Sacks-Evertson borehole extensometer

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Sacks-Evertson Borehole extensometers ( English bag-Evertson borehole strain meter ) are measured continuously, high resolution geodetic instruments for measuring volumetric deformation (extension, compression) of the earth's crust . The instruments measure relative volume changes in the nano range (0.01 mm / km or 10 −8 ) at a frequency of 60 Hz. They are primarily used in the monitoring and early detection of seismic and igneous activity in the subsurface and contribute significantly to a better understanding of the causes Processes and control mechanisms.

Construction and installation

A strain gauge consists of a steel cylinder that is approx. 4 m long and 11 cm in diameter. About 90% of the inside of the cylinder is occupied by a reservoir which is completely filled with silicone oil, an incompressible liquid. The upper end of this oil reservoir is coupled to a narrow expansion bellows. When the cylinder is compressed by pressure, oil is forced from the reservoir into the bellows. When the pressure is released, the oil flows out of the bellows back into the reservoir. The change in length of the bellows is measured and converted into an electrical signal, whereby the relative change in volume of the reservoir can be determined.

Strain gauges are installed at the bottom of an approx. 200 m deep borehole . The base of the borehole is filled up to approx. 5 m with expanding cement and the strain gauge is sunk into it. As the cement hardens, the extensometer is directly connected to the surrounding rock and can therefore record minimal changes in pressure that act on the rock. The electrical signal is transmitted to the earth's surface via cables.

Once installed, the sensitivity of each instrument is calibrated using tidal waves and seismic waves from major global earthquakes. The specific calibration factor obtained for the individual devices then guarantees the comparability of measured signals in a network.

History and dissemination

The strain gauges were developed and first developed by Dale W. Evertson and I. Selwyn Sacks in the late 1960s at the Department of Terrestrial Magnetism , Carnegie Institution for Science (Washington DC, USA) and Applied Research Laboratories, University of Texas (Austin, USA) Tested in Japan in 1971. Initially in explosion experiments and later by monitoring the earthquake activity, it was possible to show that Sacks-Evertson borehole strain gauges can record sudden, rapid compression / strain waves in the earth's crust significantly more precisely than the rod or wire-type tensometers that were conventional up to that point.

Since then, the instruments have been continuously developed and adapted to smaller drill hole diameters and developed as 3-component instruments. Today, Sacks-Evertson downhole strain gauges are installed in many igneous and seismically active regions around the world. These include the volcanoes Soufrière Hills (Montserrat), Etna , Stromboli , Vesuvius and the Phlegraean Fields (Italy), Miharayama (Japan), Katla and Hekla (Iceland) as well as seismogenic areas in California (USA), the Aegean Sea (Turkey), China and Taiwan. In a large-scale deep-sea drilling project, Sacks-Evertson borehole strain gauges have been installed off Japan for several years in order to enable the early detection of potentially destructive earthquakes.

findings

The many times higher temporal and volumetric resolution of deformations in the earth's crust by borehole strain gauges makes it possible to make signals visible that cannot be resolved with other observation techniques . This enabled significantly new insights into the processes that underlie and control seismic and volcanic activity. This includes:

  • Evidence of aseismic stress redistribution (offset along fault zones ) in earthquake regions as a forerunner of large seismic earthquakes (first detected with a strain gauge, later also with GNSS )
  • Identification of typhoons as a potential cause of aseismic displacement
  • Findings about control mechanisms of cyclical eruption dynamics (Hekla)
  • Discovery of a new trigger mechanism in Vulcanian explosions (Soufrière Hills)
  • detailed resolution of the structure of igneous systems ( magma chambers , tunnels, chimneys ) in the subsurface (Hekla, Soufrière Hills, Miharayama)
  • Detection of magma migration in the subsurface
  • Quantification of the vertical mass distribution in volcanic eruption columns due to pressure waves in the atmosphere
  • Early warning of volcanic eruptions (Hekla)
  • early estimation of the duration of sequences of Strombolian activity (Etna)

particularities

Due to the direct coupling with the earth's crust at a depth of two hundred meters, the instruments are not exposed to near-surface disturbances. In contrast to other measuring instruments, Sacks-Evertson borehole extensometers are calibrated in situ. The sensitivity of the sensors takes into account the nature of the immediate surrounding rock.

The effort and cost of installing Sacks-Evertson borehole strain gauges are very high due to the drilling required and the cementing of the devices in the ground. The devices cannot be removed again and used for other purposes. As a result, Sacks-Evertson downhole strain gauges are not currently standard in geophysical monitoring networks.

Web links

Individual evidence

  1. ^ A b Evelyn A. Roeloffs, Alan T. Linde: Borehole observations of continuous strain and fluid pressure . In: Daniel Dzurisin (Ed.): Volcano Deformation - Geodetic Monitoring Techniques . ISBN 978-3-642-51763-1 , Chapter 9, pp. 305-322 .
  2. ^ A b c I. Selwyn Sacks, Shigeji Suyehiro, Dale W. Evertson: Sacks-Evertson Strainmeter, its installation in Japan and Some Preliminary Results Concerning Strain Steps . In: Proceedings of the Japan Academy . tape 47 , no. 9 , 1971, p. 707-712 , doi : 10.2183 / pjab1945.47.707 .
  3. ↑ Japan Agency for Marine-Earth Science and Technology (JAMSTEC): Frontal Thrust Long-Term Borehole Monitoring System. In: Press Release JAMSTEC. 2018, accessed June 29, 2020 .
  4. ^ I. Selwyn Sacks, Alan T. Linde, Shigeji Suyehiro, J. Arthur Snoke: Slow earthquakes and stress redistribution . In: Nature . tape 275 , no. 5681 , October 1978, p. 599-602 , doi : 10.1038 / 275599a0 .
  5. ↑ I.Selwyn Sacks, Shigeji Suyehiro, Alan T. Linde, J.Arthur Snoke: stress redistribution and slow earthquakes . In: Tectonophysics . tape 81 , no. 3-4 , 1982, pp. 311-318 , doi : 10.1016 / 0040-1951 (82) 90135-4 .
  6. ↑ ChiChing Liu, Alan T. Linde, I. Selwyn Sacks: Slow earthquakes triggered by typhoons . In: Nature . tape 459 , no. 7248 , June 2009, p. 833-836 , doi : 10.1038 / nature08042 .
  7. ↑ Stefanie Hautmann, I. Selwyn Sacks, Alan T. Linde, Matthew J. Roberts: Magma buoyancy and volatile ascent driving autocyclic eruptivity at Hekla Volcano (Iceland) . In: Geochemistry, Geophysics, Geosystems . tape 18 , no. 9 , September 2017, p. 3517-3529 , doi : 10.1002 / 2017GC007061 .
  8. ↑ Stefanie Hautmann, Fred Witham, Thomas Christopher, Paul Cole, Alan T. Linde: Strain field analysis on Montserrat (WI) as a tool for assessing permeable flow paths in the magmatic system of Soufrière Hills Volcano . In: Geochemistry, Geophysics, Geosystems . tape 15 , no. 3 , March 2014, p. 676-690 , doi : 10.1002 / 2013GC005087 .
  9. ↑ a b c Erik Sturkell, Kristján Ágústsson, Alan T. Linde, Selwyn I. Sacks, Páll Einarsson: New insights into volcanic activity from strain and other deformation data for the Hekla 2000 eruption . In: Journal of Volcanology and Geothermal Research . tape 256 , April 2013, p. 78-86 , doi : 10.1016 / j.jvolgeores.2013.02.001 .
  10. ↑ a b Stefanie Hautmann, Dannie Hidayat, Nicolas Fournier, Alan T. Linde, I. Selwyn Sacks: Pressure changes in the magmatic system during the December 2008 / January 2009 extrusion event at Soufrière Hills Volcano, Montserrat (WI), derived from strain data analysis . In: Journal of Volcanology and Geothermal Research . tape 250 , January 2013, p. 34-41 , doi : 10.1016 / j.jvolgeores.2012.10.006 .
  11. ↑ a b Alan T. Linde, Osamu Kamigaichi, Masaaki Churei, Kenji Kanjo, Selwyn Sacks: Magma chamber recharging and tectonic influence on reservoirs: The 1986 eruption of Izu-Oshima . In: Journal of Volcanology and Geothermal Research . tape 311 , February 2016, p. 72-78 , doi : 10.1016 / j.jvolgeores.2016.01.001 .
  12. ↑ Peter G. Baines, Selwyn Sacks: Chapter 8 Atmospheric internal waves generated by explosive volcanic eruptions . In: Geological Society, London, Memoirs . tape 39 , no. 1 , 2014, ISSN  0435-4052 , p. 153-168 , doi : 10.1144 / M39.8 .
  13. ^ Alan T. Linde, Kristjan Agustsson, I. Selwyn Sacks, Ragnar Stefansson: Mechanism of the 1991 eruption of Hekla from continuous borehole strain monitoring . In: Nature . tape 365 , no. 6448 , October 1993, ISSN  0028-0836 , p. 737-740 , doi : 10.1038 / 365737a0 .
  14. ↑ A. Bonaccorso, S. Calvari, A. Linde, S. Sacks: Eruptive processes leading to the most explosive lava fountain at Etna volcano: The 23 November 2013 episode: Bonaccorso et al .: Etna most explosive lava fountain . In: Geophysical Research Letters . tape 41 , no. 14 , July 28, 2014, p. 4912–4919 , doi : 10.1002 / 2014GL060623 .