Review of Subsidence Analyses 1995-2015 & Proposal to New
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Review of subsidence analyses 1995-2015 & Proposal to new analysis procedure 2017 Prepared by: Well Engineering Partners, Hoogeveen Author: Version: 2 (final) Publication date: 4 January 2018 Approved: Frisia Zout B.V. Review of subsidence analyses 1995-2015 & Proposal to new analysis procedure 2017 v.2 Pagina 1 Review of subsidence analyses 1995-2015 & Proposal to new analysis procedure 2017 v.2 Contents Management summary .............................................................................................................. 3 Introduction ................................................................................................................................. 4 1 First production period 1995-1999 (Frima Zoutindustrie) .................................................. 4 2 Second production period 2000-2013 (esco/Frisia Zout) .................................................. 5 2.1 First phase of Gaussian subsidence modeling ................................................................ 5 2.2 Iterative Gaussian subsidence modeling ......................................................................... 6 2.3 Further subsidence-monitoring & analysis developments ............................................... 7 2.4 Interfering autonomous benchmark-subsidence .............................................................. 9 3 Third phase with Frisia’s monitoring & control protocol 2014 ........................................... 9 3.1 Execution of control cycles 2013 and 2015 ..................................................................... 9 3.2 Revision of 2015 control cycle ....................................................................................... 10 3.3 Iterative Gaussian modeling unsuitable to the purpose................................................. 11 4 New approach to benchmark subsidence contouring ..................................................... 11 4.1 Previous experience with conventional contouring ........................................................ 11 4.2 Flexible benchmark inclusion applied to Kriging interpolation ....................................... 12 4.3 From total subsidence to salt-extraction induced subsidence ....................................... 13 4.4 Geodetic reinforcement of benchmark survey network ................................................. 14 4.5 New layout for mapping GPS subsidence data ............................................................. 15 5 References ....................................................................................................................... 16 Attachment 1: Subsidence bowl cross section July 1999 ....................................................... 18 Attachment 2: Modified widening-parameter γ in 2004 ........................................................... 19 Attachment 3: Contribution of BAS-3 cavern convergence to total subsidence ..................... 20 Attachment 4: Vermilion gas production impact zone ............................................................. 21 Attachment 5: Benchmarks used in subsidence 2006 analysis iteration v14 ......................... 22 Attachment 6: Subsidence cross section from BAS-1/2 to BAS-3, 2006, iteration v14 .......... 23 Attachment 7: Merged 2006 Gaussian bowls BAS-1/2 and BAS-3, iteration v14 .................. 24 Attachment 8: Remaining subsidence 1995-2006 not attributable to salt extraction .............. 25 Attachment 9: Shifted GPS trendline based on iteration v14 .................................................. 26 Attachment 10: Monitoring & control cycle Frisia Barradeel II ................................................ 27 Attachment 11: Subsidence at the GPS location Barradeel (BAS-1/2) .................................. 28 Attachment 12: Remaining subsidence 1995-2015 not attributable to salt extraction ............ 29 Attachment 13: Primary benchmark subsidence 1988-2006 gas and salt production ........... 30 Attachment 14: Benchmark network of 2015 leveling survey and GPS locations .................. 31 Attachment 15: Holocene structure observation points in and around Frisia license areas ... 32 Attachment 16: Profile AA’ of Holocene layer structure above Frisia caverns ....................... 33 Attachment 17: Construction sketch of GeoDelft subsurface benchmark .............................. 34 Attachment 18: New layout of permanent GPS monitoring data ............................................ 35 Pagina 2 Review of subsidence analyses 1995-2015 & Proposal to new analysis procedure 2017 v.2 Management summary According to State Supervision of Mines, the growing systematic deviations between benchmark movements, measured by periodic leveling surveys, and Frisia’s Gaussian iterative forward modeling of land subsidence need to be addressed. In this report, Frisia has performed an in-depth analysis of the feasibility of the currently applied Gaussian modeling method. The methods, subsequently used by Frisia since salt production start in 1995 for measuring, mapping and monitoring land subsidence caused by salt extraction, have been inventoried and analyzed. Based on the analysis, Frisia has concluded that the Gaussian-based method has gradually become inadequate for reliably deriving the maximum subsidence above salt-producing caverns from benchmark movements. As spin-off, this learning phase from past experiences has generated a modified subsidence analysis method better fit for purpose. As follow-up method, Frisia now proposes to (re)start making conventional contourmaps of salt- production induced subsidence, using periodic benchmark height differentials corrected for autonomous movements not caused by salt production. A new element of the method is the ‘flexible benchmark inclusion’, applied to Kriging interpolation. This procedure enables the successive smooth inclusion of new, secondary benchmarks into the set of original primary benchmarks. Before contouring the specific salt-extraction caused subsidence, autonomous benchmark movements and gas production impact are removed from the benchmark-differentials registers representing the total regional subsidence. Frisia aims at correcting for autonomous benchmark movements on a very local scale and based on dedicated geodetic stability studies. The on-line monitoring of subsidence by means of GPS measurements at or near the deepest points of the three subsidence-bowls will be continued as usual. However, GPS reporting layout is modified, by entirely removing references to leveling-based subsidence results. Pagina 3 Review of subsidence analyses 1995-2015 & Proposal to new analysis procedure 2017 v.2 Introduction According to State Supervision of Mines (SSM), more investigations are needed to explain the growing systematic deviations between benchmark movements, measured by periodic leveling surveys, and Frisia’s Gaussian iterative forward modeling of land subsidence (refs.1, 2). Consequently, Frisia has performed an in-depth analysis of the feasibility of the currently applied modeling method (ref.3). Based on the analysis, Frisia has concluded that the method has gradually become inadequate for reliably deriving the maximum subsidence above salt- producing caverns from benchmark movements. This report provides an overview of subsequent methods used by Frisia since salt production start in 1995 for measuring, mapping and monitoring land subsidence caused by salt extraction. As spin-off, learning from past experiences has lead to a modified subsidence analysis method better fit for purpose. As follow-up method, Frisia now proposes to (re)start making conventional contourmaps of subsidence, using periodic benchmark height differentials corrected for autonomous movements not caused by salt production (ref.3). Additional analytical modeling is not needed anymore. 1 First production period 1995-1999 (Frima Zoutindustrie) At start of salt production from cavern BAS-1 in 1995, yearly salt production of 1.2 million tons was planned from cavern BAS-1 and from a next constructed nearby-cavern BAS-2. Land subsidence was predicted to be circa 0,5 cm per year in the deepest point of the merged subsidence bowls of the two caverns (ref.4). To monitor expected subsidence a network was set up in 1995, consisting of circa 50 fixed benchmarks. The initial benchmark heights were measured by second-order leveling techniques, according to instructions of the NAP department of Rijkswaterstaat. The heights (in m NAP) were adjusted relative to one stable reference benchmark by means of standard software MOVE3. The data were first checked by the NAP department before publication in an official survey register in compliance with mining legislation. In September 1997, a repeat leveling survey showed maximum height differentials relative to the 1995-baseline of up to 2.8 cm, whereas less than 1 cm was expected. Because of this early trend breach SSM urged immediate extension of the network and benchmark densification in the center of the combined bowl. The enlarged network consisted of circa 100 benchmarks and had a maximum diameter of circa 7.5 km. An early control leveling-survey in May 1998 confirmed the intensified subsidence trend above the twin caverns and revealed a bowl with deepest point of 4.8 cm. The fourth repeated survey in July 1999 showed a maximum subsidence of 11.4 cm. The subsidence development was only elementarily monitored. The amount of maximum subsidence was derived from height differentials of a limited number of representative benchmarks along a SW-NE line across the subsidence