brick wall? Even when there are no holes in the bricks for thermal insulation, there will be voids much bigger than 10 cm³ where mortar misses. (Mortar never fills a brick wall perfectly!). For that reasons all but X-rays will not work. – Georg Nov 3 '11 at 14:03
This is off topic because it's basically a shopping recommendation in disguise, not a question about physics. If you were to ask about what physical principles could be used to detect open spaces within a wall (which is more like what Alexander answered), then it might be more appropriate. – David Z Nov 3 '11 at 17:36
Since the existing answer already addresses the question you "should" be asking, I'd say you're fine to go ahead and reword this one. Keep in mind that any mention of a budget (e.g. "less than £/$/€ 100,000") or of non-operational properties of a specific device (e.g. "portable enough that it doesn't need a crane to manoeuvre it") is a sign of an off-topic question. – David Z Nov 3 '11 at 17:41
That's good enough that it doesn't need to stay closed, I suppose. – David Z Nov 3 '11 at 18:19

There are several ways to do that, none that I can think of at the moment that are easy, reliable and cheap at the same time:

  • Ultrasound
  • X-ray
  • Inductive systems
  • Radar

From these systems the the inductive monitors are available in most hardware stores but are not really suited to detect voids. They are regularly used to find pipes and electrical lines. So if your wall is made for example from steel reinforced concrete you might have a chance with these devices. The basically contain a coil and some electronics and measure the energy loss which is influenced by any conducting material nearby.

X-ray systems are routinely used to find small cracks and holes in metal rods, for example for oil pipelines (Pipeline Inspection). There are transportable devices available, they might reach your budget limit though. If X-ray is suited also depends a lot on your wall material.

Ultrasound is often used in non-destructive evaluation for all kinds of materials but the dampening in such a thick wall will make this approach difficult.

With radar you can basically see through the wall and depending on the frequency, antenna used and other parameters you might be able to detect voids but as far as I know this is more an research area than a reliable method which is currently used.

Other alternatives are even more obscure like low-field MRI, sending shockwaves through the wall, that are only used in very special circumstances.

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given that the material is brick & mortar, with varying degrees of moisture content, any thoughts on likely success with radar or ultrasound? – EnergyNumbers Nov 3 '11 at 13:22
@EnergyNumbers: A brick and mortar wall of up to 50 cm cannot be penetrated by ultrasound. With radar you will see something but I doubt it that it can detect a 10cm^3 hole. I would agree with Georg that X-ray is most likely the best choice as you need both spatial resolution and a long penetration depth. – Alexander Nov 4 '11 at 10:55

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