Acoustic Correlating Logger Technology

ZCorr Digital Correlating Loggers

ZCorr is a portable leak detection system that combines a network of digital correlating loggers (DCLs), a computer docking station, and easy-to-use sound analyzing software. This technology enables us to plan the placement of the loggers, listen and record multiple soundings, and analyze the results on a laptop computer, making it possible to find leaks in situations where the acoustic ground microphone is unsuccessful. Because the theory behind this technology is rather complex, first we will explain how it is used to find a leak, then we will follow up with an explanation of the theory behind it.

Conducting a Correlating Logger Survey

The survey is broken down into the following 4 step process: (1) deploy, (2) retrieve, (3) analyze, and (4) designate. A detailed description of each step of the process is shown below:

How Digital Correlating Logger Technology Works

The ZCorr system has several patented innovations, such as synchronization (timing), data recording, processing, and PC software analysis. If you would like to learn more about how this technology works and don't mind a deep technical discussion, please continue reading. Otherwise, please turn back.

Real-Time Clock

At the heart of the ZCorr logger is an ultra low-power, temperature-compensated Real-Time Clock (RTC), which is used to synchronize loggers via the docking station. Over an 18-hour period, between deployment and retrieval for an overnight recording, an ordinary wrist watch would drift in time by up to 50 parts per million (ppm), i.e. about 3 seconds. In a correlation analysis this could lead to errors of 10,000 feet or more in the pinpointed location of a leak. Using a combination of ultra-low power electronic and software techniques, timing drifts are reduced by a factor of 2,500 times. The patented method of timing compensation restricts errors in leak location to typically less than 3 feet.

Recording

The ZCorr logger is fitted with a low-noise piezo-ceramic acceleration sensor, known as an accelerometer, which senses vibration and converts it to a tiny electrical charge. Sensitive conditioning electronics amplify and filter this electrical response. As with audio and other amplifiers the accuracy, precision, and intrinsic noise level of the amplifier is more important to the recording quality than the gain or volume level, per se. In an accelerometer, intrinsic noise that is often defined by resolution and gain is referred to as sensitivity. The ZCorr sensor has a sensitivity of 1V/g at a resolution of 0.025 μg / √Hz. Additional electronic gain gives a pre-digitizing sensitivity of 200 V/g.

The earth’s gravitational field is defined as having a constant acceleration of 1 g. In contrast, leak signals are sensed from pipes as a varying pattern of acceleration as small as microg’s, i.e. millionths of g’s. These are imperceptible levels of vibration to humans and require very low-noise, precise electronic amplification and filtering to preserve the leak signal. In addition, in a correlating logger the sensor is positioned very close to the logger’s electronics. Most real-time correlators can advantageously separate the sensor and recording electronics by a long cable. To overcome this potential interference problem, the ZCorr logger uses a custom-designed discrete amplifier and filter circuit, integrated very closely with the sensor. 

Digitizing of the analog signal is performed with a 23-bit ADC. Most acoustic loggers use between 8 and 12 bits, allowing a digitizing resolution of between 1 part in 256 and 4096. Vibration signals on pipes have a useful dynamic range of up to 80 dB, i.e. one part in 10,000. A 23-bit ADC, with a resolution of 1 part in over 500,000, is therefore adequate and necessary to preserve the fine detail of pipe vibrations and the leak signal.

Data Processing

After digitizing, the pipe vibration signal is encoded and saved in low-power memory. The ZCorr data processor is programmed to employ special encoding techniques in different situations. For example, different pipe materials (plastic, cement, or metal), large-diameter pipes, and long distances between loggers (with the presumption of very small leak signals), are all situations where the encoding is set to enhance the anticipated leak acoustic signature.

Data Downloading

Data is downloaded to a PC via the ZCorr docking station. A waterproof connector facilitates a high-speed data link between the logger, docking station, and PC. This is the final link in the chain of high resolution recording, processing, and data transfer for correlation analysis.

Data Analysis via ALFA Optimal Correlation

Itron has developed an advanced leak noise correlation method called ALFA, or Automatic Leak Frequency Analysis. ALFA works by identifying a leak acoustic signature in 2-logger recordings. The leak sound may be very different between the 2 loggers, both in sound level and frequency pattern, and is often not audible or otherwise discernible to a human user. The ALFA process identifies the acoustic signature of the leak sound at each logger and designs an optimal correlation filter that is unique for that 2-logger recording. The ALFA filter concentrates on the leak sounds and ignores other vibrations; for example, those due to normal flow, transient usage, and environmental sounds such as traffic. ALFA is particularly useful with large-diameter pipes, multiple leaks, and noisy environments. The graphic below shows a correlation analysis with peaks displaying the most likely location of a leak within a 720-foot span of pipe.

Another benefit of ALFA is enhanced productivity. Recordings in a zone with one or more leaks may readily produce a total of 28 correlations, one for every possible pair of loggers. ALFA produces 28 optimal correlations, each ranked by Correlation Score, without the need for time-consuming experimentation with manual filter settings.

Data Analysis via Enhanced Listening Analysis

Unlike listening loggers, which only store statistics, ZCorr stores actual CD quality recorded sound. A Leak Index (a 0 – 100 ranking) for each logger shows the sound levels present at the quietest time of the recording. As shown in the figure below, each logger is represented by a bar (red = abnormal leak index; green = normal). The listening and sound analysis provide additional capabilities that can supplement the correlation information.

Data Analysis via Enhanced Listening Analysis

Unlike listening loggers, which only store statistics, ZCorr stores actual CD quality recorded sound. A Leak Index (a 0 – 100 ranking) for each logger shows the sound levels present at the quietest time of the recording. As shown in the figure below, each logger is represented by a bar (red = abnormal leak index; green = normal). The listening and sound analysis provide additional capabilities that can supplement the correlation information.