An evaluation of airborne SWIR imaging spectrometers for CH4 mapping: Implications of band positioning, spectral sampling and noise

Rebecca Del’Papa Moreira Scafutto,Harald Van Der Werff,Wim H Bakker,Freek Van Der Meer,Carlos Roberto De Souza Filho

doi:10.1016/j.jag.2020.102233

Abstract

The development of instruments and methods to assist in methane (CH4) emissions mapping is essential to the fossil fuel industry. Early identification of local CH4 sources can benefit both petroleum hydrocarbon prospecting and environmental monitoring. Generic airborne imaging spectrometers operating in the shortwave infrared (SWIR) wavelength range (1000−2400 nm) have shown their suitability for this task. However, to date, there is no airborne scanner specifically designed for the detection of CH4 plumes. To overcome current handicaps and achieve better results at local scale, further investigation in sensor design is needed to evaluate which components can be adjusted to improve CH4 mapping with airborne sensors. Here, we focus on the evaluation of currently operational airborne imaging spectrometers for CH4 mapping in the SWIR range. Data acquired over areas with known CH4 emissions by scientific and industry-grade airborne hyperspectral sensors were examined. The research was conducted in three steps: analysis of sensor design, image processing and noise simulation. In the first step, differences in the spectral sampling between the sensors were analyzed. A reference CH4 signature from HITRAN spectral database was resampled to the spectral sampling of each airborne sensor. The center wavelength of diagnostic CH4 absorption features (identified in the convolved signatures) in relation to the position of band centers from each equipment was examined. For the image processing stage, a new CH4 index and a classic matched filtering were used to map CH4 plumes. To assess the impact of the signal-to-noise ratio (SNR) of the airborne sensors on CH4 plume mapping, white noise was added to the data to simulate images with varying SNR levels. Results demonstrated that the wavelength position of band centers is a key for CH4 mapping. The CH4 plumes could be mapped only with scientific-grade sensors, in which the band centers were closer to the center of CH4 features. Simulations with the addition of random noise demonstrated that a noisier signal is probably the reason why the industry-grade sensor tested here failed to map CH4 plumes, given that all instruments have a comparable spectral sampling. Furthermore, the simulations also demonstrated that the density of the plume has also a weight on the mapping of CH4 sources, once the image that captured the densest plume requested a higher addition of noise to be lost. The overall investigation indicates that a hyperspectral airborne sensor with bands properly positioned and scientific-grade SNR would better resolve the narrow CH4 features in the SWIR range.

Highlights

Methane (CH4) is a hydrocarbon gas relevant for petroleum explo ration and assessing environmental impacts
Looking at the simulated images from AVIRIS-NG (Fig. 9a) and AVIRIS-NG resampled to the IG sensor resolution (Fig. 9b), it can be noticed that the CH4 plume virtually disappears with additions of 7% and 5% of noise, respectively, which is consistent with the simulations done with the resampled HITRAN CH4 spectra (Fig. 10)
Three operational airborne imaging spectrometers were evaluated for CH4 plume mapping

Summary

Introduction

Methane (CH4) is a hydrocarbon gas relevant for petroleum explo ration and assessing environmental impacts. Mapping point sources of natural (geological) seepage and anthropogenic leakage as sists the fossil fuel industry in the exploration of new sites and the identification and control of gas leaks occurring in production, trans mission and handling sites. Asadzadeh and Souza Filho, 2017; Calin et al, 2020; Dierks and Kroll, 2017) Orbital sensors such as SCIAMACHY/ENVISAT (Bovensmann et al, 1999; Frankenberg et al, 2011) and TANSO-FTS/GOSAT (Kuze et al, 2020 and references therein), for example, provide global maps of CH4 concentration. Despite providing estimates of total column gas in the atmosphere, these sensors lack a spatial resolution suitable for mapping spatially-restricted CH4 plumes on land. The Methane Airborne MAPper (MAMAP - Gerilowski et al, 2011), for example, is a non-imaging spectrometer able to retrieve column concentration and gas flux estimates from CH4 plumes in small scales (meters). Neverthe less, this sensor is better suited for monitoring known CH4 sources rather than finding unknown seeps and leaks

Objectives

Methods

Findings

Discussion

Conclusion