GPR survey and field work summary in Siilinjärvi mine during July 2014

Transcription

1 GEOLOGICAL SURVEY OF FINLAND Groundwater Espoo /2016 GPR survey and field work summary in Siilinjärvi mine during July 2014 Samrit Luoma, Juha Majaniemi, Tiina Kaipainen, and Antti Pasanen

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3 1 Index Documentation page 1 Introduction 2 2 Study area 2 3 Geology of the study area 3 4 Methods GPR survey and interpretation Interpolations 9 5 Results and discussions GPR Interpretation Bedrock surface Quaternary deposit 13 6 Summary 16 7 References 16 Appendix 1: Interpreted GPR profiles Appendix 2: Photographs from field work in Siilinjärvi mine during

4 2 1 INTRODUCTION WaterSmart Project (Management of water balance and quality in mining areas), is a two-year project ( ) that has been mainly funded by Tekes, The goal of the project is to improve the awareness of actual quantities of water, and water balances in mine areas to give possibility to forecast the amounts of water in the future. The main aims are to exploit on-line water quantity and quality measurements, to develop mathematical models used to calculate water balance, and to sort out how the monitoring and modelling tools can be integrated into the management system and process control. GTK's responsibility is on WP2 for the data collection and monitoring of the geological and hydrogeological parameters that have to be collected manually. This report is one of the results from the WP2 by providing a summary and interpretation of data from field investigation in the Siilinjärvi Mine area during The objective of this study is to determine the bedrock surface topography and the characteristics of the Quaternary sediment based on the Ground Penetrating Radar (GPR) survey and the available hydrogeological data. GPR is one of the main geophysical methods that can be used to classify the sediment types by confirmation of data from a controlled wells and also indicate the internal structure of sedimentary deposit in the study area. GPR is based on a pulse of electromagnetic energy that is transmitted into the subsurface. The energy is reflected back from the electrical boundaries and the amplitudes and the two-way travel time in nanoseconds is recorded. The electrical boundaries are created at the interfaces of the geological materials with different dielectrical properties (Hänninen 1991; Pasanen 2009). The differences in dielectrical properties are mainly caused by changes in water content, lithology and material density (Hänninen 1991). For the porous and dry material, such as sand layers, the depth of investigation of GPR could reach about 50 m deep and in most cases the average depth of investigation ranges between m. However, in materials that has high conductivity, such as clay or high conductive groundwater, the attenuation of the signal is high and the penetration depths are low. The information of bedrock surface, bedrock geology, Quaternary sediments, groundwater level, and hydrogeological parameters received from the field investigation will be used for the groundwater flow models and the study of interactions between surface water and groundwater in the catchment area. These data are important for developing a better understanding of the occurrence and flow system of groundwater in the study area, groundwater discharge conditions and its interaction to the surface water, e.g. streams, rivers, lakes and wetlands, which are useful information for the water balance modelling and water management and planning in the mining area. 2 STUDY AREA The Siilinjärvi Mine is located in the eastern Finland, approximately 400 km north of Helsinki and 28 km north-eastern of Kuopio (Figure 1). The Siilinjärvi Mine is a

5 3 phosphate mine from the Siilinjärvi carbonatite complex (Puustinen, 1971). Open pit mining for phosphorus ore was started in Currently the production at the Siilinjärvi mine is about 9.2 Mt of ore per annum from the operations of two open pits: Särkijärvi in the south and Saarinen in the north-eastern of the mining area. The mining area consists of two tailings ponds, Musti and Raasio, located in the northern part of the mine area. The bulk of the tailings material is discharged and pumped into the Musti tailings pond. Water from these ponds and the drainage water from the open pit are recirculated to the concentration plant. The surplus water is treated with water purification chemicals and adjusted to ph 7 before release to the nearby lake. The rock/overburden and gypsum piles locate around the Särkijärvi open pit area, while the calcine pile locates nearby the factory, south of the open pit. Based on LIDAR data, the topographic elevation in the study area varies between 84.0 m a.s.l. and m a.s.l. with the mean value of m a.s.l. The topography decreases from north to south (Figure 2) in valleys close to the lakes. 3 GEOLOGY OF THE STUDY AREA Bedrock deposit in the study area consists of the Siilinjärvi carbonatite complex with was formed about million years and is one of the oldest carbonatite deposits in the world. The in situ grade of the ore is 4.2% phosphate (P2O5). The deposit mainly consists of three rocks: phlogopite- and apatite-rich glimmerite, syenite and carbonaterich carbonatite. The apatite bearing host rocks consist of a steeply dipping lenticular body roughly 16 km long, up to 650 m width, with a maximum depth of 1.5 km and a surface area of 14.7 km 2 (Figure 3).

6 4 Figure 1. Location map of the study area in the Siilinjärvi Mine - Yara Suomi Oy. Basemaps: National Land Survey of Finland. The Quaternary deposit overlying the Precambrian bedrock mostly consists of basal meltout till in the mining area and fine-grained sediments of silt and clay in the south around the lakes (Figure 4). 4 METHODS The study comprises of two parts: 1) field investigation with the GPR survey, GPR data processing and interpretation, and 2) interpretations of bedrock surface, Thickness of Quaternary sediments and hydrogeological data. Data used for the survey and interpretations consist of bedrock map scale 1: , Quaternary geologic map scale 1:20 000, LIDAR topographic map data, well information data including groundwater level, sediment description and geochemical analysis data received from Yara Suomi Oy.

7 5 4.1 GPR survey and interpretation The GPR survey was conducted during 8-18 July 2014 by using the Malå - Ramac ProEx equipment and Rough Terrain Antenna (RTA) with 25 and 100 MHz centre frequencies. The global positioning system (GPS) device was used for the horizontal positioning. A total of 41 km in 47 GPR survey lines was carried out throughout the main geological deposits and mining activities areas in the Yara Oy Siilinjärvi Mine including tailings, gypsum pile, calcine pile areas, and rock/overburden dumping areas (Figure 5). The GPR data processing and interpretation were done in the GeoDoctor (version 2.546) software. The data processing consists of the filtering to enhance the signal to noise ratio, topographic correction to compensate for changes in surface elevation, and depth conversion. Data from drilled wells such as top bedrock, descriptions of sediments, groundwater level and chemical analysis of groundwater were used for the calibration and interpretation of the GPR data. The 100 MHz antenna provides a better balance between the penetration depth and vertical resolution in this areas. Thus, the data set from 100 MHz antenna was interpreted and presented in this report. The interpretation results consist of bedrock top, and if the radar profile quality is sufficient, the internal structure of the Quaternary sediment that overlain the top bedrock surface.

8 6 Figure 2. Topographic elevation LIDAR map of the Siilinjärvi Mine area. Basemaps: National Land Survey of Finland.

9 Figure 3. Bedrock geological map of the Siilinjärvi Mine area. Bedrock deposit database Geological Survey of Finland. 7

10 8 Figure 4. Quaternary geological deposit in the Siilinjärvi Mine area. Quaternary deposit database Geological Survey of Finland and Basemaps: National Land Survey of Finland.

11 9 4.2 Interpolations Top bedrock surface elevation map Top bedrock surface elevation map was produced from the interpolation of the top bedrock data points from various data including top bedrock from GPR data, drilled well, outcrop, and bedrock polygon data from the Quaternary geological map scale 1: The data points were first interpolated using kriging with 5 m grid size for the whole study area using the ArcMap (version 10.1). The interpolated surface was calibrated with those top bed rock data points (the control points). For the interpolated surface areas that are shallower than the control points, the interpolated data was replaced by the control points and the top bedrock surface was re-interpolated using inverse distance weighting (IDW) method in the ArcMap program. Quaternary thickness map The Quaternary thickness map was calculated from the subtraction of the topographic map by the bedrock surface elevation map. The topographic map used was LIDAR 2mx2m grid, which was re-gridded into 5 m grid size in order to preserve the same grid size with the top bedrock surface. The distributions of the Quaternary sediments were interpolated based on the data from drilled well, soil samples, outcrop and the Quaternary geologic map scale 1: from map sheet number , (Huttunen, 2000; 2002). The sediments data provide important information on the hydrogeological characteristics, e.g. number of layers of an existing aquifers (sand, gravel), aquitards (fine-grain sediments, e.g. clay, silt, till) and it was also used to estimate percolation times and infiltration rates of water from surface to groundwater table based on the hydraulic properties of those sediments. 5 RESULTS AND DISCUSSIONS 5.1 GPR Interpretation GPR survey lines throughout the study area are shown in Figure 5. The interpretations of ground surface and top bedrock surface in each survey line is shown in Figures A1-1 to A1-3 in the Appendix 1. The GPR survey was conducted successfully in most areas and with the calibration from the top bedrock from well data, the bedrock surface can be identified (e.g. Figures 6&7). However, in some particular areas such as in the calcine pile and the gypsum pile areas, GPR survey lines show no reflections in the radar profiles (Figure 8). Well data in gypsum pile area indicate the Quaternary sediment is mainly clay. Both the calcine pile and the gypsum pile areas contain high electrical conductivity groundwater up to 1040 ms/m and 428 ms/m, respectively. Data acquisition and quality of GPR is limited by high conductive media, e.g. clay layer and high conductivity water content. Top bedrock elevation from wells and outcrop data were used for the bedrock surface interpolation.

12 Bedrock surface Bedrock surface elevation in the Siilinjärvi Mine area (excluding tailings, rock piles and quarry areas) varies between to m a.s.l. with a mean value of m a.s.l (Figure 9). Bedrock topography decreases from north to south, following the topographic elevation. Bedrock surface has a sharp contact with the overlying Quaternary sediment and in most places the bedrock exposures contains fracturing (Appendix 2). No fault zones were observed from the GPR profiles, however, based on the exposures in the open pits, interpretation of geophysical data, and bedrock geologic map, the fracture patterns can be observed in many directions including 325, and 12-22, with an overall fracture directions: 311, 0, and 18, follow a relatively broad deep fracture zone is located in the direction NW- SE. The fracture direction (311 ) is approximately the same as the most common direction of the Quaternary continental ice movements. This fact obviously increases the apparent frequency of this direction since fractures in this direction would have been more easily hollowed out by the moving ice (Puustinen, 1971). Based on those data the fault zones can be observed following the directions of streams, creeks and lakes trends (Figure 9). 325, and 12-22, with an overall fracture directions: 311, 0, and 18, following a relatively broad, deep fracture zone with a strike in NW- SE direction. The fracture direction (311 ) is approximately the same as the most common direction of the Quaternary continental ice movements. This fact obviously increases the apparent frequency of this direction since fractures in this direction would have been more easily hollowed out by the moving ice (Puustinen, 1971). Based on the data, the fault zones can be observed following the directions of streams, creeks and lakes trends (Figure 9).

13 Figure 5. Map presenting the GPR survey lines in Siilinjärvi Mine area during July Detailed maps of the GPR survey lines in the different areas are presented in Appendix 1. 11

14 12 Figure 6. GPR profile from survey line 632 along the west boundary of rock pile, south of the Raasio tailings pond. Red dots represent interpreted bedrock surface. Figure 7. GPR profile along the survey line 73, south-east of Sikopuron allas artificial lake. Red dots represent interpreted bedrock surface.

15 13 Figure 8. GPR profile along the survey line 56, north of the gypsum pile. 5.3 Quaternary deposit Thickness of Quaternary sediment varies between 1.05 m to 17.7 m with mean thickness of 2.30 m (Figure 10). The Quaternary deposit mainly consists of fine-grained till to basal melt out till which occupy approximately 60.0 % of the total area, while 13.8% is surface water (stream, rivers and lakes) and 11.1 % is bedrock exposures (cf. Figure 4). Fine-grained sediments such as clay, silt and peat are distributed sporadically along the watercourse areas and in valleys close to lakes, covering approximately 8.3 % of total area. Based on the well data, in many places the finegrained sediments overly the fine-grained till. The thickness of fine-grained sediments vary between less than one meter to 17.8 m, with mean thickness of 1.70 m. The thickest clay deposit was observed south of the Sulkavanjärvi Lake. Fine-grained till and fine-grained sediments have generally low permeability ( m/d), which may provide a low infiltration rate of water in to the ground. This probably can be observed also from the existence of rainwater puddles on ground surface for few days after raining (Appendix 2).

16 Figure 9. Bedrock surface elevation map (m a.s.l.) in the Siilinjärvi Mine area. 14

17 Figure 10. Thickness map of the Quaternary deposit in the Siilinjärvi Mine area. 15

18 16 6 SUMMARY This report summarizes the results of the GPR survey and field work during July 2014 in the Yara Suomi Oy - Siilinjärvi Mine, where the 41 km of GPR survey lines were measured throughout the mine area. Top bedrock information from GPR survey was integrated with geological data and well data to identify the bedrock surface elevation, which was later used for the geological and groundwater flow modelling of the area. Bedrock surface elevation varies between to m a.s.l. with mean value of m a.s.l. Bedrock topography decreases from north to south, following the topographic elevation. Bedrock surface has a sharp contact with the overlying Quaternary sediment. The bedrock exposures covers approximately 11.1% of total area and contains high fracturing. No fault zones were observed from the GPR profiles, however, based on the rock exposed in the open pits, interpretation of geophysical data, and bedrock geological data, an overall fracture directions in the study area are 311, 0, and 18, following a relatively broad deep fracture zone in the NW- SE direction. Thickness of Quaternary sediment is quite thin, varying between 1.05 m to 17.7 m with mean thickness of 2.30 m. It mainly consists of fine-grained till and it covers approximately 60.0 % of total area, while 13.8% is surface water. Finegrained sediments such as clay, silt and peat distribute sporadically along the watercourse area and in valleys close to lakes, covering approximately 8.3 % of total area. Fine-grained till and fine-grained sediments have generally low permeability, which provide low infiltration rate of water in to the ground. GPR is a useful tool for investigating the shallow sub-surface when used in the appropriate environment. In the Siilinjärvi mine area, GPR survey can be used to determine the bedrock surface and for characterising of the Quaternary deposits. However, in the areas that contain fine-grained sediments, e.g. clay, or high electrical conductivity groundwater, the GPR profiles show no reflections and cannot provide the information of the internal structure of the Quaternary deposit or top bedrock surface. 7 REFERENCES Hänninen, Pekka, Maatutkaluotaus maaperägeologisissa tutkimuksissa. Summary: Ground penetrating radar in Quaternary geological studies. Report of investigation 103. Geological Survey of Finland. Espoo. 33 p. Huttunen, Timo., Siilinjärvi. Suomen geologinen kartta 1:20 000, maaperäkartta, lehti Geologian tutkimuskeskus. Huttunen, Timo., Kolmisoppi. Suomen geologinen kartta 1:20 000, maaperäkartta, lehti Geologian tutkimuskeskus. Kauko, Puustinen, Geology of the Siilinjärvi carbonatite complex, Eastern Finland, Bulletin de la Commission Geologique de Finlande N: 249, 43p. Pasanen, Antti, The application of ground penetrating radar to the study of Quaternary depositional environments.res Terrae, Ser.A No. 29. Oulu.

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20 18 APPENDIX 1. GPR RESULTS Ground surface and top bedrock surface in each survey line as shown in Figures A1-1 to A1-3.

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27 25

28 26

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30 28

31 29

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36 Figure A1-1. GPR survey lines in the Musti-tailings area. 34

37 Figure A1-2. GPR survey lines around the Raasio tailings pond area. 35

38 36 Figure A1-3. GPR survey lines around the calcinate pile and gypsum pile areas. APPENDIX 2: PHOTOGRAPHS FROM FIELD WORK IN SIILINJÄRVI MINE DURING

39 37 Figure A2-1. Malå GPR Equipment assembly with 2 antenna slots: A (long line, 25 MHz) and B (short line, 100 MHz). Figure A2-2. West-East Cross section of a small sand rich till in front of the artificial Sikopuro artificial lake. GPR line 72 was taken in this section. The total length of the cross section as seen in photo is 100 m and the thickness of the till deposit is 5 m. Scale: Tiina Kaipainen height is 177 cm.

40 38 Figure A2-3. V- Gauging in the creek in east of Saarinen Quarry. Saarinen Quarry is behind the trees. Upper photo was taken in summer ( ) and bottom in spring ( ).

41 39 Figure A2-4. Creek ( ) next to the V- Gauging in Figure A2-3, east of Saarinen Quarry.

42 40 Figure A2-5. The fracture bedrock with double ditches collecting runoff and seepage waters around of the gypsum pile.

43 41 Figure A2-6. Basal till in north of Musti-tailing area. Yellow dashed line represents a contact between fractured bedrock and thin till layer.

44 42 Figure A2-7. Typical bedrock exposed in the mining area with high fracturing and thin cover of fine-grained till.

45 43 Figure A2-8. Boulder rich till in the area north of Musti-tailing area. The northern tailing boundary is shown in the upper part of the photo. Figure A2-9. Typical stratigraphy of Quaternary deposit in Siilinjärvi Mine: boulder rich till mixed with fine-grained sediments of glacial fine sand or silt. Photo was taken in the

46 44 area between south of Saarinen quarry and Raasio areas. Water in the lower photo was received from rainfall from the previous day.