Introduction

Conventional leak noise correlators are limited by:

• Their need for manual selection of discrete filters at the base station.
• Disturbance of the correlation from flow noise, usage, traffic and other environmental noise
• Loss of the leak noise correlation due to limited recording resolution

The need for discrete filters has significantly limited the usefulness of conventional leak noise correlators. A skilled operator must sometimes experiment with a selection of filters either to detect a leak or to pinpoint the leak accurately. Occasionally a leak is missed (does not correlate) because no single (parametric) linear bandpass filter is able to isolate a coherent leak sound at both sensors. This process can be time-consuming for the user and requires their attention after each recording. Multiple leaks present in one recording can be missed.

How ALFA Works

DigiCorr and ZCorr uses a system of Automatic Leak Frequency Analysis (ALFA) to detect and pinpoint leak sounds with no user intervention. ALFA is a digital signal process capable of isolating (multiple) leak sounds individually at either sensor and statistically optimizing their correlation at each possible value of time delay. It can be thought of as constantly scanning the data arriving at each sensor, looking for energy that is coherent in time and frequency between the two sensors.

Unlike conventional filters, ALFA is able to extract leak sounds from the recording even when much greater noise may be present at the same frequencies. It is able to collect leak sound energy from many different frequencies – not just a single frequency band - and to discriminate among multiple sources of leak sounds.

ALFA tracks the spectral patterns of leak sound over time to ‘tune in’ to the leak sound even when other noise dominates the recording. Once ALFA has registered the characteristics of the leak sound, the results of the correlation process will be significantly less susceptible to disruption by ambient noise, such as traffic, or changing flow noise due to usage variations. A key aspect of ALFA is that this tracking and characterization of leak sounds is performed uniquely and optimized in every recording.

The Benefits of ALFA

With ALFA an accurate registration and pinpointing of each leak sound occurs automatically with every recording. This includes multiple leak sounds within a correlation span and leak sounds that would have been lost in analog FM radio transmission). When no leak is present no correlation occurs. It is not necessary to re-record with many different filter settings to ensure that ‘no leak’ is truly present.
The benefit of ALFA is shown in Figure 1. The screen above and left depicts the digital correlation result over 869 feet of 4” diameter cast iron pipe. No correlation peak is observed. The screen below and right shows exactly the same leak noise data file but processed using ALFA filtering. A correlation peak indicates a leak at 91 feet from the red sensor.