Investigation of the correlation of fracture frequency and electric resistivity in impact craters in crystalline rocks

Abstract: Impact craters are formed when a large meteorite or comethits the Earth. At the impact a shock wave is released causingabundant fracturing in the surrounding bedrock. This type offracturing is intense and occurs throughout a very large volume(>100 km3) of the bedrock. Fractures of this type have beenobserved in deep drilling, to 5 km depth, in thePuchezh-Katunki Impact Crater. At theses depths the ambienttemperature is high. Thus impact structures are candidates forpotential heat-exchange sources for extraction of geothermalenergy.There is a relation between fracture intensity and electricresistivity in bedrock predominated by impact-generatedfractures. In crystalline bedrock changes in electricresistivity is mainly due to fracturing which is the mainsource of porosity in these rocks. Electric resistivity methodsare highly sensitivity to porosity. Furthermore highfracture-intensities have generally been associated with lowelectric resistivity. Electro-magnetic methods like Very LowFrequency Resistivity (VLF-R) and Magnetotellurics (MT) canindirectly measure electric resistivity to relatively largedepths in the bedrock.This study will quantify the relationship between fractureintensity and electric resistivity which can be used as aprospecting tool for geothermal energy resources at largedepth.To meet that end, a method for fracture mapping on outcropsin Swedish terrain and a method to calculate thethree-dimensional fracture frequency from two-dimensionalfracture data has been developed. The fracture traces measuredin two dimensions on outcrops are assumed to represent avertical surface and must be converted to a three-dimensionalmeasure of the fracture frequency per unit volume. Spacing, dipand trace length of fractures have been accounted for. Thebiases associated with the mapping method have also beenaccounted for (II).The correlation between impact-induced fracturing andelectric resistivity in crystalline rocks in the Lockne Areashows that the extent of impact fracturing in crystalline rockscan be measured with electro-magnetic or electric techniques.In addition the electric resistivity of crystalline basementand impact generated Tandsby Breccia from the Lockne Craterwere determined (I).The relation between fracture frequency and electricresistivity in fresh water conditions using the VLF-R method isestablished from data collected from both two drill holes andfrom numerous outcrops in the Björkö region. Apreliminary quantification of the fracture frequency has beenmade. The MT resistivity models, related to the two drillholes, show that porosity and mineral-conductivity variationsof the bedrock affect this relation more than the salinityvariations in the bore-hole fluid. Further research is neededto establish a firm relation between fracture frequency,salinity of rock fluid, conductivity and porosity in order tovalidate the MT resistivity models (III).Keywords:Electric resistivity, Fracture frequency,Impact generated fractures, Electro-magnetic techniques, VLF-Rmethod, MT method, Window-mapping technique, Three-dimensionalfracture calculations, heat-exchange structure, geothermalenergy.