Helicopter-borne Leak Detection Instrument Undergoes Field Testing In Windy Conditions

A new instrument called realSens™ has been designed by Synodon Inc. for the remote sensing detection of leaked natural gas.

It employs a well-established remote sensing technology known as Gas-Filter Correlation Radiometry (GFCR), also known as Correlation Spectroscopy. Operationally, the instrument mounts on a helicopter flying at an altitude of 300 m (1000 feet). It is a passive push-broom imaging instrument, using reflected sunlight to make measurements of ethane with a spatial resolution of 2 m (6 feet) on the ground.

This article details the first field test of the realSens™ instrument, which occurred in Southern Arizona. The results of these test showed that the instrument in its initial configuration is capable of detecting leaks of natural gas as low as 100 scfh.

Gas-Filter Correlation Radiometry (GFCR)
Gas-Filter Correlation Radiometry (GFCR), also known as Correlation Spectroscopy, is a well-established remote sensing technology, first developed in the 1960s and first employed for atmospheric remote sensing of the atmosphere from space in 1970 (Smith and Pidgeon, 1964, and Abel et al., 1970).

Since its invention, a number of practical realizations of GFCRs have been developed, including the original Selective Chopper radiometers (SCR), pressure modulator radiometers (PMR), and length modulated radiometers (Abel et al., 1970, Taylor et al., 1972, Drummond et al., 1978, Drummond, 1989, and Tolton and Drummond, 1997).

For the realSens™ instrument, a new form of GFCR has been developed, known as a Simultaneous View Correlation Radiometer (SVCR) (Miller et al., 2004).

The main principle of a GFCR is to use a sample of the gas of interest as a spectral filter for the gas. As a consequence, it combines the advantages of the high-resolution spectral selectivity (to the gas of interest) of a spectrometer with the energy grasp of a low-resolution radiometer. The principles of the realSens™ GFCR are shown in Figure 1. Incoming radiation is first passed through a narrow bandpass filter to select radiation from a spectral band of the gas of interest. The beam is then split by a beam splitter along two paths--one path containing a gas cell filled with the gas of interest (known as the correlation cell) and the other path containing no gas (known as the reference cell).

The radiant flux in each path is then measured by infrared detectors, and the signals are analyzed. The difference in the transmission along the two paths corresponds primarily to the absorption of the gas along the correlation cell path. Two signals from the detectors can be defined: an average signal (Savg) corresponding to the average radiant fluxes on the two detectors, and a difference signal (Sdiff) corresponding to the difference in the radiant fluxes. In operation, it is often convenient to define an instrument signal from a GFCR as the ratio of the Sdiff to the Savg (Sd/a), because it is unitless.