# What instrument will measure voids in a brick wall non-destructively?

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
• other

The objective is to find these things out about the voids larger than $10 cm^3: • dimensions • locations Indications as to the resolution (accuracy / precision / confidence intervals) of the measurements, appropriate frequencies, and any constraints re penetrating depth would be very welcome - Did You ever rip down a brick wall? Even when there are no holes in the bricks for thermal insulation, there will be voids much bigger than 10 cm³ where mortar misses. (Mortar never fills a brick wall perfectly!). For that reasons all but X-rays will not work. – Georg Nov 3 '11 at 14:03 This is off topic because it's basically a shopping recommendation in disguise, not a question about physics. If you were to ask about what physical principles could be used to detect open spaces within a wall (which is more like what Alexander answered), then it might be more appropriate. – David Zaslavsky Nov 3 '11 at 17:36 Since the existing answer already addresses the question you "should" be asking, I'd say you're fine to go ahead and reword this one. Keep in mind that any mention of a budget (e.g. "less than £/$/€ 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 Zaslavsky Nov 3 '11 at 17:41
That's good enough that it doesn't need to stay closed, I suppose. – David Zaslavsky 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