Analysis using surface wave methods to detect shallow manmade tunnels

Multi-method seismic surface wave approach was used to locate and estimate the dimensions of shallow horizontally-oriented cylindrical voids or manmade tunnels. The primary analytical methods employed were Attenuation Analysis of Rayleigh Waves (AARW), Surface Wave Common Offset (SWCO), and Spiking Filter (SF). Surface wave data were acquired at six study sites using a towed 24-channel land streamer and elastic-band accelerated weight-drop seismic source. Each site was underlain by one tunnel, nominally 1 meter in diameter and depth. The acquired surface wave data were analyzed automatically. Then interpretations compared to the field measurements to ascertain the degree of accuracy. The purpose of this research is to analyze the field response of Rayleigh waves to the presence of shallow tunnels. The SF technique used the variation of seismic signal response along a geophone array to determine void presence in the subsurface. The AARW technique was expanded for practical application, as suggested by Nasseri (2006), in order to indirectly estimate void location using a Normalized Energy Distance (NED) parameter for vertical tunnel dimension measurements and normalized Cumulative Logarithmic Decrement (CALD) values for horizontal tunnel dimension measurements. Confidence in tunnel detects is presented as a measure of NED signal strength. Conversely, false positives are reduced by AARW through analysis of sub-array data. The development of such estimations is a promising tool for engineers that require quantitative measurements of manmade tunnels in the shallow subsurface --Abstract, page iii

Eliminate P-wave Direction in Application of Exploration Seismology

Exploration seismology has substantially contributed to finding and developing giant field worldwide. The technology has evolved in single to two, three dimensional methods, and later added a fourth dimension for reservoir monitoring [1]. In land seismic acquisition, a variety of innovative single, three or multiple geophone has been developed by some manufacturer. In this project, the experiment will demonstrate numerous applications for single component geophone (SM – 24) using different composition of sand and rock proportion. The source vibration will generate by a geophone (SM-24) which supplied external frequency from the function generator. The resulting output by single component geophone only provides the output voltage of the propagating elastic wave in one directio

Constrained Parameterization of the Multichannel Analysis of Surface Waves Approach with Application at Yuma Proving Ground, Arizona

Field data from Yuma Proving Ground, Arizona was used to test the feasibility of merging common multichannel analysis of surface waves (MASW) processing routines with mode- consistent shear-wave refraction traveltime tomography and synthetic modeling to optimize and constrain inversion results. Shear-wave first-arrival refraction tomography was used to enhance layer-model resolution and refine the MASW layer model with independent body-wave information. Shear-wave tomograms suggested a high-velocity layer, not found in initial `smooth' MASW velocity sections that were used as initial models for tomographic inversion. Increasing the stratification of the MASW layer model, to generally match tomogram structure, resulted in a higher-resolution MASW model constrained through joint analysis. This mutual analysis of shear-wave velocity (Vs) provided multiplicity to the structural interpretation of the site. Constrained-parameterization MASW results, compressional-wave tomography (Vp:Vs ratio), and density well logs populated a 2D model for numerical modeling, which was manually updated over several iterations to converge upon the site's first-arrival and dispersion characteristics. Further evaluation of the synthetic seismograms gave insight into the relationship between acquisition geometry (offset selection) and the associated dispersion-image character. Furthermore, modeling gave a secondary measurement on depth to half-space, velocity structure, and relative Vp:Vs ratios, which formulated a final MASW profile. The gradual change of the earth model, given an evolving hierarchy of constraint, is seen as the main finding of this thesis. The calculated movement towards a higher-resolution inversion based on joint geophysical measurements, analysis, and interpretation, engenders a constrained-parameterization solution with highest confidence

The use of seismic techniques to identify hazardous ground conditions associated with cavities

The identification of civil engineering hazards such as cavities, mine shafts, etc., is an integral part of site investigations carried out prior to the construction of roads, tunnels and other civil engineering structures. The use of geophysical methods to identify these hazards is becoming increasingly important. An investigation into the effectiveness of three seismic methods to delineate the possible anomalous ground conditions associated with the presence of cavities has been evaluated. The three methods are: surface, interborehole and a single hole method. The surface seismic method (refraction) was used successfully over a disused railway tunnel to identify the presence of a cavity, and a technique was developed to generate images of a theoretical model, which can be compared to a field survey. The interborehole seismic method was used across a disused railway tunnel to study the effect of a cavity on changes in seismic parameters, such as compressional wave velocity and attenuation. Both of these parameters were sensitive to the presence of a large cavity, and the successful application of the technique is demonstrated in the Maidstone survey, where the presence of loosely packed sand (due to sinkholes in the vicinity) was located under a house. The use of the single hole method for detecting cavities is a new technique (ACDER) in seismics, and it is analogous to methods of radar detection. A sparker source, directional receiver and associated instrumentation were developed in the laboratory before field trials at East Fleet. Of the three methods, the interborehole technique was the most successful, followed by the surface method. The single hole method looks promising but requires further work in the design of directional receiving transducers, followed by more field trials

Underground Vertical Seismic Profiling with Conventional and Fiber-Optic Systems for Exploration in the Kylylahti Polymetallic Mine, Eastern Finland

Seismic reflection methods have been used for the exploration of mineral resources for several decades. However, despite their unmatched spatial resolution and depth penetration, they only have played a minor role in mineral discoveries so far. Instead, mining and exploration companies have traditionally focused more on the use of potential field, electric and electromagnetic methods. In this context, we present a case study of an underground Vertical Seismic Profiling (VSP) experiment, which was designed to image a (semi-)massive sulfide deposit located in the Kylylahti polymetallic mine in eastern Finland. For the measurement, we used a conventional VSP with three-component geophones and a novel fiber-optic Distributed Acoustic Sensing (DAS) system. Both systems were deployed in boreholes located nearby the target sulfide deposit, and used in combination with an active seismic source that was fired from within the underground tunnels. With this setup, we successfully recorded seismic reflections from the deposit and its nearby geological contrasts. The recording systems provided data with a good signal-to-noise ratio and high spatial resolution. In addition to the measurements, we generated a realistic synthetic dataset based on a detailed geological model derived from extensive drilling data and petrophysical laboratory analysis. Specific processing and imaging of the acquired and synthetic datasets yielded high-resolution reflectivity images. Joint analysis of these images and cross-validation with lithological logging data from 135 nearby boreholes led to successful interpretation of key geological contacts including the target sulfide mineralization. In conclusion, our experiment demonstrates the value of in-mine VSP measurements for detailed resource delineation in a complex geological setting. In particular, we emphasize the potential benefit of using fiber-optic DAS systems, which provide reflection data at sufficient quality with less logistical effort and a higher acquisition rate. This amounts to a lower total acquisition cost, which makes DAS a valuable tool for future mineral exploration activities.Peer reviewe

Geophysical methods to detect tunnelling at a geological repository site : Applicability in safeguards

ABSTRACT Generating power with nuclear energy accumulates radioactive spent nuclear fuel, anticipated not to be diversified into any unknown purposes. Nuclear safeguards include bookkeeping of nuclear fuel inventories, frequent checking, and monitoring to confirm nuclear non-proliferation. Permanent isolation of radionuclides from biosphere by disposal challenges established practices, as opportunities for monitoring of individual fuel assemblies ceases. Different concepts for treatment and geological disposal of spent nuclear fuel exist. Spent nuclear fuel disposal facility is under construction in Olkiluoto in Southwest Finland. Posiva Oy has carried out multidisciplinary bedrock characterization of crystalline bedrock for siting and design of the facility. Site description involved compilation of geological models from investigations at surface level, from drillholes and from underground rock characterization facility ONKALO. Research focused on long term safety case (performance) of engineered and natural barriers in purpose to minimize risks of radionuclide release. Nuclear safeguards include several concepts. Containment and surveillance (C/S) are tracking presence of nuclear fuel through manufacturing, energy generation, cooling, transfer, and encapsulation. Continuity of knowledge (CoK) ensures traceability and non-diversion. Design information provided by the operator to the state and European Commission (Euratom), and further to IAEA describes spent nuclear fuel handling in the facility. Design information verification (DIV) using timely or unannounced inspections, provide credible assurance on absence of any ongoing undeclared activities within the disposal facility. Safeguards by design provide information applicable for the planning of safeguards measures, e.g., surveillance during operation of disposal facility. Probability of detection of an attempt to any undeclared intrusion into the repository containment needs to be high. Detection of such preparations after site closure would require long term monitoring or repeated geophysical measurements within or at proximity of the repository. Bedrock imaging (remote sensing, geophysical surveys) would serve for verifying declarations where applicable, or for characterization of surrounding rock mass to detect undeclared activities. ASTOR working group has considered ground penetrating radar (GPR) for DIV in underground constructed premises during operation. Seismic reflection survey and electrical or electromagnetic imaging may also apply. This report summarizes geophysical methods used in Olkiluoto, and some recent development, from which findings could be applied also for nuclear safeguards. In this report the geophysical source fields, involved physical properties, range of detection, resolution, survey geometries, and timing of measurements are reviewed for different survey methods. Useful interpretation of geophysical data may rely on comparison of results to declared repository layout, since independent understanding of the results may not be successful. Monitoring provided by an operator may enable alarm and localization of an undeclared activity in a cost-effective manner until closure of the site. Direct detection of constructed spaces, though possible, might require repeated effort, have difficulties to provide spatial coverage, and involve false positive alarms still requiring further inspection

Detection of cavities by a continuous-wave seismic method

Imperial Users onl

Eliminate P-wave Direction in Application of Exploration Seismology

Exploration seismology has substantially contributed to finding and developing giant field worldwide. The technology has evolved in single to two, three dimensional methods, and later added a fourth dimension for reservoir monitoring [1]. In land seismic acquisition, a variety of innovative single, three or multiple geophone has been developed by some manufacturer. In this project, the experiment will demonstrate numerous applications for single component geophone (SM – 24) using different composition of sand and rock proportion. The source vibration will generate by a geophone (SM-24) which supplied external frequency from the function generator. The resulting output by single component geophone only provides the output voltage of the propagating elastic wave in one directio

Identifying shallow subsurface stratigraphy and voids using dispersive characteristics of electromagnetic and surface waves

This dissertation presents the results of three manuscripts on spectral analysis of electromagnetic and seismic surface waves to detect subsurface stratigraphy and voids. In the first manuscript, a new technique was developed by utilizing spectral analysis of surface waves to detect subsurface openings. This technique applied the concept of group delay to evaluate the effects of subsurface voids on the phase shift domain. Seismic data sets were acquired at different sites where the shape, size, and depth of the void varied. In all surveys, the time delay technique precisely identified the locations of the subsurface openings. The second manuscript presents the results of attenuation analyses of seismic surface waves to identify buried concrete conduits. A conduit of known size and depth was chosen for the study. Vertical and horizontal component geophones with different frequencies (14-, and 100-Hz) were employed for this study. The third manuscript presents the results of a study on dispersive electromagnetic signals to identify the thickness and dielectric permittivity of thin soil layers using Ground Penetrating Radar (GPR) techniques. Data for this experiment were acquired in a large experimental tank, where the permittivity and thickness of each soil layer was controlled. Six different experiments were carried out with three soil types, and variable-offset GPR data were acquired using four frequencies (100-, 250-, 500-, and 1000-MHz). Dispersive GPR signals were analyzed using inversion of dispersion curves to estimate the permittivity and thickness of the overlying soil layers. The accuracy of these estimates were analyzed as a function of layer thickness, antenna frequency, and permittivity. Both the thickness and permittivity estimates were most accurate when the overlying layer had a low permittivity, and higher frequencies usually had more accurate results --Abstract, page iv