Effect of the aperture on the SNR
Let's look at the detection of a fatigue crack (roughly a triangular shape 2-mm side) initiated at the borehole of a titanium beta component. The grain size is between 0.5 and 1.5 mm.
We compare apertures of 64, 32 and 16 elements all focused along the borehole at 65 mm. The probe is a 5 MHz, 64 elements, pitch 0.6 m. We use a multi-group configuration on the Gekko (64 elements on the left, 32 in the middle and 16 on the right).
You can see the defect along the circle of the borehole . The big echo on the left is from the borehole. The ascans corresponds to the maximum amplitude from the defect.
As we can see, as the aperture increases the SNR improves as well. A 16-element aperture cannot detect the defect, with a 32-element the defect could be missed as we only have a 2-to1 SNR and a 64-element gives a sufficient 12 dB SNR allowing the detection of the defect.
A bigger aperture can lower the minimum detectable in noisy materials by improving the SNR.