Comparison of GNSS Processing Methodologies for Subsidence Monitoring

Abstract

Gas production and the resulting reduction of reservoir pressure cause compaction of the reservoir formation. This is expressed as subsidence at surface, which e.g. requires measures to maintain the ground water level in the area above the Groningen gas field. Subsidence measurements are also used to
determine the compaction of the gas reservoir, which drives seismicity in the Groningen area. Monitoring of subsidence is therefore an important activity for NAM. Different techniques are used to monitor subsidence: levelling surveys, GNNS-measurements and InSAR satellite observations.

The Study and Data Acquisition Plan for Winningsplan 2016 (Ref. 1 and 2) included a research program into the monitoring of subsidence aiming to improve the processing and interpretation of the GNSS (Ref. 3 and 4) and In-SAR technologies (Ref. 5 and 6). The goal of the NAM GNSS Alternative Processing Methodologies project is to compare existing GNSS processing methodologies, to investigate potential biases in the solutions and to obtain transparent time
series estimates (decomposition of signals) for NAM monitoring stations, with the final aim to detect deformation trend changes with predefined confidence levels.

In the current report three GNSS processing methods have been investigated: State Space modeling (SSR, currently used by NAM), regional network processing with the Bernese GNSS Software (BSW), and Precise Point Positioning (PPP).