Geophysical methods in bedrock groundwater studies

Antti Pasanen. Geological Survey of Finland, P.O. Box 1237, FI-70211 Kuopio, FINLAND, e-mail: antti.pasanen(at)gtk.fi

Introduction

The major problem in bedrock groundwater studies is the large heterogenity in hydraulic properties when working in hard, plutonic or metamorphic bedrock, like in most mining areas in Finland. The primary effective porosity of such bedrock is low and the water flow is channeled in secondary structures such as joints, fissures and fractures which have  higher hydraulic conductivity than in unfractured bedrock. When studying the transport of water and potentially harmful substances or e.g. mine water management in hard rock bedrock the three dimensional locations of secondary porosity is the key to succesful study.

The geophysical methods, both airborne and land-based, are  invaluable toosl when pinpointing the secondary porosity and drilling locations. The geophysical methods gives 2D data in most cases, which can be interpreted in pseudo-3D or 2,5D to give an relatively accurate information in the resolution of the study of the areas where the hydraulic properties of the bedrock are such, that they can affect the environment or mine water management. In some cases, a true 3D survey is needed.

Geophysics is always ambiguous, and therefore, the use of several geophysical methods and other geological data is needed. This Wiki-article describes the methodology used in Talvivaara mining region to pinpoint the fracture zones and to design the locations of bedrock groundwater wells for geochemical sampling and measurements of hydraulic conductivity. The overall design requirements are also discussed.

Designing the study

The scale of the study need to be selected the first before choosing the instrumentation. Like in hydraulic properties the scale of the secondary porosity structures vary from single fractures to fracture zones and major fault systems. When working at the mining site the input and the possible output of the data need to be carefully considered. The smaller details need higher resolution equipment and slower measurements adding to the cost, whereas too overall studies may lead to unuasable information. In Talvivaara case the resolution was selected to identify the strike of the main fracture zones in the area. The horizontal fracturing or single major fractures and joints could not be studied due to financial restrainments and lack of equipment, such as 3-component seismic reflection. The scale of fracture zones is justified because they are the main hard rock aquifers and transport routes in Finnish mining areas, whereas the quantity of water able to transport through single fractures or relatively unfractured bedrock is many times very small compared to fracture zones.

When selecting the equipment for hard rock hydrogeological survey not only the geology, but vegetation, equipment availability and cost vs. benefit must be considered. For example the electrical conductivity of the bedrock and overburden governs the possibility to use galvanic and inductive methods, the vegetation governs the equipments weight and bulk, but also the coupling between ground and the equipment. If the equipment is available and its purchase or rental cost is appropriate for the budget the main cost comes from the personnel; how many persons  is needed to operate the equipment and how long the survey lasts versus the measured line kilometres. The fracture zones are elongated, mainly vertical to slighty subvertical structures, which also sets limitations for the equipment. For example reflection seismic survey is unsensitive  to vertical structures. With reflection seismics, the poisson ratio (Poisson, 1829; Barton, 2007) can be applied in the estimation of rock quality when using 3-component equipment.

The design of the measumerement lines always depends on the objective of the survey. The same aspects than selecting the equipment need to be considered, but also the possibility of the measuring crew to move on the terrain. When performing a land-based 2D survey on fracture zones the measurement lines should be designed transverse to fracture zones, if possible, and all the fracture zones should be measured at least in two locations. Prior information of the strike of the fracture zones is a prerequisite for this kind of measurement line design. The preliminary strike of the fracture zones can be investigated with lineament study if aeromagnetic measurements and digital elevation model  (DEM) or topographic map is available or with field survey. Another alternative is to perform a preliminary, land based survey with an inexpensive geophysical method, such as gravimetric.The existing data should always be taken into account when designing the survey. E.g. drillings are excellent data and, if possible, the lines should be designed near the drillings.

The motivation behind the measument line desing, selected equipment and scale must be strict positioning of the sampling locations in fracture zones. Because of the morphology of the fracture zones, they usually are very narrow features, tens to hundreds metres, and thus the incorrect positioning of the sampling locations may lead to sampling e.g. background groundwater from relatively unfractured bedrock. Anyhow, the geophysics can give valuable information of the internal structure of the fracture zones and the groundwater in it. The following chapters descripes the methods, design and interpretation used at the Talvivaara mining site.

Geophysical studies at Talvivaara

The main aim with geophysical studies in Talvivaara gypsum pond leakage study was to identify the strike of the fracture zones, which were estimated as the highest risk for possibly contaminated water to transport outside the mining area and to pinpoint the groundwater standpipes in fracture zones, but also in relatively unfractured bedrock for background sampling. The area had some standpipes for bedrock groundwater monitoring already installed (Fig.1), which also guided the design of the measurement lines along with the predetermined location of the fracture zones and the terrain (Fig. 2).

Figure 1. Sampling locations in Talvivaara gypsum pond area (Pasanen et al. 2013). The letters in sampling point names refers to the time and technique of the drilling (Px = diamond coring, bedrock, made before the study, FIDxx = diamond coring, bedrock, made during the study, Rx = sediment boring, sediment, made during the study, all the other = diamond coring XXXKA, sediment boring XXXMAA, made before the study).

Figure 2. Geophysical survey lines in Talvivaara Gypsum pond area. The inexpensive methods, gravimetry and ground penetrating radar (GPR) was measured first and interpreted before the more expensive methods, seismic refraction and electrical resisitivity tomography (ERT), were designed in most interesting areas. Many of the survey lines coincide because of the terrain restrictions and to reduce ambiguity of the results by investigating with several methods.

Methods

Lineament study

The study was started with defining lineaments from aeromagnetic and LIDAR-DEM data. The so called lineament study can give approximate locations of fracture zones from large area quickly. The lineament study in Finnish conditions always have to be considered as preliminary investigation and should not be used in defining the drilling points. The fracture zones in Figure 2 are defined using the lineament study, but The drilling points do not coincide with the lineaments because the drillings locations are refined with ground based geophysics.

The data used in lineament study consist of aeromagnetic data which is available from the whole mainland Finland and Lidar-DEM data which is available from many parts of Finland. The intepretation of lineaments is based on interpretation of valleys and aeromagnetic minimum anomalies. The overburden in Finland is usually thin, except in many glaciofluvial formations, and reflects the topography of the underlaying bedrock. Finland has been covered with continental glaciations several times during the Quaternary causing a massive bedrock erosion. The fracture zones usually are more easily eroded than the unfractured bedrock and they are often seen as valleys. The overburden in valleys is thicker than outside the valley but this usually do not mask the valley. The bedrock movements that cause the fracture zones crush rock and minerals. This crushing causes the volume of magnetic minerals to diminish which is seen in aeromagnetic data as minimum anomalies.

The main sources of error are the masking of valleys and other geological process caused valleys which genesis is not fracture zone erosion. The magnetic anomalies can also mask the anomalies from fracture zones. One of the main reasons why the lineament study should be treated as preliminary is the lack of accurate airborne positioning systems during the time of the measurement causing even 200 metre location errors for the aeromagnetic data. Also the valleys can be wider than the fracture zones.

Gravimetric measurement

Gravimetric measurement is done with extremely accurate scale which position and altitude is know in each location. The scale measures the earth gravity field under the scale and when done in lines or grid the anomalies can be detected. In the detection of fracture zones the line-form gravimetric data is inverse modelled to estimate the overburden thickness and the bedrock topography. The indicators for possible fracture zones are thicker overburden and lower bedrock level which is many times abrupt (Fig. 3). Also low gravity anomalies are used in the interpretation that possibly show the lower density of crushed rock in fracture zones.

The method is inexpensive and fast and was used to guide more detailed geophysical investigations. The low cost allows large areas to be covered in reasonable time. The main errors are caused by possible location errors and scale errors due to changing conditions, e.g. temperature. The measures to avoid errors include marking the survey lines beforehand and measuring the locations with centimetre accurate GPS (real-time kinematic, RTK) and during the measurement with water scale for very accurate relative altitude. In this kind of measurements where relative anomalies are the most important and the survey lines are relatively short the instrumentation errors can be deemed small.

Figure 3. Gravimetric measurement from Talvivaara mining region (Forss 2013). The upper graph shows the measured gravity values in blue and the calculated Bouguer anomaly values in red. The lower graph show the ground topography in green and the bedrock topography in red. The red circles mark the locations where possible fracture zones are interpreted.

Ground penetrating radar

Ground penetrating radar (GPR) transmits electromagnetic waves in megahertz to gigahertz range to the ground and measures the two-way travel time, amplitude and polarity of the reflected wave. The reflections are formed in the dielectric surfaces which in stratified sediments reflect the deposional surfaces or other subsurface structures and in bedrock e.g. joints and fractures. GPR is a very powerful and inexpensive method but it needs an experienced person to do the interpretation. In fracture zone studies the main interpretation element is the overburden thickness and bedrock topography (Fig. 4). Evidence of fractured bedrock can also been seen in places and this combined with thick overburden and low bedrock topography gives a clue of possible fracture zones.

The errors in GPR profiling consist mainly of instrumentation errors caused by non-subsurface reflections, such as metal fences and light posts especially with unprotected antennae and from the interpretation errors. The depth penetration varies to centimetres with air-coupled antennae to maximum 50 metres with low-frequency ground-coupled antennea (25 MHz) but depends largely on electricical properties of the subsurface and in places the bedrock surface is below the depth penetration limit. Unstratified sediments are many times difficult to define from fractured bedrock and the interface between them can be masked. In such places the interpretation goals cannot be achieved. The GPR methodology is covered in Monitoring section of this WIKI.

Figure 4. GPR interpretation image from Talvivaara mining region (Forss 2013). The upper radargram shows the greyscale image produced with the 25 MHZ centre frequensy antenna and the lower radargram with 100 MHz antenna. The red line shows the interpreted bedrock topography.

Refraction seismic

Refraction seismic survey can be performed when the seismic velocity of the subsurface increases with depth. This causes that the critical angle is reached and the wave can be refracted from the subsurface interfaces to geophones at the ground surface. The methodology is explained in the Monitoring section of this WIKI. Refraction seismic can be deemed as a mid-prize to expensive method with low to medium daily line kilometres. Therefore, it should be used as a focused method to give more information of the most important locations for the study.

Refraction seismics can be used in two ways to to study the position of the fracture zones. The measured data can be inversion modelled where the overburden, bedrock topography and possibly groundwater level can be separated when using three layer model. The model can then be interpreted for thick overburden and low bedrock topography similar to gravimetric data (Fig. 5.) The acoustic wave velocity in bedrock can also be used in the interpretation because the crushed rock in fracture zone lowers the velocity. Combining these interpretation keys the possible fracture zones can be interpreted.

Figure 5. Seismic refraction interpretation from Talvivaara mining region (Modified after Forss 2013). The upper diagram shows the inversion modelling of the ground surface, groundwater table and bedrock surface. The lower diagram shows the acoustic wave velocity in bedrock. The higher velocities ca. 5500 m/s is typical for unfractured bedrock whereas lower velocities ca. 3500 m/s can represent fractured bedrock.

Electrical resistivity tomography

Electrical resistivity tomography (ERT) is a galvanic, direct current method which reveals the apparent resistivity and its variation in subsurface. The overall principle of the resistivity method is presented in the Monitoring section of this WIKI. ERT method is one of the best methods to reveal fracture zones and and their properties in high resistivity subsurface and bedrock. The method is labour intensive and relatively expensive and, therefore, it should be used as focused method. The method is sensitive to high electrical conductivity and is unusable in such conditions.

The data is inverse modelled and the interpretation of the ERT method in low electrical conductivity bedrock is based on identification of narrow, vertical to nearly vertical, relatively lower resisitivity anomalies that may represent fracture zones (Fig. 6). In the research area the bedrock is composed of Archaean and Paleoproterozoic lithology withhigh resistivity. The resistivity of the water is much lower than in the bedrock and areas with high water content show in lower apparent resistivity. Lithological changes can also cause low resistivity anomalies, but combining the interpretation with expected shape of fracture zones and other data the fracture zones can be interpreted with reasonable reliability. The inverse modelled data can also be used similar to gravimetric and seismic refraction data to interpret thicker overburden and bedrock valleys. The electrical conductivies in possible fracture zones can also give a clue of the possible contaminiation of groundwater.

Figure 6. Electrical resistivity tomography interpretation from Talvivaara mining region (modified after Forss 2013). The low electrical conductivity or high apparent resistivity show the relatively unfractured bedrock with low water content (blue). The vertical to nearly vertical, higher electrical conductivity anomalies are interpreted as possible fracture zones that are filled with groundwater (red and yellow).

Discussion and conclusions

Recognizing and studying the secondary structures able to transport water and possibly harmful substances from an active or closed mine site through the bedrock always need extensive studies. If the structures, such as fracture zones, and the study area is close to ore body the exploration drillings can be used in recognizing these. Usually, the drillings are absent from the area of interest outside the ore body and other methods need to be used.

Geophysical surveys allow inexpensive methods to study the strike of fracture zones and pinpointing the drillings and sampling sites. The ambiquity of geophysical methods should always be reduced with the use of several methods, geophysical, geological and morphological, to avoid drillings that do not strike the fracture zone. In this study a lineament study using Lidar DEM and aeromagnetic data was enhanced with ground based geophysics, gravimetry, GPR, refraction seismic and ERT. In places and because of the financial restrictions some of the methods are unusable and less surveys can be performed. In this kind of cases it is extremely important to choose the correct methods and, if possible, test it on site. In other studies the author and his colleagues have also tested more inexpensive method, VLF-R (Very Low Frequency with Resistivity option), to replace expensive ERT method in recognizing the fracture zones. The results show that the method can detect fracture zones and the bedrock topography and overburden thickness can be modelled. The resolution depends on the distance between the measuring points but is is usually lower than with ERT.

The interpretation of the data should be performed in steps where the earlier, and more inexpensive data guides the later and more expensive measurements. In each step the data is interpreted againts all existing data. The best way to interpret data from multiple sources is to import all the data in a 3D environment where the interpretation elements can more easily be observed. This can be labour intensive because few software can natively read all the data formats used in research. In this study the interpretation was done in 2D environment on computer and with lots of large size prints. The main fracture zones were identified and the drillings were guided to proper positions (Fig. 1).

A rule of thumb states that when working with fractured aquifers one out of two drillings do not strike the fracture zone. In this case six out of eight drillings strike the fracture zone and the two that did not, were made next to each other to study the hole-to-hole hydraulic conductivity.

The study continued with water sampling for chemical and isotope analyses and the groundwater flow pattern in bedrock could be interpreted. Without the large investment to geophysical studies the outcome of the research would not have been as succesful than it is now. The study in whole is presented in Water management research and development section and its subpages.

References

Barton, N. 2007. Rock quality, seismic velocity, attenuation and anisotropy.Taylor & Francis Group, London, UK. 551 pp.

Forss, H., Lerssi, J., huotari-Halkosaari, T., Pasanen, A., Eskelinen, A., Kittilä, A. 2013. Talvivaara; Geofysikaaliset tutkimukset 2013. M21K2013. 17 p. (In Finnish)

Pasanen, A., Eskelinen, A., Räisänen, M-L., Lerssi, J. & Kittilä, A. 2014. Talvivaaran kipsisakka-altaan vuodon pohjavesivaikutusten selvitys ja leviämisen ja haitan arviointi. Geological Survey of Finland, archive report. M21K2013. 52 p. (in Finnish)

Poisson, S.D. 1829. Mémoire sur l’équilibre et le movement des corps élastiques. Mém. de l’Acad. Sci. 8, 357.