Acoustic Correlating Logger Technology

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 ZCorr Digital Correlating Logger Survey

The survey is broken down into 4 steps--deploy, retrieve, analyze, and designate. A detailed description of each step is shown below.

1. Deploy

A logger is placed on each line in question at both sides of the potential leak, and a series of field observations and measurements are made to gather the following information: the utility material, utility depth, and distance between loggers. This data is then be entered into a special software program on a laptop computer and the loggers are attached to each end of the line for one-half hour or, if the area is noisy, overnight. During this time, two sound recordings are made.

ZCorr Digital Correlating Loggers

2. Retrieve

After the the loggers have made the recordings at the user-programmed times, the vibration samples are saved into the memory of the loggers. The loggers then enter a low-power mode until they are retrieved in the field and reconnected to the docking station.

ZCorr Correlating Logger Deployment

3. Analyze

The loggers are collected, placed into a case, and the data is transferred to a laptop computer, where special software is used to analyze the sound recordings and calculate the leak’s distance from each correlating logger. A graph is then displayed, showing a peak between the loggers at the point where the leak is located.

ZCorr Correlating Logger Report

4. Designate

Once the leak's distance from each correlating logger has been calculated, the technician measures this distance from each point in the field to designate the location of the leak with pink paint and/or flags.

ZCorr Correlating Logger Leak Designation

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.


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 in 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.

ZCorr Correlating Logger Correlation Analysis

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 Analsysis

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.

ZCorr Correlating Leak Index Ranking

Advantages of Acoustic Correlating Technology 

  • Detects leak sounds incapable of being detected by the human ear.
  • Survey multiple lines at once for commercial or municipal water systems.
  • Sometimes detects leaks in non-metallic pipes.
  • Noisy areas can be surveyed by taking readings at night.

Limitations of Acoustic Correlating Technology

  • Not all leaking pipes produce leak sounds capable of being detected.
  • Cannot detect leaks unless the water pressure is high enough. 
  • Sometimes it cannot detect leaks in non-metallic pipes. 
  • Even at night some areas are too noisy to collect readings at.