Noise reduction on ultrasonic images

Electronic equipments, cables and transducers used in NDT are interfered by noise which severely limits the ability to detect defects. Noise has different origin and nature, i.e., white noise which is generated by several sources; thermal originated from electronics components, impulsive noise produced by motors and/or switchers, or power supplies, etc. Sometimes the inspection process itself can be a source of noise due to residual echoes generated in a previous shot. It is a common situation where the inspected material has a low attenuation coefficient and the pulse repetition rate is high (reverberations). Therefore, when the interference is broadband, it occupies the same bandwidth as the ultrasonic signal, so its effects can be harmful when signals are weak to detect. As a result, conventional filtering techniques should be avoided and nonlinear methods which respect the integrity of the real signals must be applied.

A simple example was carried out as follows: a methacrylate block (100x80x98mm) is inspected into an immersion system using water as coupling medium (Fig. 1a). The block has three flat-bottom holes of 5 mm diameters which have depths of 25, 50 and 75mm  (Fig. 1b). The phased array probe is moved at constant low-speed (1 mm/s) over the piece in straight line (Fig. 1c), keeping a constant water path distance between transducer and block of 52mm (Fig. 1d). 

A SITAU 111 system was configured to perform a linear sweep (B-scan) with 32-elements sub-aperture of for emitting and receiving the ultrasonic beam, deflection is not used. The active aperture is moved into the adjacent element after each trigger to cover the full phased array dimension in y-axis direction. Each B-scan is taken by encoded position with 1 mm of resolution. Moreover, in order to get the C-scan representation gates functionality must be setup and enabled.

Fig. 1. a)- Experiemental setup. b)- Mathacrylate block scheme. c)- Top view. d)- Side view. 

The switching power supply used to control the immersion system motors in this experimental arrangement introduces a high level of noise (Electromagnetic Interferences - EMI). The interferences on the image are due to motors activities which handle the probe, giving a low quality C-scan image with a high level of noise.

Fig. 2. C-scan image interfered by switching power supply noise.

These interferences could have been reduced by applying an average filter. However, this processing function requires that noise indications do not have the same position in the N-acquisitions used for averaging. Otherwise the indication only will be reduced in amplitude but not totally removed. The EMI interference has a certain spatial diversity that depends on the noise density (given by the ratio between the number of samples with noise and the trace length). Thus, the probability that two EMI peaks have the same position in an average of N-traces usually varies between 1% and 25%, so the effect of averaging is limited.

Fig. 3. Image improvement using an average filter of 8 consequtive acquisitions.

The phased array system has a hardware implemented EMI filter algorithm which takes into advantage the different spatial-temporal distribution of the signal for reducing and cancelling interferences. This functionality was enabled and configured with 8 consecutive acquisitions to carry out the algorithm. As a result, false indications disappear when the EMI filter is turned on. The resulting image is practically the same as if it was obtained without noise source.

Fig. 4. Image improvement using the EMI-filter functionality.

This powerful tool available on some phased array technologies allows to simplify tasks to NDT operators, which frequently have to face up to noise interferences that commonly appears in industrial and/or automatic inspections and those which are generated due to reverberations. So this functionality makes easier inspect and evaluate possible defects.