Fracture Zones In Crystalline Rock Research Articles

FRACTURING AND PERMEABILITY OF CRYSTAL ROCKS AND THEIR FRACTURE ZONES, HYDROGEOLOGICAL ASPECT

Fractured rock aquifers are regarded as of particular scientific and practical interest because of their importance for underground water supply, energy storage and safety radioactive waste disposal. The study of fissure waters of crystalline rocks in the context of global warming to address issues of drinking water supply is highly relevant today. It has potential practical importance for the future.The sustainable use of groundwater in fractured rock aquifers requires a detailed knowledge of their hydraulic properties. The complexity of hydrogeological conditions in massive fractured rocks results from uncertainties about the configuration of the fractures, while the potential for groundwater movement in such rocks is primarily determined by their permeability and, consequently, hydraulic conductivity. Fractures (fracture networks) have different permeability properties, which is one of the key parameters required for understanding and predicting fluid and water flow. This paper reviews and analyses the results of global studies of fracturing and permeability of crystalline rock massifs and the fault zones occurring in them. Since the study of permeability of crystalline rocks concerns many disciplines — including structural geology, tectonophysics, petrophysics, hydrogeology, and hydrology — this review highlights information about the distribution of groundwater in fractured crystalline rocks in the context of their potential exploitation. This study consists of two parts: generalization of research results concerning fracturing and permeability of crystalline rocks, and particularities of structure of fracture zones in crystalline rocks and of the presence of groundwater in them. Based on this review of the structure and development of fracture zones (including their evolution under the influence of weathering and other secondary processes), recommendations are given for using tectonophysical reconstructions to improve hydrogeological field works and mathematical modeling.

A user-driven feedback approach for 2D direct current resistivity inversion based on particle swarm optimization

We have developed an inversion approach based on particle swarm optimization (PSO) for 2D direct current resistivity data. Once the PSO algorithm is implemented, cost-function changes on constraints and misfit criteria can be easily performed and an approximate invariance of the Lagrange multipliers is obtained if each term of the cost function is properly normalized. More importantly, the interpreter can divide the mesh into partitions and apply a different constraint or combination of constraints in each partition. We explore several restrictions on the log-resistivity variation, either in single or in joint versions, including spatial continuity constraints in the [Formula: see text]-and [Formula: see text]-norms, total variation, sparsity constraints using the discrete cosine transform and Daubechies bases, and minimum moment of inertia constraints to impose concentration of resistive or conductive materials along target axes. In the latter case, the earth surface might be used also as a target axis. In addition, the final state of the PSO algorithm, once a stopping condition has been reached, contains not only the best solution but also a cluster of good suboptimal solutions that can be used for uncertainty analysis. As a result, the interpreter has the flexibility to perform an interpretation process based on a feedback trial-and-error inversion approach, in a similar manner that he/she has when using a friendly forward modeling software. The interpreter can then drive the tentative solutions obtained along the inversion process to incorporate their conceptions about the geologic environment, besides appraising misfit and stability of the solutions. We evaluate synthetic and field data examples. In the first case, we determine how the interpreter can drive the inversion process in a karst environment to image dissolution features. In the second case, we explore a similar situation aiming to optimize borehole locations in fracture zones in crystalline rocks.

Shallow 3D seismic-reflection imaging of fracture zones in crystalline rock

A limited 3D seismic-reflection data set was used to map fracture zones in crystalline rock for a nuclear waste disposal site study. Seismic-reflection data simultaneously recorded along two roughly perpendicular profiles (1850 and [Formula: see text] long) and with a [Formula: see text] receiver array centered at the intersection of the lines sampled a [Formula: see text] area in three dimensions. High levels of source-generated noise required a processing sequence involving surface-consistent deconvolution, which effectively increased the strength of reflected signals, and a linear [Formula: see text] filtering scheme to suppress any remaining direct [Formula: see text]-wave energy. A flexible-binning scheme significantly balanced and increased the CMP fold, but the offset and azimuth distributions remain irregular; a wide azimuth range and offsets [Formula: see text] are concentrated in the center of the survey area although long offsets [Formula: see text] are only found at the edges of the site. Three-dimensional dip moveout and 3D poststack migration were necessary to image events with conflicting dips up to about 40°. Despite the irregular acquisition geometry and the high level of source-generated noise, we obtained images rich in structural detail. Seven continuous to semicontinuous reflection events were traced through the final data volume to a maximum depth of around [Formula: see text]. Previous 2D seismic-reflection studies and borehole data indicate that fracture zones are the most likely cause of the reflections.

A multidimensional horizontal-loop controlled-source electromagnetic inversion method and its use to characterize heterogeneity in aquiferous fractured crystalline rocks

SUMMARY The controlled-source dual horizontal-loop harmonic electromagnetic (HLEM) profiling method is well suited to the problem of investigating fracture zones in crystalline rocks but there are still limitations in the way that experimental data are currently interpreted—the use of 1-D data inversion leads to inaccurate determination of geological structure. To allow accurate characterization of zones of fractured rock especially underneath heterogeneous overburden, we have developed an efficient 2.5-D regularized inversion method for reconstructing subsurface electrical resistivity distributions from multifrequency HLEM data, with the forward problem solved in 3-D using a staggered-grid finite-difference method. The inversion method is validated using a synthetic example and practical data sets from four borehole sites in a granitic terrain in northeast Brazil. An appraisal of our results for sites with boreholes sited using conventional data analysis procedures shows that we can distinguish between optimally located productive wells in fracture-zone lineaments and those with diminished yields in weathered layer with no major underlying fracture zones. We suggest that 2.5-D inversion can aid in developing better strategies for sustainable groundwater resource development in the basement terrains.

Imaging of reflection seismic energy for mapping shallow fracture zones in crystalline rocks

The high‐resolution reflection seismic technique is being used increasingly to address geologic exploration and engineering problems. There are, however, a number of problems in applying reflection seismic techniques in a crystalline rock environment. The reflection seismic data collected over a fractured crystalline rock environment are often characterized by low signal‐to‐noise ratios (S/N) and inconsistent reflection events. Thus it is important to develop data processing strategies and correlation schemes for the imaging of fracture zones in crystalline rocks. Two sets of very low S/N, high‐resolution seismic data, previously collected by two different contractors in Pinawa, Canada, and the island of Äspö, Sweden, were reprocessed and analyzed, with special emphasis on the shallow reflection events occurring at depths as shallow as 60–100 m. The processing strategy included enhancing the signals hidden behind large‐amplitude noise, including clipped ground roll. The pre‐ and poststack processing includes shot f-k filtering, residual statics, careful muting after NMO correction, energy balance, and coherency filtering. The final processed seismic sections indicate that reflected energy in these data sets is closely related to rock quality in Äspö data and fracturing in Atomic Energy of Canada, Ltd. (AECL) data. The lithologic boundaries are not clearly mappable in these data. When thickness of the reflection zone is of the order of a wavelength, the top and bottom of the zone may be resolved. The major fracture zones in crystalline rocks correlate closely with the well‐log data and are usually characterized by very low velocity and produce low‐acoustic‐impedance contrasts compared to those of surrounding rocks. Because the incidence angles vary rapidly for shallow‐reflection geometries, segments of major fracture zones can effectively be analyzed in terms of reflectivity. Reflection images of each fracture zone were investigated in the common‐offset section, where each focused event was associated with a consistent incidence angle on the reflectivity map. The complex attributes of the data indicate that strong reflectors at shallow depth coincide with intensely fractured zones. These correlate well with instantaneous amplitude plots and instantaneous frequency plots. The instantaneous phase plot also identifies the major and minor fractures.

Application of high resolution seismic reflection techniques for fracture mapping in crystalline rocks

Surface reflection seismic techniques have the capability of mapping subsurface geological features without disturbing the rock mass. They also have an added capability of penetrating to a much deeper depth than any other geophysical technique, including the ground probing radar. However, the successful application of reflection seismic techniques in crystalline rocks has in general been more difficult than in sedimentary basins, because of the irregular geometry and low acoustic impedance contrasts across geological boundaries. In this paper, we describe the imaging of fracture zones in crystalline rocks. Effective data processing, carefully modified from the conventional approaches, was applied on two high-resolution field data previously collected by different contractors. The strategy included enhancement of the signal hidden under the large-amplitude scattering noise, through pre- and post-stack processing such as shot f-k filtering, residual statics and careful muting after NMO correction. Two sets of low S/N test data from Canada and Sweden are analyzed in this research. The reflected energy in these data sets appeared to be more closely related to fracturing than to lithologic boundaries. The major fracture zones at shallow depth have been mapped with the desired resolution and can be correlated to the available well-log and seismic crosshole tomographic data. Once the surface waves were removed, shallow reflectors in the fracture zones could be identified and analyzed on the field records. Focusing analysis of the seismic image was performed in the constant-offset section to investigate the trends of major fracture zones. The complex attributes were also analyzed to determine whether they could be applied to the shallow fracture zones. Instantaneous frequency plots outline the intense fracturing zone and instantaneous phase plots identify the major and minor fractures, and other coherent events with different dip attitudes which interfere with each other.

Borehole radar applied to the characterization of hydraulically conductive fracture zones in crystalline rock : Olsson, O; Falk, L; Forslund, O; Lundmark, L; Sandberg, E Geophys ProspectV40, N2, Feb 1992, P109–142

Borehole radar applied to the characterization of hydraulically conductive fracture zones in crystalline rock : Olsson, O; Falk, L; Forslund, O; Lundmark, L; Sandberg, E Geophys ProspectV40, N2, Feb 1992, P109–142

Electrical borehole measurements for the mapping of fracture zones in crystalline rock : Jamtlid, A; Magnusson, K A; Olsson, O; Stenberg, L GeoexplorationV22, N3–4, July 1984, P203–216

Electrical borehole measurements for the mapping of fracture zones in crystalline rock : Jamtlid, A; Magnusson, K A; Olsson, O; Stenberg, L GeoexplorationV22, N3–4, July 1984, P203–216

Investigations of Fracture Zones in Crystalline Rock by Borehole Radar

AbstractA borehole pulse radar system has been developed as part of the International Stripa Project with the objective to identify and characterize fracture zones at a considerable distance from boreholes. The radar uses very short pulses, which are transmitted and received by dipole antennas inserted into the boreholes. The pulses are extremely broadband with center frequencies of 25–60 MHz corresponding to wavelengths of a few meters in the rock. At 25 MHz the attenuation in the Stripa granite is 28 dB/100 m and the pulse velocity is approximately 128 000 km/s. Reflection measurements have been used to identify fracture zones and determine their position and orientation. The zones often cause strong and well-defined reflections. Improvements in the pulse form and numerical filtering of the data have consequently made the radar a very efficient instrument for locating fracture zones. During measurements in Stripa reflections from fracture zones have been observed more than 100 m from the borehole.

Electrical borehole measurements for the mapping of fracture zones in crystalline rock

The mise-à-la-masse method has been used for delineation of fracture zones in crystalline rock. Measurements have been carried out at two different sites in Sweden, Finnsjö and Stripa. Measurements in Finnsjö show that the mise-à-la-masse method can be used to interpret the orientation and the dimensional shape of a fracture zone. The greatest distance over which a significantly large fracture zone could be detected from the current electrode was 150 m. In the Stripa mine mise-à-la-masse measurements have given valuable information on the geometry of a large fracture zone reached at the bottom of a vertical borehole even though the measurements were to be made in a mine where the possible locations for the potential electrode were limited to a few boreholes. In Stripa the distance between the current electrode and the potential electrode was at most about 500 m. An apparent resistivity has been defined and used for qualitative interpretation of the data. The qualitative interpretation was successful in the Finnsjö measurements where potential data were available over a large surface. Interpretation has also been made with a semi-quantitative model where the conductivity, width and orientation of a conductive sheet has been fitted to the data. In Stripa this model has given an indication of the orientation of a large fracture zone.

A vertical borehole geophone array

This note describes a vertical borehole geophone array which can be used in slim boreholes. The array was designed as part of a research program in Sweden to study the potential of seismic crosshole techniques for mapping cracks and fracture zones in crystalline rock. The design meets not only the constraints imposed by the slim boreholes (diameter 56 mm) available in Sweden for crosshole surveys, but also the desire to make array recordings with three component locked geophones. The use of vertical arrays in boreholes has been discussed by Galperin (1974).

Electrical cross-hole measurements for the mapping of fracture zones in crystalline rock

Electrical cross-hole measurements for the mapping of fracture zones in crystalline rock