Tu Myce 08 Detection and Characterization of Fracture Zones in Bedrock - Possibilities and Limitations

Transcription

1 Tu Myce 08 Detection and Characterization of Fracture Zones in Bedrock - Possibilities and Limitations G.A. Tassis* (Norges Geologiske Undersokelse (NGU)), P.I. Tsourlos (Aristotle University of Thessaloniki), J.S. Rønning (Geological Survey of Norway) & T. Dahlin (Lund University) SUMMARY In Norway, resistivity measurements have already been tested in marine environments in order to detect subsea fracture zones. However, most of these data have been processed without taking into account the special conditions the presence of seawater creates. More recent studies worldwide have also applied ERT in marine conditions, but under more favorable conditions nevertheless since they dealt with brackish water of considerably higher resistivity than pure seawater. This study summarizes our efforts to establish basic rules when considering whether or not pure sea water ERT can satisfactorily detect weak zones inside resistive bedrock, a problem engineers in Norway usually come up against in tunnel construction sites. The scope for this study is related to the construction of a sub-sea tunnels and the potential application of ERT to detect fractured zones as part of the geotechnical study. Our results indicate that ERT surveys for fracture zone detection in Norwegian marine environments can be promising under certain conditions but at the same time ambiguous since they suffer from reduced resolution and major artificial effects. Based on the modeling results, we were able to improve interpretations of ERT measurements made across the straits at Kvitsøy and plan further investigations in southern Norway.

2 Introduction In Norway, resistivity measurements have already been tested in marine environments in order to detect subsea fracture zones (Lile et al. 1994; Dalsegg, 2012). However, most of these data have been processed without taking into account the special conditions the presence of seawater creates. More recent studies in several localities (Tsourlos et al., 2001; Satriani et al., 2011; Rucker & Noonan, 2013; Dahlin et al., 2014) have also applied ERT in marine conditions. However, most of these cases were performed under more favorable conditions since they had to deal with brackish water of considerably higher resistivity than pure seawater (an example of the effect of brackish water shown in figure 1). This study summarizes our efforts to establish basic rules to be applied when considering whether or not pure sea water ERT can satisfactorily detect weak zones inside resistive bedrock which is possibly overlain by sedimentary formations, a problem engineers in Norway usually come up against in tunnel construction sites. Figure 1 The response of Dipole-dipole array to various seawater resistivities. A higher water resistivity (increasingly brackish water) yields better results. The scope for this study is related to the construction of a sub-sea tunnels and the potential application of ERT to detect fractured zones as part of the geotechnical study. This is of particular interest to the Norwegian Public Roads Administration and is practically a continuation of previous ERT measuring (Rønning et al., 2009; Ganerød et al., 2006; Dalsegg, 2012) and modelling work (Reiser et al., 2007) carried out at the Geological Survey of Norway (NGU) for locating and characterizing fractured zones in bedrock. Based on the modeling results, we were able to improve interpretations of ERT measurements made across the straits at Kvitsøy and plan further investigations in southern Norway. Modelling parameters and feasibility study With salinity 3.5% and temperature 5 o C, the sea-water resistivity is 0.30 Ohm m: a salinity of 4% and temperature of 10 o C reduces the sea-water resistivity to 0.23 Ohm m. In our modeling we have used a sea-water resistivity of 0.25 Ohm m which is a reasonable assumption for Norwegian conditions. Bedrock in the coastal areas of Norway consists mostly of crystalline basement. Resistivity in subsea crystalline basement is not well known in Norway, but a few observations do exist: gabbro varies from 700 to 1500 Ohm m (Elvebakk & Saintot 2011) while gneiss varies from 1500 to 2500 Ohm m (Dalsegg & Elvebakk 2012). Based on these observations, a resistivity for unfractured subsea crystalline bedrock of 1000 Ohm m seems reasonable. Resistivity in the fractured bedrock however, is the most uncertain parameter in our marine resistivity modeling work. It could be that resistivity in subsea fracture zones of 50 Ohm m is too high however, modeling with different resistivity contrasts between the fracture zone and the host rock produce only slightly better results for a contrast of 100 (10 Ohm m in fracture zone) compared with contrast of 20.

3 According to Reiser et al. (2009), when investigating fracture zones on land, the optimal arrays for this task are multiple gradient and dipole-dipole. However, the presence of an extremely conductive medium such as the sea water causes several of the standard arrays to be inapplicable due to high inversion instability which leads to low quality inversion results. Moreover, a feasibility investigation should be carried out to verify whether the voltages produced in our theoretical models are measureable by actual commercial instruments. Using array settings of 81 electrodes and 5.0 m spacing and testing a large variety of modeling scenarios (effect of fixing the water layer before inversion, effect of seawater depth, effect of seawater resistivity, etc.), we have determined that in an idealized noise free 2D environment a dipole-dipole configuration is the preferable array for marine ERT. In a real case the small signal-to-noise level of the dipole-dipole array can be a limiting factor leading to choice of e.g. multiple gradient array instead. Figure 2 The effect of removing immeasurable voltages (below 0.5 mv) in the effectiveness of dipoledipole array with increasing seawater depth. Robust inversion has been employed in all cases - model setting as in figure 1. Figure 2 displays the results of removing voltages below 0.5 mv and running inversion on the remaining apparent resistivity points versus the result of the inversion without any filtering. Dipoledipole array although being the theoretically preferred array in our study, has a significantly lower voltage range than multiple gradient which in increasing seawater depth conditions can measure an equally increasing number of voltages below 0.5 mv. At 10 m depth for example, two thirds of the produced voltages are below 0.5 mv, most of which being around 0.1 mv. This means that in order to approach the modeled conditions, we should insert a current 5 times stronger than in our ideal case (5 Amps instead of 1). This will not make our fracture zone any more detectable than in the modeled case, but it will enable immeasurable voltages to become measurable and therefore the whole inversion process to be more reliable. That is not the case with multiple gradient in which voltages remain within the measurable range for all depths up to 10 meters of seawater.

4 Real case scenario In ERT surveys in Norway, the proposed line occasionally has to cross a strait of sea water. This means that there is an area within the cross section where floating electrodes are used and are in direct contact with highly conductive seawater. As part of a project, NGU measured 5 resistivity lines at the island of Kvitsøy north of Stavanger during spring 2012 (Dalsegg 2012). The purpose of this work was partly to map fracture zones that could cause problems during construction of the tunnel to Kvitsøy, and partly to test the effect of seawater in narrow straits as a part of the project presented here. One of the measured lines crossed a strait in a case similar as described below. Figure 3 Effect of fixing the water layer resistivity and dimensions prior to inversion tested on both synthetic and real data.the array used is Multiple gradient while robust inversion has been employed in all cases. V/H filter equals 1. To investigate whether the lack of fixing the seawater body in resistivity and dimensions before inversion could mask an otherwise existing fracture zone, we decided to model such a case with two different settings. The first setting included a 15 m wide 50 Ohm m fracture zone outside the influence of water. The second setting placed a second fracture zone underneath the strait, completely inside the zone of influence of the overlying water layer which was 3.5 m deep and had 0.25 Ohm m resistivity. The response of both these models was tested using both aforementioned software for the Multiple gradient which has been the employed array in Kvitsøy. It has been proven that if the water layer isn't fixed prior to inversion, the seawater body causes a low resistivity artificial effect in the area below it, which could be interpreted as a fracture zone regardless of one existing there or not. However, if we do fix the water layer the artificial effects are removed and the interpretation becomes much clearer as seen in figure 3. Conclusions It is safe to say that accurate monitoring of sea water resistivity and sea bottom topography is essential when performing floating electrode mode measurements. Furthermore, we note that warmer and more saline waters reduce the method's resolution capability while colder and less saline conditions increase our chances of detecting underwater fracture zones in their true extent and dimensions. On the other hand, setting a general depth limitation where ERT works is not as a simple task. However, repeated modeling tests have shown that a water depth of 10.0 m is the limit for informative marine ERT surveys when sea-bottom measuring mode is used. Accordingly in case of floating electrodes, a typical water depth of 5 m can be considered the limit above which ERT surveys cannot provide

5 useful results. Our modeling has shown that electrical resistivity traversing is a challenge in marine environments. The seawater conducts away a large portion of the injected current, and a fracture zone in bedrock is not visible when seawater depth exceeds 10 meters. However, in the field study at Kvitsøy, we could locate a fracture zone in bedrock under ca. 4 meters of seawater. To do so it was necessary to have information about seawater resistivity and depth in order to fix those parameters during inversion. For dipole-dipole, a higher current can enable several immeasurable theoretical voltages to become measurable. However, this cannot affect the overall performance of the inversion. Acknowledgments This work is supported by the Norwegian Public Roads Administration (Statens Vegvesen) as part of the ForForUT project (Forbedrede Forundersøkelser UTbygning, NGU, project no ). References Dahlin, T., Loke, M.H., Siikanen, J. and Höök, M. [2014] Underwater ERT Survey for Site Investigation of a New Line for the Stockholm Metro. 31 st Nordic Geological Winter Meeting, Lund, Sweden, January Abstract in conference proceedings. Dalsegg, E. [2012] Geofysiske målinger på Kvitsøy, Kvitsøy kommune, Rogaland. NGU Report , Dalsegg, E. and Elvebakk, H. [2012] Geofysiske målinger i forbindelse med undersjøisk tunnel til Bjarkøy, Troms. NGU Rapport , Elvebakk, H. and Saintot, A. [2011] Geofysisk logging av borehull på Alstein, Randaberg kommune, Rogaland. NGU Report , Ganerød, G.V., Rønning, J.S., Dalsegg, E., Elvebakk, H., Holmøy, K., Nilsen, B. and Braathen, A. [2006] Comparison of geophysical methods for sub-surface mapping of faults and fracture zones in a section of the Viggja road tunnel, Norway. Bull. Eng. Geol. Env., 65, ISSN: (Paper) (Online). Kim, J.H. [2012] DC 2DPro v User's Guide. Lile, O.B., Backe, K.R., Elvebakk, H. and Buan, J.E. [1994] Resistivity measurements on the seabottom to map fracture zones in the bedrock underneath sediments. Geophysical Prospecting, 42(7), Loke, M.H. [2002] RES2DMOD ver 3.01 Geoelectrical Imaging 2D & 3D. Instruction manual. Loke, M.H. [2010] RES2DINV ver Geoelectrical Imaging 2D & 3D. Instruction manual. Reiser, F., Dalsegg, E., Dahlin, T., Ganerød, G. and Rønning, J.S. [2009] Resistivity Modelling of Fracture Zones and Horizontal Layers in Bedrock. NGU Report , Rønning, J.S., Dalsegg, E., Elvebakk, H., Ganerød, G.V. and Heincke, B.H. [2009] Characterization of fracture zones in bedrock using 2D resistivity. Proceedings from 5 th Seminar on Strait Crossings, Trondheim, June , (SINTEF/NTNU). Rucker, D.F. and Nooman, G.E. [2013] Using marine resistivity to map geotechnical properties: a case study in support of dredging the Panama Canal. Near Surface Geophysics, 11(6), doi: / Satriani, A., Loperte, A. and Proto, M. [2011] Electrical Resistivity Tomography for coastal salt water intrusion characterization along the Ionian coast of Basilicata Region (Southern Italy). Fifteenth International Water Technology Conference, IWTC , Alexandria, Egypt. Tassis, G., Tsourlos, P., Rønning, J.S. and Dahlin, T. [2014] Detection and characterization of fracture zones in bedrock in a marine environment - possibilities and limitations. NGU Report , Tsourlos, P.I., Tsokas G.N. and Albanakis, K. [2001] Geoelectrical surveys in marine environments. Geowaters Project Report, 2001.