Shallow Seismic Refraction Data Research Articles

Geophysical and structural analyses of the Middlesboro impact structure, Kentucky, USA: Reactivation of a thrust detachment of the Appalachian foreland fold‐and‐thrust belt

AbstractThis paper reports on the Middlesboro impact structure (36°37′03″ N; 83°43′39″ W), a complex impact structure located in the Appalachian foreland fold‐and‐thrust belt of southeast Kentucky, USA. The structure forms a basin approximately 5.5 km in diameter exposing intensely deformed Pennsylvanian sediments. Based on field data, microstructural observation, and geophysical analyses we qualitatively assess degree and distribution of fragmentation of target rocks within the impact structure. Shock deformation features, especially feather features, are reported for the impact structure. Shallow seismic refraction data were acquired along a radial profile from outside the impact structure to the center of the structure. Seismic (P‐wave) velocity trends indicate that fracture intensity increases toward the center, reducing the overall seismic velocity. However, intense rock sealing in some parts of the central uplift increases the strength of rock samples and locally increases seismic velocities. We present a modified geologic map of the Middlesboro impact structure based on high‐resolution lineament analysis, field work, and reevaluation of existing structural data from geologic maps. The modified map suggests that crater circularity at Middlesboro is skewed by the reactivation of pre‐existing zones of structural anisotropy in the target. We propose that due to an oblique impact from northerly direction the sub‐horizontal Pine Mountain thrust plane beneath the impact structure became reactivated with a top‐to‐the south shear component. The impact‐induced formation/reactivation of the Doublings Fault Zone as a thrust ramp appears to have deflected the otherwise straight strike of the Cumberland Mountains southward. The Rocky Face fault approaches the crater rim fault, and we propose that this sinistral strike‐slip fault was formed, or at least reactivated, by the impact and accommodated the oblique impact's horizontal momentum.

Evaluating the structural model from ambient vibration recordings and shallow seismic refraction techniques. A case study of Zagazig City, Egypt

This study uses the seismic refraction and noise measurements to investigate the velocity structure of the subsurface and emphasize the advantage of ambient vibration over the conventional seismic refraction technique. Field measurements were carried out at nine sites in and around Zagazig city. Shallow seismic refraction data were interpreted using the delay time method to obtain the two-dimension ground model at each site. Ambient vibration arrays are used to infer the one-dimensional compressional and shear wave velocity profiles through two main steps. The first step is to derive the dispersion curve from the recorded signals using the frequency-wavenumber method. The second is to invert the dispersion curve to obtain the site velocity profiles. The results of the compressional wave velocities obtained from seismic refraction technique showed that the subsurface consists of a number of layers ranging from two to four layers and give a good agreement with the results of the seismic wave velocities obtained from the ambient vibration arrays. The ambient vibration arrays gave a deeper depth of penetration than the other method, providing more information on the subsurface structure without any disturbance to the environment. This work provides reliable estimates of the seismic velocity structures of both shallow and deep sedimentary layers within the area of interest.

Geophysical investigation in the Northwestern part of the Gulf of Suez, Egypt

Vertical Electrical Soundings (VES) and shallow seismic refraction data interpretation have been used to delineate groundwater aquifer and fault elements which are dissecting the northwestern part of Gulf of Suez. Thirty-five Vertical Electrical Soundings (VES) were carried out and inverted through 3-D VES inversion to determine the subsurface stratigraphy, structures and groundwater aquifer potentialities. The results of VES inversion indicate that the study area consists of four geoelectrical units interpreted as surficial dry sand and gravels deposits, underlain by fresh water bearing zone, salt water bearing unit and limestone layer, at the bottom. The 3-D VES inversion indicates that the area is dissected by normal fault of NE-SW direction. Thirty-four shallow seismic refraction profiles of 94m spread length have been used to delineate the geotechnical characteristics of surface layers, subsurface structures and subsurface lithology. The results of shallow seismic refraction indicate that the shallow part of the subsurface section consists of three layers, the first soil layer (dry gravels and sands of the recent deposits) underlain by the second soil layer (sands and gravels of the Pleistocene) while the third layer is the bedrock layer of sandstone and shale belonging to the Middle Miocene.

Estimating the near-surface site response to mitigate earthquake disasters at the October 6th city, Egypt, using HVSR and seismic techniques

The damage caused by earthquake occurrences in different localities necessitates the evaluation of the subsurface structure. A priori estimation of the site effects became a major challenge for an efficient mitigation of the seismic risk. In the case of moderate to large earthquakes, at some distances from large events, severe damage often occurred at zones of unfavorable geotechnical conditions that give rise to significant site effects. The damage distribution in the near-source area is also significantly affected by fault geometry and rupture history. The microtremor (background noises) and shallow seismic surveys (through both the seismic refraction and Multi-channel Analysis of Surface Waves (MASW)) were carried out in a specific area (The club of October 6 city and its adjacent space area). The natural periods derived from the HVSR (Horizontal to Vertical Spectral Ratio) analysis vary from 0.37 to 0.56s. The shallow seismic refraction data, which were conducted at the area, are used to determine the attenuation of P-waves (Qp) in different layers, using the pulse-width technique. The evaluation of the site response at the studied area yields amplification factor of the ground motion, ranging between 2.4 and 4.4.

Attenuation measurements on shallow seismic refraction data in the Kocaeli region, Turkey

Attenuation of P-waves (Qp) in different geologic units in the Kocaeli region, NW of Turkey, was determined using both the pulse-broadening and the spectral amplitude ratio techniques. With that aim, shallow seismic refraction data were conducted at total 97 points at the Sirintepe, Akveren, Izmit, Korucu, Arslanbey, Sopali, Peksimet and the Cinarlidere formation sites, which are generally in dry states located in the vicinity of the North Anatolia Fault within the Kocaeli segment. The Qp values determined from both techniques were correlated. The wide variation of the computed Qp values indicated that the spectral technique can found to be generally slightly better than the pulse-broadening method. In addition to these studies, the dynamic elastic properties were figured out using P and velocities determined from seismic refraction studies, and the natural water content and density of the subsoil layers were determined from the soil samples collected from total 36 boreholes drilled in the formations to observe their variation with Qp values. The dynamic elastic properties determined using P- and S-wave velocities in empirical equations were correlated with the quality factors. Generally, the physical parameters did not represent any linear relations with the quality factors which demonstrated that the attenuation characteristics of seismic waves cannot be used to derive dynamic elastic parameters of the subsurface sediments.

Seismic refraction characteristics of the Elura ore body and regolith

Major exploration difficulties occur when the distinctive characteristics of ore targets are obscured or modified by physical property variations within the regolith. Improved exploration success in the highly variable Australian regolith requires greater understanding of this medium which can only be achieved with new or improved exploration technologies.The Cobar District of New South Wales is one of the world’s most active mineral exploration regions with a variable regolith. This region has yielded substantial mineral wealth and the Elura ore body is one of the largest massive sulphide deposits to have entered the Cobar regolith. Previous pre-mining, shallow seismic refraction data over this ore body have been re-interpreted using visual interactive ray tracing and wavepath eikonal tomography. This improved interpretation approach has been integrated with the most recent geological knowledge, weathering history and the seismic properties of the shallow ore and host rocks to refine the seismic characteristics of the Elura ore body and regolith. The interpretations have confirmed the earlier qualitative interpretation that the Elura gossan and the altered ore zone form a local, low-velocity plug extending to a depth of ~100 m within the shallower, higher velocity weathered and fresh siltstone host rocks. The margins of this plug are well defined in the refraction interpretation as they form strong seismic wave diffraction sites at the base of the surrounding regolith. The base of this plug, representing the altered massive sulphide ore, also tends to have a lower seismic velocity than the fresh host rocks. Velocity information on the deeper gossan, supergene zone could not be obtained directly from the first-arrival seismic data as this region is laterally hidden. It is clear from this interpretation that the base of the regolith over the Elura ore body and margins are highly irregular and not well represented as a single continuous refractor as required by less sophisticated refraction interpretation approaches.This case study shows that detailed seismic refraction, supported by improved interpretation techniques and petrophysical testing, provide detailed regolith information and have increased exploration potential for massive sulphide targets that enter or are close to the regolith.

Application of Amplitudes in Shallow Seismic Refraction Inversion

I generate three starting models for the inversion of a set shallow seismic refraction data using wavepath eikonal traveltime tomography. Two models are generated with the generalized reciprocal method (GRM), which uses a multi-layer model with discrete velocity changes while the other is generated with a one dimensional (1D) inversion algorithm similar to the tau-p method, which uses a model with vertical velocity gradients. There is little correlation between the results in either the absolute depths, the relative depths or the refractor velocities. I attribute the differences in the absolute depths to the occurrence of a velocity reversal in the overburden, the existence of which is indicated by the concave-upwards shape of the traveltime graphs and by the extremely rapid decrease in amplitudes. I attribute the differences in the relative depths and seismic velocities to the poor starting model generated by the 1D tau-p inversion algorithm, which is unable to resolve the fundamental ambiguity between seismic velocity and structure on refracting interfaces. The GRM-derived refractor velocities are compatible with the head coefficients determined with amplitude products. The existence of velocity gradients is suggested by the convergence of the traveltime graphs with increasing shot-to-detector distance. I attribute this convergence in part to the large variations in signal-to-noise ratios, which affect the accuracy of the traveltime data. The shot amplitudes decrease rather than increase with distance, which supports the occurrence of distinct interfaces with velocity gradients over relatively small depths. Therefore, while velocity gradients are likely to occur, the results generated with 1D tau-p tomography are probably not representative of the subsurface conditions. The considerable differences between the results generated with the GRM and those obtained with 1D tau-p tomography are compelling demonstrations of the non-uniqueness of model-based inversion and of the importance of the selection of the initial model. Accordingly, all methods of model-based inversion of shallow refraction data could usefully include starting models generated with the GRM or the refraction convolution section.

A meaningful correlation between shallow seismic refraction data to borehole data in site investigation work

In site investigation work the subsurface characteristic is mainly determined from boreholes data such as the bore log, standard penetration test and other suitable test. Surface geophysical survey including seismic refraction has been routinely used to compliment the borehole data. In many small scale site investigation boreholes are made mainly using the wash boring method and are normally very shallow. The standard penetration test value N = 50 is used to end the borehole and it occurs normally within the weathered zone or above the bedrock surface. The seismic work determines the depth and characteristic of the bedrock as well as the strata above the bedrock. The seismic refraction data is compared to the boring data and often used to extrapolate the subsurface condition between the boreholes. Correlation of seismic refraction data to wash boring data is often difficult because the wash boring bore log description is mainly grain sized based and not lithological based. Comparison to both the bore log and the standard penetration test values allows a better correlation. In addition, at the shallower level the Mackintosh probe test value and the water table has been used. The two sites studied show that comparison of the seismic refraction data to an integrated data gives a more meaningful and useful correlation. Abstrak: Dalam kerja penyiasatan tapak ciri ciri subpermukaan biasanya di tentukan dari data lubang bor termasuk log lubang bor, ujian tusukan piawai atau dari ujian ujian lain yang bersesuaian. Survei geofizik permukaan termasuk seismos biasan selalu di gunakan bersama dengan lubang bor. Dalam penyiasatan tapak skala kecil pengrudian biasanya mengunakan kaedah pemboran basuhan dan hujung lubang bor biasanya cetek sahaja. Nilai ujian tusukan piawai N = 50 digunakan mengakhiri pemboran dan biasanya ia berada di zon luluhawa dan di atas paras batudasar. Kerja seismos biasan menentukan kedalaman batudasar dan ciri strata di dari batudasar. Data seismos di bandingkan kapada data lubang bor dan biasanya di gunakan bagi menganggarkan ciri subpermukaan antara lubang bor. Pengkaitan antara data seismos kapada data lubang bor dari pemboran basuhan sahaja biasanya sukar kerana penerangan log lubang bor dari kaedah ini adalah berdasarkan kapada saiz butiran dan bukan litologi. Perbandingan data seismos kapada data lubang bor bersama nilai ujian tusukan piawai memberikan perkaitan yang lebih baik. Pada kedalaman cetek perbandingan juga di buat kapada uji prob Mackintosh dan aras air tanah. Kajian di dua tapak menunjukan perbandingan data seismos kepada data bersepadu lubang bor dapat memberikan perhubungan yang lebih bermakna dan berguna.

A new direction for shallow refraction seismology: integrating amplitudes and traveltimes with the refraction convolution section

The refraction convolution section (RCS) is a new method for imaging shallow seismic refraction data. It is a simple and efficient approach to full‐trace processing which generates a time cross‐section similar to the familiar reflection cross‐section. The RCS advances the interpretation of shallow seismic refraction data through the inclusion of time structure and amplitudes within a single presentation. The RCS is generated by the convolution of forward and reverse shot records. The convolution operation effectively adds the first‐arrival traveltimes of each pair of forward and reverse traces and produces a measure of the depth to the refracting interface in units of time which is equivalent to the time‐depth function of the generalized reciprocal method (GRM). Convolution also multiplies the amplitudes of first‐arrival signals. To a good approximation, this operation compensates for the large effects of geometrical spreading, with the result that the convolved amplitude is essentially proportional to the square of the head coefficient. The signal‐to‐noise (S/N) ratios of the RCS show much less variation than those on the original shot records. The head coefficient is approximately proportional to the ratio of the specific acoustic impedances in the upper layer and in the refractor. The convolved amplitudes or the equivalent shot amplitude products can be useful in resolving ambiguities in the determination of wave speeds. The RCS can also include a separation between each pair of forward and reverse traces in order to accommodate the offset distance in a manner similar to the XY spacing of the GRM. The use of finite XY values improves the resolution of lateral variations in both amplitudes and time‐depths. The use of amplitudes with 3D data effectively improves the spatial resolution of wave speeds by almost an order of magnitude. Amplitudes provide a measure of refractor wave speeds at each detector, whereas the analysis of traveltimes provides a measure over several detectors, commonly a minimum of six. The ratio of amplitudes obtained with different shot azimuths provides a detailed qualitative measure of azimuthal anisotropy and, in turn, of rock fabric. The RCS facilitates the stacking of refraction data in a manner similar to the common‐midpoint methods of reflection seismology. It can significantly improve S/N ratios.Most of the data processing with the RCS, as with the GRM, is carried out in the time domain, rather than in the depth domain. This is a significant advantage because the realities of undetected layers, incomplete sampling of the detected layers and inappropriate sampling in the horizontal rather than the vertical direction result in traveltime data that are neither a complete, an accurate nor a representative portrayal of the wave‐speed stratification. The RCS facilitates the advancement of shallow refraction seismology through the application of current seismic reflection acquisition, processing and interpretation technology.

Attenuation measurements on shallow seismic refraction data : Hatherly, P J Geophysics V51, N2, Feb 1986, P250–254

Attenuation measurements on shallow seismic refraction data : Hatherly, P J Geophysics V51, N2, Feb 1986, P250–254

Attenuation measurements on shallow seismic refraction data

Techniques of making seismic attenuation measurements are of interest in engineering geophysics because they allow rock type and quality to be estimated. The measurements may be made on field data from spectral amplitudes, amplitude decay curves, or observed pulse broadening. With shallow seismic refraction data, attenuation is best measured from the pulse broadening. I discuss the problem and demonstrate a computer technique for making routine measurements.

On: “Refraction Determination of Water Table Depth and Alluvium Thickness” by James Otto Duguid (GEOPHYSICS, June 1968, p. 481–488)

Mr. Duguid’s study provides an opportunity to compare borehole information with shallow seismic refraction data. The single geophone technique is well adapted to very shallow refractors such as those present in this study.