When performing a close interval survey (CIS), good practice has the surveyor using an oscilloscope to obtain a waveform reading. The main purpose of obtaining a waveform is to establish that correctly timed interruption is taking place. However, further analysis in the field or at the office can provide more insight into the rectifier operation and other electronic sources affecting the pipe. Examining frequency analysis using the Fast Fourier Transform (FFT) algorithm provides insights about the frequency of interference. Graphing the FFT results against time can confirm proper interruption in noisy circumstances where simple visual analysis fails.
Close Interval Surveys (CIS) are frequently performed on pipelines to measure the pipe to soil potential. While the instrument is connected to the pipe it is possible to obtain additional types of readings that can be analyzed to provide further information. Many modern electronic instruments are capable of functioning as oscilloscopes in addition to obtaining synchronized CIS readings. The information captured by the oscilloscope function provides a waveform for further analysis. The waveform is captured as a raw, unfiltered signal with at least 2000 data points per second for the duration of the interruption cycle or longer.
The introduction of computerized data loggers with graphical screens in the 1990's has allowed field surveyors to obtain, store and visualize waveforms in the field. While initially limited in display resolution and difficult to use, portable tablet and computer technology has expanded the potential to use waveforms. Modern survey instruments also record the waveform with ephemerous information such as the date, time and location taken. This allows the data to be recorded accurately for later comparison with internal and external data. However, a lack of knowledge and understanding of the usefulness of observing and storing waveforms has led to this tool being under-utilized on most surveys.
