Four Decades of Gravity Monitoring of the Groningen Gas Field

First, basics of modern microgravity monitoring and accuracies of alternative techniques are discussed. The history of gravity monitoring at the Groningen gas field is reviewed, with first survey aquired in 1978 and the most recent in 2015. The 2015-survey was aquired in a 24/7 operation, at a densified grid of 98 stations. Station uncertainties are estimated at 1.2 microGal (standard deviation). Gravity reductions of up to 84 microGal are estimated in the time-span 1978-2015, likely caused by the gas extraction. The most recent gravity survey on Groningen shows the feasibility of achieving 1-2 microGal in land surveys when using multiple relative meters, multiple visits, 24/7 operation, reference stations outside the reservoir and ties to absolute gravity stations.