The guided wave testing report should be easily understood by facility owners. It should include the following information: table of findings, data plot with indications, and backup data.
Table of Findings
The table of findings includes basic information, data analysis information, test summary, result table as shown in the figure. The basic information includes site, pipe ID/location, pipe size, inspector, analyzer, client, procedure, inspection date, guided wave testing system, and analysis software. Data analysis information includes wave mode, velocity, reference percent reflection, attenuation, threshold level, and inspection range. The two values of percent reflection, attenuation, and threshold levels are shown because the data analysis of MsS guided wave testing uses data acquired at two center frequencies for detecting defects of different sizes. For a relation of defect size and operating frequency, please check relation between operating center frequency and defect size. The test summary includes details of test pipe condition, note of visual observation of pipe, and summarization of the test result. The result table includes symbols, distance, percent reflection (or percent cross-sectional area (CSA) relative to the total pipe wall cross-sectional area), and comments of all indications and geometric features in the data plot.
Data Plot with Indications
The indications of data were marked in the Time-Corrected Gain (TCG) plot. The TCG data plot is a mixture of both the distance amplitude correction (DAC) curve and original data. The TCG plot is simply made by compensating the data with the local attenuation due to large geometric features and general attenuation of the pipeline. The following figure shows TCG and DAC data plot. The DAC curve line in DAC data is at a constant level because the TCG data has compensated for the attenuation. In this plot, the 2-percent threshold line was marked with a green solid line in TCG plot and DAC plot.
The ultrasonic guided wave testing finds an indication with signal-to-noise ratio (SNR) of 2-to-1 (or 6 dB). The guided wave testing draws the threshold level at 2 or higher amplitude of noise level. All indications of threshold level or higher amplitude are automatically marked by the analysis software. The data analyzer should be able to find each indication and sort out signals from imperfect direction control and multiple reflections. The threshold level of guided wave testing should be lowered as the pipe diameter increases. The relationship between the cross-sectional area (CSA) of defect and defect depth should be considered when selecting the threshold level in guided wave testing.
The displaying indications in TCG plot has the following merits instead of displaying in DAC plot:
- The TCG plot amplifies the signal that is reflected from the far distance and thus shows the clear view of a signal
- The TCG plot allows checking both the signal and noise amplitudes which satisfies the ultrasonic testing rule of signal-to-noise ratio of 2-to-1
- The TCG plot shows the background noise level clearly so the inspection range can be decided with the background noise level
- The TCG plot can show the data clearly with percent reflection y-scale that is proportional to the CSA percent of a/the defect. (Refer to the relation between signal amplitude and CSA of defect).
Backup Data Plot
The last page of the report includes backup data plots that include rectified rf data and spectrogram data. The below figure shows two data sets acquired at two different center frequencies: 128 and 64 kHz. The original data were plotted in TCG rectified data and spectrogram data. The original rf data shows the spatial resolution of MsS data along the X-axis. The MsS system acquires data at 1 or 2 cycles of tone-burst electric pulse so it has a high special resolution and a wide frequency spectrum as shown in the spectrogram plot below. The spectrogram plot is a 3 dimensional data plot where the amplitude is displayed in red-blue-green color with the red color being the highest amplitude. The signals from a real indication and geometric features are shown as a vertical line in the spectrogram plot because the data were acquired with fundamental non-dispersive (constant velocity at any frequency).