Geometry and topology of multiscale fracture networks - Methodology and characterization

The study of bedrock fracture networks is essential for applications such as modelling fluid flow, predicting radionuclide and geothermal heat transfer as well as overcoming bedrock engineering challenges since the fractures affect a number of physical characteristics of the bedrock. However, the multiscale nature of these fracture networks poses challenges in observation and analysis as fractures occur at all scales from centimeter-scale features on outcrops to kilometer-scale tectonic margins. To overcome this challenge, this doctoral thesis focuses on the geometry and topology of two-dimensional multiscale fracture networks and presents methods for their multiscale investigation – both existing, from the literature, but also novel methods developed under the course of this work. Scale-independent methods are introduced and employed, allowing for the direct comparison of data collected at different scales. Geometric fracture network properties encompass fracture lengths, orientations, and intensity i.e. the spatial distribution of fractures. Furthermore, the analysis of topological properties focuses on the connectivity and fracture relationships, offering valuable information about the flow potential within the fracture network and potential age relations between fractures, respectively. This thesis comprises three articles that both address a specific research question in fracture studies while presenting novel methods for these purposes. Research issues include the identification of faults, their damage zones and fracturing that is kinematically unrelated, determination of the data requirements for accurate fracture network characterization and prediction of fracture properties, such as length, for scales of observation, where data collection is challenging. As a result of applying the presented methods, it was found how pre-existing faults affect later fracturing and how through subsampling of fracture trace data the accuracy of fracture data can be assessed. Furthermore, the thesis comprised fracture data collection in length ranges that had previously lacking data and provided estimations on predicting fracture lengths within crystalline rocks

Scalability of lineament and fracture networks within the crystalline Wiborg Rapakivi Batholith, SE Finland

Multiscale lineament and fracture extraction conducted within the Wiborg Rapakivi Batholith offers insights both into the brittle bedrock structures of the batholith and to the scale-dependence of lineament and fracture analysis results. Multiscale fracture studies from crystalline rocks are sparse even though brittle structures in the crystalline bedrock significantly affect the flow models of fluids, hydrothermal heat and hydrocarbons, and are the main factor controlling the permeability in crystalline rocks. The main goal of this study is to assess the scalability of lineament and fracture networks through statistic characterization of lineament and fracture datasets extracted from four scales of observation using geometric and topological parameters, and by studying the subsequent correlations between the dataset characterizations. The parameters are acquired from both the individual lineaments and fractures and from their respective networks. Brittle bedrock structures were extracted manually using two principle methods: lineament traces were digitized from Light Detection And Ranging (LiDAR) digital elevation models and fracture traces were digitized from drone-based orthophotography of bedrock outcrops. Both extractions result in two-dimensional datasets and, consequently, all characterizations of these datasets along with the scalability analysis results are limited to two dimensions. The crystalline Wiborg Rapakivi Batholith is structurally isotropic and lithologically sufficiently homogeneous so that the effect of both precursor fabrics and lithological variations can be ignored when considering the genesis and emplacement of brittle bedrock structures in the batholith. Scalability analyses conducted within this investigation revealed that the results of lineament and fracture network extractions are always dependent on the scale of observation. Even dimensionless parameters of networks, such as connectivity, were found to follow a scale-dependent trend: The apparent connectivity of a lineament or fracture network decreases as the scale of observation increases. The characterizations of the datasets were used for the interpretation of Wiborg Rapakivi Batholith fracture patterns and paleostresses, which could be compared to Olkiluoto site studies of paleostresses in southern Finland.Viipurin rapakivibatoliitin alueella useassa mittakaavassa tehty lineamenttien ja rakojen kartoitus antaa tietoa sekä batoliitin hauraista kallioperän rakenteista että lineamentti- ja rakokartoituksen tulosten skaalariippuvuudesta. Useassa mittakaavassa tehtävät rakotutkimukset kiteisistä kivistä ovat harvinaisia, vaikka kiteisen kallioperän hauraat rakenteet vaikuttavat vahvasti nesteiden, kaasujen, hydrotermisen lämmön ja hiilivetyjen virtausmalleihin ja ne ovat kiteisen kallioperän permeabiliteetin tärkein kontrolloija. Tämän tutkimuksen tärkein tavoite on lineamentti- ja rakoverkkojen skaalautuvuuden tutkiminen. Tutkiminen tapahtuu ensin karakterisoimalla tilastollisesti lineamentti- ja rakoaineistoja neljästä eri mittakaavasta käyttäen geometrisiä ja topologisia parametrejä, ja sitten tutkimalla aineistojen karakterisointien välisiä korrelaatioita. Parametrit ovat sekä yksittäisten lineamenttien ja rakojen että lineamentti- ja rakoverkkojen parametrejä. Kallioperän hauraat rakenteet kartoitettiin kahdella eri metodilla: lineamenttiviivat digitoitiin laserkeilauskorkeusmalleista (LiDAR DEMs) ja rakoviivat digitoitiin lennokilla otetuista kalliopaljastumien ortomosaiikkikuvista. Molempien kartoitusten tuloksena oli kaksiulotteisia aineistoja, ja tämän takia myös kaikki aineistojen karakterisoinnit ja skaalautuvuusanalyysien tulokset ovat kaksiulotteisia. Kiteinen Viipurin rapakivibatoliitti on rakenteellisesti isotrooppinen ja litologisesti riittävän homogeeninen, jotta sekä edeltävät rakenteet että litologiset vaihtelut voidaan jättää huomioimatta, kun tutkimuksen kohteena on batoliitin hauraiden rakenteiden syntyminen. Tämän tutkimuksen puitteissa tehdyt skaalautuvuusanalyysit osoittivat, että lineamentti- ja rakoverkkokartoitusten tulokset ovat aina riippuvaisia kartoituksen mittakaavasta. Jopa yksiköttömät verkkojen parametrit, kuten verkottuneisuus, seurasi skaalariippuvaista trendiä: Näennäinen lineamentti- tai rakoverkon verkottuneisuus pienenee, kun mittakaava suurenee. Lineamentti- ja rakoaineistojen karakterisointeja käytettiin Viipurin rapakivibatoliitin rakojen muodostamien kuvioiden ja paleostressien tulkintaan. Paleostressitulkintoja voi verrata Olkiluodossa tehtyihin tutkimuksiin paleostresseistä eteläisessä Suomessa

A new subsampling methodology to optimize the characterization of two-dimensional bedrock fracture networks

This paper introduces a new subsampling method to determine the empirical relationships between the areal sampling coverage and the topological-geometric parameters resulting from characterization of two-dimensional outcrop fracture networks. We further developed these relationships into correlations between the total sampling coverage and variance of the characterization result, hence providing insight to the objective uncertainties related to fracture network characterization. The analyses were conducted using two new open-source Python packages: fractopo and fractopo-subsampling, designed for fracture network analysis and subsampling, respectively. We conducted the study on the well-exposed crystalline outcrops of Getaberget, Åland Islands, Finland, where a total of 42499 fracture traces were manually digitized from 13 circular target areas. For the purposes of subsampling, we conducted fracture network characterization for randomly located and sized subsample areas, which locate within the larger target areas.Based on our subsampling results we provide recommendations for the preliminary optimization of areal coverage used in outcrop fracture sampling and the use of our subsampling method for assessing the precision related to the areal fracture network characterization in other previously uncharacterized areas. As an example, we recommend using a total sampling area of 8000with 8 circular sampling areas to define the power-law exponents of fracture traces when conducting outcrop fracture network characterization with drone-based methodology done with similar initial sampling setup in comparable geological environments, as this coverage has shown an acceptable level of precision.</p

Fault-induced mechanical anisotropy and its effects on fracture patterns in crystalline rocks

We used drone-acquired orthophotographs to map the 2D-networks of fractures transecting the Mesoproterozoic Rapakivi granites in southern Finland. The work aims at understanding i) how discrete faults and the kine-matically linked syn-fault extension fractures originate in crystalline rocks, and ii) how the resulting structural anisotropy controls the patterns of later-formed regional fractures. We recognized incipient faults with associated distinct damage zones (DZ; wall-, tip-, bend- and linking damage) indicating that faulting occurred within mesoscopically isotropic material devoid of  pre-existing fabrics. Moreover, fault-induced extension fractures occur also outside the normal DZs indicating much wider DZs than that predicted by scaling laws. Extension fractures within the wide DZs are the result of linkage between sub-parallel segments of the dominantly sinistral and ~N-S trending (sub-) vertical faults. The resulting syn-fault network of N-S, NW-SE and NE-SW trending fracture sets contributed to the mechanical anisotropy that controlled the patterns of the later-formed regional fractures. Specifically, i) narrow fault reactivation damage zones caused deviations in the patterns of otherwise systematic near-orthogonal regional fractures, and ii)  the  NNW-SSE trending regional fractures are  poorly developed in  areas where either several N-S  faults or  sub-parallel NW-SE syn-fault extension fractures are dominant. The latter applies in particular to a large left step-over zone between sinistral faults, which we infer to provide the first-order control over the development of contrasting fracture domains. The results of this study further indicate that a strike-slip paleostress event affected the crust after ca. 1.65 Ga.</p