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

*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
 A: 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. 
