ON THE USE OF POINT TARGET CHARACTERISTICS IN THE ESTIMATION OF LOW SUBSIDENCE RATES DUE TO GAS EXTRACTION IN GRONINGEN, THE NETHERLANDS

Delft Institute of Earth Observation and Space Systems; Delft University of TechnologyKluyverweg 1, 2629 HS Delft, The NetherlandsV.B.H.Ketelaar@lr.tudelft.nlABSTRACTThe subsurface of the north-eastern part of the Netherlands contains large gas reservoirs that have been takeninto production since the early 1960s. The resulting subsidence has been precisely and reliably estimated usingmeasurements from repeated leveling campaigns and is now for the first time unambiguously observed usingPS-InSAR. The subsidence rates are small (< 1 cm/year) and the PS density is relatively low due to the ruralcharacter of the area. Particularly it is difficult to distinguish the PS deformation components due to gas extrac-tion from other possibly superposed deformation regimes. Besides a strict quality description of the estimatedparameters, the characterization of physical PS properties is therefore of major importance.In this paper we focus on the interpretation of PS-InSAR deformation estimates in Groningen by investigatingthe scattering properties of the detected PS. Through combining the shape of their complex spectrum and esti-mated heights from PS-InSAR processing, the nature of the PS is investigated. Important issues for performingthe scatterer characteristics analysis, such as coregistration and SAR amplitude calibration, are addressed.After analyzing the effect of temporal and perpendicular baseline and Doppler frequency variations, the PS-InSAR deformation estimates from different reflection types are evaluated on their performance for estimatingsubsidence due to gas extraction.1 THE GRONINGEN GAS FIELDThe north-eastern part of the Netherlands contains large gas reservoirs at a depth of approximately 3 kilome-ters below ground level. The largest is the Groningen gas field with a diameter of roughly 30 kilometers. Thereservoir itself consists of a 100-200 m thick porous sandstone layer. Since the early 1960s it has been takeninto production by the Nederlandse Aardolie Maatschappij B.V. (NAM). During the production, the reservoirpore pressure is decreasing resulting in a compacting layer. As the layers on top of the compacting reservoirsare subsiding as well, the ground level is subsiding. Surface deformation due to gas extraction has the shapeof a spatially smooth ellipsoidal subsidence bowl [1], depending on the depth and shape of the reservoir. Thesubsidence rate is approximately linear, with a decreasing velocity further away from the center of the bowl.The NAM is legally obliged to monitor the subsidence due to gas extraction, to be able to assess the influence ofgas extraction on water management and environmental issues. Hence leveling campaigns have been performedsince the start of production. Geodetic adjustment and testing techniques have been applied to estimate subsi-dence due to gas extraction from height difference observations. Besides tracing measurement errors and pointmisidentifications also tests on autonomous benchmark behavior are evaluated. The total estimated subsidencein the center of the bowl is 24.5 cm for the period up to 2003, which implies an average subsidence rate of 7.5mm/year.Currently, the application of PS-InSAR for estimating subsidence due to gas extraction is evaluated. Com-plicating factors are the low subsidence rates (< 1cm/year), the rural character of the area and atmosphericdisturbances. Besides these issues, a major challenge lies in the interpretation of the estimated PS velocities.Contrary to traditional geodetic techniques using well-defined benchmarks, the PS measurement point – theeffective reflectivity center – is less well known. As a result it may be difficult to discriminate which PS displace-ments are caused by a certain deformation regime (gas extraction, shallow compaction, structural instabilities).A way to gain more insight on PS properties, is to investigate if they can be classified in specular and dihedralreflections. With the objective to be able to discriminate subsidence due to gas extraction from other defor-mation regimes, the PS reflectivity patterns are analyzed. Methods as described in [2] are evaluated for theGroningen gas field.