Comparing Shear Wave Refraction and Continuous Surface Wave Surveys in Sand and Gravel
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The aims of this study were to evaluate some surface wave based methods and their limitations with regard to aggregate variability and thickness determinations. We compared the results of field assessments of sand and gravel sequences using two different surface wave survey approaches. The first, followed a seismic refraction approach, and the second, a CSW survey methodology. Further probing using an ultra-lightweight cone penetrometer provided verification of results, and also, an active extraction programme at the field site provided the opportunity to directly observe the subsurface geology post-survey.Keywords:
Penetrometer
Seismic refraction
Field survey
Shallow seismic reflection method is a commonly used technique in urban active fault detection,however,special geotectonic environment may sometimes make reflection survey inapplicable.In such cases,high-resolution seismic refraction could be a feasible option.In this study,we use the finite difference method as the main technique and the conventional methods of refraction data interpretation as auxiliary means in the interpretation of high-resolution shallow refraction data for active fault detection in Lanzhou area.After a comprehensive analysis of first-break refraction travel-time characteristics,the velocity structure and interface structure along each profile have been obtained.A detailed description of the detection results from SS04-1 and SS11-2 seismic profiles is presented in this paper.The main stratigraphic interfaces and tectonic features identified by the two profiles are quite consistent with the results from drilling surveys along the profiles.Our results indicate that high-resolution seismic refraction is an effective replacement in areas where reflection seismic survey is hard to carry out.
Seismic refraction
Reflection
Seismic survey
Synthetic seismogram
Vertical seismic profile
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The refraction method of seismic exploration was initiated in 1952 in the Northern Sahara after several difficulties were encountered with the reflection method. The first tests showed the existence of a deeper marker bed having a velocity of about 6,000 m/sec (20,000 ft/sec) which later proved to be the eroded surface of the basement. Now refraction can be employed, at least in certain regions, in detailed surveys although many difficulties in interpretation still exist. Both field practices and methods of interpretation are discussed.
Seismic refraction
Prospecting
Reflection
Basement
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The theory and practice of using the refraction seismograph for shallow, subsurface investigations is summarized. This is 1ntended to be a guide to the application of the technique and not a comprehensive analysis of every aspect of the method. The fundamentals are presented and their use in time-intercept calculations and interpretations using delay times are discussed. The limitations of this exploration tool are discussed, and other applications of the equipment, such as uphole surveys, are described. Field procedures for carrying out refraction surveys are also recommended. (auth)
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Seismic refraction
Seismic exploration
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Seismic refraction imaging is a technique that has seen an increase of applications in engineering during recent years. In the work presented here a case of refraction tomography in the city of Copenhagen is discussed. The survey included two modalities; 1. a surface survey where 13.9 kilometers of crooked lines along segments of the planned underground metro were mapped, 2. borehole “walk-away” seismic refraction surveys in twenty nine boreholes located in proximity to the surface lines. The overall aim was to map the extension of the near-surface unconsolidated sediments and their interface with underlying sequences of limestone. The results showed it was possible to map the unconsolidated sediments and the underlying limestone. This led to a more reliable interpretation of the surface results along the sections where neither geology nor borehole data was available.
Seismic refraction
Geophysical Imaging
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The historical Christus church in Schwelm, Western Germany, shows structural damages at its front side and its southern tower, which are supposed to be due to the geological situation in the area. To describe the shallow underground below the church and to detect locations of anomalies, a refraction seismic program which uses the complete information of the refracted waves consisting of the travel times as well as the phase and amplitude characteristics in the recorded seismic data, was performed. At first a joint application of Common Midpoint- (CMP-) refraction seismics, which uses first break phases and the GRM was applied. In a second step the resulting intercept time sections were depth converted using the results of the GRM and the refraction tomography, which was performed as well. With the combination of These 3 different refraction seismic processing techniques anomalous zones below the church were detected and classified. These anomalies were due to weak zones and karstified cavities in the limestone below the church, as well as to anthropogenic structures. The anomalous zones were located horizontally and vertically, interpreted and thus, together with the results of refraction seismic, a model of the shallow underground below the church was developed.
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Subsurface structure can be mapped using refraction information from marine multichannel seismic data. The method uses velocities and thicknesses of shallow sedimentary rock layers computed from refraction first arrivals recorded along the streamer. A two‐step exploration scheme is described which can be set up on a personal computer and used routinely in any office. It is straightforward and requires only a basic understanding of refraction principles. Two case histories from offshore Peru exploration demonstrate the scheme. The basic scheme is: step (1) shallow sedimentary rock velocities are computed and mapped over an area. Step (2) structure is interpreted from the contoured velocity patterns. Structural highs, for instance, exhibit relatively high velocities, “retained” by buried, compacted, sedimentary rocks that are uplifted to the near‐surface. This method requires that subsurface structure be relatively shallow because the refracted waves probe to depths of one hundred to over one thousand meters, depending upon the seismic energy source, streamer length, and the subsurface velocity distribution. With this one requirement met, we used the refraction method over a wide range of sedimentary rock velocities, water depths, and seismic survey types. The method is particularly valuable because it works well in areas with poor seismic reflection data.
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Reflection
Vertical seismic profile
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