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Repeatability of CSEM Measurements on Land and 4D Inversion Concepts
- Publisher: European Association of Geoscientists & Engineers
- Source: Conference Proceedings, 80th EAGE Conference and Exhibition 2018, Jun 2018, Volume 2018, p.1 - 5
Abstract
“We report on Controlled Source Electromagnetics (CSEM) measurements acquired across an oil field on land in three consecutive surveys. As conductivity changes in the reservoir structure are not expected for this time frame, the data sets provide an excellent basis to study the repeatability of such measurements over a time span of 2.5 years. Our results suggest that repeatability of CSEM measurements depends on source-receiver distances, component of the transfer function, source-polarisation, and relocation errors, in particular at sites close to the source. Best repeatability was observed for receiver stations at 2–4 km distance from the source and frequencies < 20 Hz. At these stations, TF phases and amplitudes usually agreed within ±1° and ±5 % between measurements. The aim of time-lapse inversion is to obtain a series of 3D conductivity models which represent individual snapshots of the conductivity structure and which allow to track changes in the subsurface. For the inversion of our time-lapse data, we use Minimum Support regularisation and a model weighting scheme giving larger model weights to an area loosely encompassing the reservoir structure. Our time-lapse inversion can recover subtle changes within thin layers at depth if the background resistivity structure is well known.”