Contrasting ground movements revealed by radar interferometry over abandoned coal mines (Campine, Belgium)

Transcription

1 Contrasting ground movements revealed by radar interferometry over abandoned coal mines (Campine, Belgium) Xavier Devleeschouwer*, Pierre-Yves Declercq*, Michiel Dusar*, Annekatrien Debien^ *Geological Survey of Belgium, Rue Jenner 13, BE-1000 Brussels, Belgium* ^KU Leuven student, Leo Meulemansstraat 56, B-3020 Herent, Belgium

2 Geological Survey of Belgium 2/15 Devonian/Carboniferous subcrop map of the Campine basin (Belgium)

3 Geological Survey of Belgium* 3/15 Colliery André Dumont Colliery Eisden Colliery Houthaelen Colliery Winterslag Colliery Zolder Voort mineshaft

4 Geological Survey of Belgium* 4/15 (1) (3) (2) 0 m -500 m m (1) Quaternary and Tertiary units (2) Cretaceous units (3) Paleozoic (Namurian to Westphalian)

5 Geological Survey of Belgium* 5/15 (1) (2) 0 m -500 m (3) m (1) Quaternary and Tertiary units (2) Cretaceous units (3) Paleozoic (Namurian to Westphalian)

6 Geological Survey of Belgium 102 scenes used from 23 April 1992 to 11 September 2005!Satellite data used from ERS1/ERS2 & ENVISAT (ASAR)!Number of PS identified: around 36,000 " PS density: highly variable PS/km!!Master Scene Date: 9 August 1998!Georeference (X,Y) accuracy: ± 4m, ± 10 m 6/15

7 Geological Survey of Belgium 7/15

8 Geological Survey of Belgium* 8/15 30 Annual average velocity (mm/yr) The variogram is being modelised from a gaussian model and a nugget effect (C0). Variogram C0 correspond to the!(h) value when h=0. It happens when PS are close but their respective VEL is different. According to the gaussian model equation the parameters are: C0=0.5 a=5000 C= Gaussian :! (h)= C (1-e " ^2 ) Variography and kriging in Campine Lag Distance (m)

9 Geological Survey of Belgium 9/15 Kriging interpolation in the Campine coal basin

10 Geological Survey of Belgium 10/15 Kriging interpolation and standard deviation contours in the Campine coal basin

11 Geological Survey of Belgium 11/15 Limits of the coal concessions superimposed to the kriging interpolation in the Campine coal basin

12 Geological Survey of Belgium* 12/15 Isolines of the estimated subsidence in the coal basin - less than 4 meters in the western subbasin - up to 8 meters in the eastern subbasin

13 Geological Survey of Belgium* Conclusions 13/15 - Contrasting ground motions were revealed in the Campine coal basin: - subsidence in the western subbasin (-4 -> -14 mm/yr) - uplift in the eastern subbasin (3 -> 23 mm/yr) -These ground movements are clearly centred on the coal mine regions - The main subsidence phenomenon is observed during the activity of the coal mine exploitation and occurs simultaneously with strong groundwater pumpings, - Residual subsidence is known a few years after the end of the exploitation activities, - After several years of stability since the groundwater pumpings were switched off, a groundwater mining rise is expected resulting in an elastic rebound of the previously subsiding basin, - The uplift is normally centred in the area influenced by strong subsidence values as those observed in the eastern subbasin and in the Dutch coal mines. -> the uplift (25 cm) amounts to 2-4% of the subsidence (Pöttgens & van Herk, 1995, 2000) for the Dutch mines -> the depth of the coal exploitation ranges between -400 and m (western side) and between 0 and -500 m (eastern side)

14 Geological Survey of Belgium Conclusions 14/15 -> the groundwater mining rise was monitored during years (since 1974 when the last coal mine stopped) For the Belgian coal mines, we can thus expect an uplift of 8-16 cm in the western subbasin and cm in the eastern subbasin - more or less concordant with the PS data in the eastern subbasin -> cm measured in the blue (uplift) cores (8-23 mm/yr) during only 13.5 years - not with the western side characterized by strong subsidence velocities (time series indicate progressive and continuous subsidence) A difference of 5 years in the closure dates cannot explain such contrasting ground movements ->> how to explain the subsidence phenomenon? residual subsidence still active (??) other groundwater pumpings in the Cretaceous and Tertiary strata (??) the Donderslag fault zone act as barrier and the rise in the western subbasin will start in the future with a specific rate (??) Need to go deeper in details, to collect other data: piezometric evolution in different wells, to use levelling and GPS station to compare the exploited coal panels with the PS data #

15 Geological Survey of Belgium* 15/15