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Abstract

The invention is a method for monitoring subsidence of the sea-bed (14) of a survey area (8) caused by compaction of an underground hydrocarbon reservoir (1), and comprises the following steps: Conducting at least two series (S.sub.1, . . . ,S.sub.i, . . . ,S.sub.m) of time-indexed depth measurements (13a, . . . ,13n), with separation in time .DELTA. between the measurement series characteristic of a significantly detectable long-term change of seafloor elevation due to compaction to take place in the reservoir. Measurements are time-indexed and corrected for tidal depth variations. Depth measurements (13) are conducted on survey stations (2) arranged on benchmarks (6) which have settled in the locally consolidated seabed (14). To handle short-term depth variations several stationary time-indexed short-time local reference depth measurement series (19r) are conducted at short-term local reference stations (18r) at benchmarks (6) during each separate measurement series (S.sub.i). The reference depth measurement series (19r) are continuous for correcting each depth measurement (13) for short-time tidal depth variations occuring during each separate measurement series (S.sub.i). To monitor compaction or seafloor subsidence during the series of measurements, the depth measurements (13, 19r) are conducted relative to a depth measurement (13r) during each series (S) at reference station (9) arranged on the seabed (14r) at a distance from the reservoir (8) being sufficiently far to be unaffected by long-term effects taking place due to compaction in reservoir (1).

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