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.     

Uncertainty management in integration of exploration data using the belief function

The evidential belief function (EBF) provides an adequate theoretical basis for managing uncertainties in exploration data integration. The EBF can be used to represent uncertainties in the reasoning process and provides the capability of distinguishing between lack of information and negative information. This capability is desirable when combining diverse data sets, which often vary in spatial resolution and spatial extent. The uncertainties associated with data and propositions can be represented naturally and consistently using belief functions. Hence, using the EBF approach can provide a realistic quantitative picture of the target proposition.     

Crustal Structure of the Southern Korean Peninsula from Seismic Waves Generated by Large Explosions in 2002 and 2004

In order to investigate the velocity structure of the southern part of the Korean peninsula, seismic refraction profiles were obtained along a 294-km WNW-ESE line and a 335-km NNW-SSE line in 2002 and 2004, respectively. Seismic waves were generated by detonating 500–1000 kg explosives in drill holes at depths of 80–150 m. The seismic signals were recorded by portable seismometers at nominal intervals of 1.5–1.7 km. Separate velocity tomograms were derived from first arrival times using a series expansion method of travel-time inversion. The raypaths indicate several mid-crust interfaces including those at approximate depths of 2–3, 15–17, and 22 km. The Moho discontinuity with refraction velocity of 7.8 to 8.4 km/s has a maximum depth of 37–39 km under the southern central portion of the peninsula. The Moho becomes shallower as the Yellow Sea and the East Sea are approached on the west and east coasts of the peninsula, respectively. The depth of the 7.6 km/s velocity contour varies from 29.4 km to 36.5 km. The discrepancy in depth between the seismological Moho and the interpreted critically refracting interface may result from the presence of a gradual transition between the crust and mantle. The velocity tomograms show particular crustal structures including (1) the existence of an over 70-km wide low-velocity zone centered at 6–7 km depth under the Okchon fold belt and Ryeongnam massif, (2) existence of high-velocity materials under the Gyeongsang basin, and (3) the downward extension of the Yeongdong fault to depths greater than 10 km.     

Distribution of gold in different size fractions of stream sediments as a guide to bedrock gold mineralisation along the Shyok Suture Zone and adjacent areas of the Kohistan Island Arc,Pakistan

Arabian Journal of Geosciences - This paper describes a new exploration methodology using stream sediments in remote and glaciated regions of northern Pakistan. Gold concentrations in 268 samples...     

Sustained Ability of a Natural Microbial Community to Remove Nitrate from Groundwater

Microbial-mediated nitrate removal from groundwater is widely recognized as the predominant mechanism for nitrate attenuation in contaminated aquifers and is largely dependent on the presence of a carbon-bearing electron donor. The repeated exposure of a natural microbial community to an electron donor can result in the sustained ability of the community to remove nitrate; this phenomenon has been clearly demonstrated at the laboratory scale. However, in situ demonstrations of this ability are lacking. For this study, ethanol (electron donor) was repeatedly injected into a groundwater well (treatment) for six consecutive weeks to establish the sustained ability of a microbial community to remove nitrate. A second well (control) located upgradient was not injected with ethanol during this time. The treatment well demonstrated strong evidence of sustained ability as evident by ethanol, nitrate, and subsequent sulfate removal up to 21, 64, and 68%, respectively, as compared to the conservative tracer (bromide) upon consecutive exposures. Both wells were then monitored for six additional weeks under natural (no injection) conditions. During the final week, ethanol was injected into both treatment and control wells. The treatment well demonstrated sustained ability as evident by ethanol and nitrate removal up to 20 and 21%, respectively, as compared to bromide, whereas the control did not show strong evidence of nitrate removal (5% removal). Surprisingly, the treatment well did not indicate a sustained and selective enrichment of a microbial community. These results suggested that the predominant mechanism(s) of sustained ability likely exist at the enzymatic- and/or genetic-levels. The results of this study demonstrated the in situ ability of a microbial community to remove nitrate can be sustained in the prolonged absence of an electron donor.     

Assessing the validity of facilitated-volunteered geographic information: comparisons of expert and novice ratings

Facilitated-voluntary geographic information (f-VGI) is a promising method to enable systematic collection of data from residents about their physical and social environment. The method capitalizes on ubiquitous mobile smartphones to empower collection of geospatially-referenced data. It is important to evaluate the validity of user-generated content for use in research or program planning. The purpose of this study was to test whether the aggregated environmental (“bikeability”) ratings from novice community residents converges with ratings from experts using a robust research-based, paper audit-tool (the established Pedestrian Environment Data Scan (PEDS) tool). Equivalence testing statistically showed overall agreement between the composite ratings of bikeability within the novice group. Agreement in categorical ratings between novices and experts were examined using the summary agreement index, which showed substantial agreement across the 10 locations rated by 11 novices using an f-VGI mobile application and four experts using PEDS; variability depended on the nature of the specific questions asked. Results reveal overall substantial agreement between novice and expert ratings for both composite scores and individual categorical ratings. However, additional research is needed to refine the methodology for use in formalized research applications.     

Practical considerations for the segmented-flow analysis of nitrate and ammonium in seawater and the avoidance of matrix effects

In this study we describe measures taken in our laboratory to improve the long-term precision of nitrate and ammonia analysis in seawater using a microflow segmented-flow analyzer. To improve the nitrate reduction efficiency using a flow-through open tube cadmium reactor (OTCR), we compared alternative buffer formulations and regeneration procedures for an OTCR. We improved long-term stability for nitrate with a modified flow scheme and color reagent formulation and for ammonia by isolating samples from the ambient air and purifying the air used for bubble segmentation. We demonstrate the importance of taking into consideration the residual nutrient content of the artificial seawater used for the preparation of calibration standards. We describe how an operating procedure to eliminate errors from that source as well as from the refractive index of the matrix itself can be modified to include the minimization of dynamic refractive index effects resulting from differences between the matrix of the samples, the calibrants, and the wash solution. We compare the data for long-term measurements of certified reference material under two different conditions, using ultrapure water (UPW) and artificial seawater (ASW) for the sampler wash.