Background Suppression in EnMAP Methane Retrieval Maps with a Gated U-Net Denoiser

High-resolution imaging spectrometers such as EnMAP can map methane point-source plumes, but retrieval maps often contain plume-like background artifacts associated with surface reflectance, snow, water, built surfaces, terrain-dependent optical paths, residual clouds and haze, and instrument effects. We present a gated U-Net denoiser that post-processes a single-band, ppm-equivalent methane enhancement map. The network predicts a pixel-wise retention factor that is multiplied by the positive part of the raw matched-filter map; it can therefore suppress or retain existing positive enhancement but cannot create or amplify it. Training combines semi-synthetic patches generated by injecting simulated methane absorption into real EnMAP L1C radiance, auxiliary EnMAP quality channels, and real-scene hard negatives. On a held-out set of 680 semi-synthetic patches, the selected gated U-Net reduced mean absolute background enhancement from 0.071 to 0.014 ppm and the false-positive rate at 0.05 ppm from 0.247 to 0.068, reductions of approximately 80% and 72%, respectively. Plume MAE decreased from 0.107 to 0.067 ppm, precision increased from 0.024 to 0.049, and average precision increased from 0.086 to 0.112. The denoised maps retained 70.4% of the injected plume-enhancement sum. Fixed-threshold recall decreased from 0.101 to 0.041 and FSS7 from 0.079 to 0.064, showing that weak plume margins remain vulnerable to suppression. The results demonstrate substantial background reduction with improved ranking and retention of most plume enhancement. Application of the frozen model to an independent EnMAP controlled-release acquisition retained a plume-like enhancement at the documented source, although the absence of a registered physical plume mask precluded pixel-level or emission-rate validation. The method is therefore a promising constrained post-processing step, but broader independent real-plume validation is required before operational screening or quantitative emission-rate applications.