Refraction Interpretation Research Articles

The sensitivity of seismic refraction velocity models to survey geometry errors, assessed using Monte Carlo analysis

Seismic refraction models should routinely be reported with their associated uncertainty. Tomographic solutions are widespread, but estimating uncertainties in these via Monte Carlo simulation places great demands on computer resource, hence this task is often omitted. By considering the Plus-Minus method of seismic refraction interpretation, we use Monte Carlo simulations to evaluate the uncertainty in seismic refraction results and determine the sources of uncertainty that are most impactful on the reliability of the output model. Our analysis considers the impact of survey mislocation (i.e., geophones misplaced from a planned position) and interpretational problems (i.e., misidentification of first-break picks and uncertainty in identifying crossover distances) on the overall uncertainty in inferred unit thicknesses and seismic velocities. These are considered for synthetic data with varying subsurface velocity structure, and for field data collected at a shallow (< 50 m) bedrock site in north Wales (UK). Analysis of synthetic data shows that the impact of the aforementioned errors on thickness estimates is ∼1000 times that on velocity estimates. Of all permutations tested, the most significant impact on thickness uncertainty was the accuracy of first-break picks, with the variance in target thickness estimates increasing roughly exponentially with first-break pick uncertainty. It is therefore prudent to minimise such uncertainty through appropriate survey practice (e.g., maximising source energy, taking multiple shots for stacking) and to properly define the resultant uncertainty in unit thickness and velocity estimates. The simplicity of the Plus-Minus method makes it an effective tool for highlighting the errors that would impact more sophisticated interpretation approaches, such as tomography or Full Waveform Inversion. The results from such analysis can be directly applied in straightforward environmental or engineering investigations and can be used to inform more advanced refraction methods. As such, the practice we highlight should be considered for any refraction interpretation.

Flexible layer-based 2D refraction tomography method for statics corrections

Standard refraction traveltime tomography solutions for statics corrections do not allow irregular parameterization of the model, simultaneous inversion of velocity and depth parameters, or selection of the types of first arrival times to be inverted. An algorithm widely used to study and build crustal structure and velocity models from wide-angle seismic data was applied for the modeling and inversion of the velocity and structure of near-surface layers. This flexible tomographic solution allows the definition of complex heterogeneous models through trapezoids of irregular size to parameterize the model, and it allows the simultaneous inversion of head-waves and turning-rays, which can be defined independently for each layer interface. This work presents a study of the application of this flexible tomographic inversion algorithm to obtain the velocity model and geological structure of the near-surface weathering zone. The proposed workflow was evaluated on two synthetic models built with real topography, and geophysical and geological information was obtained from real cases. The first is a model with three layers with constant velocities, and the second is a model with two heterogeneous layers with lateral and vertical velocity variations in the weathering zone. For the second model, turning-rays crossing the weathering layer are considered in the tomography process. Synthetic seismic data were generated, and an initial model was built using traditional seismic refraction interpretation methods. Thereafter, inversion was performed using a 2D ray-based tomography algorithm to minimize first-arrival traveltime misfits by inverting the seismic velocities and the vertical position of the refractors. It was verified that the proposed workflow obtained the smallest relative errors in most parameters of the velocity model and consequently, for the statics corrections.

An interpretation of wave refraction and its influence on foreshore sediment distribution

To analyze the grain size and depositional environment of the foreshore sediments, a study was undertaken on wave refraction along the wide sandy beaches of central Tamil Nadu coast. The nearshore waves approach the coast at 45° during the northeast (NE) monsoon, at 135° during the southwest (SW) monsoon and at 90° during the non-monsoon or fair-weather period with a predominant wave period of 8 and 10 s. A computer based wave refraction pattern is constructed to evaluate the trajectories of shoreward propagating waves along the coast in different seasons. The convergent wave rays during NE monsoon, leads to high energy wave condition which conveys a continuous erosion at foreshore region while divergent and inept condition of rays during the SW and non-monsoon, leads to moderate and less energy waves that clearly demarcates the rebuilt beach sediments through littoral sediment transport. The role of wave refraction in foreshore deposits was understood by grain size and depositional environment analysis. The presence of fine grains with the mixed population, during the NE monsoon reveals that the high energy wave condition and sediments were derived from beach and river environment. Conversely, the presence of medium grains with uniform population, during SW and non-monsoon attested less turbulence and sediments were derived from prolong propagation of onshore-offshore wave process. These upshots are apparently correlated with the in situ beach condition. On the whole, from this study it is understood that beaches underwent erosion during the NE monsoon and restored its original condition during the SW and non-monsoon seasons that exposed the stability of the beach and nearshore condition.

REGIONAL STRUCTURAL‐TECTONIC ZONING OF THE UPPER CRUST OF TRANSBAIKALIA BASED ON SEISMOGRAVITATIONAL DATA ALONG REFERENCE PROFILE 1‐SB

Upper crust fault and fold patterns are studied with an innovative approach of controlled‐source seismic refraction profiling integrated with gravity data. The potentiality of the conventional method based on refrac‐ tion/wide‐angle reflection traveltimes, which has been so far applied to plane‐parallel layered earth, is extended onto deformed crust with lateral and vertical inhomogeneities due to advanced processing technologies. Performance of the approach is tested on data of the 1‐SB transect that traverses geologically studied areas and provides good refer‐ ence. Upper crust features revealed from seismic data agree with known geological structures, this indicating the basic applicability of refraction surveys to tectonically complex areas with 7 to 10 km wide low‐angle dipping fault zones between blocks marked by velocity anomalies to depths of 4–6 km. Joint interpretation of refraction and gravity patterns becomes possible within the limits of a seismic‐gravity model in which both are characterized by seismic‐ density and seismic acceleration values of the same dimension.

Application of undecimated discrete wavelet transforms (UDWT) for seismic refraction velocity analysis improvement

In this study, the wavelet transform is applied to the seismic refraction data to provide an accurate first break picking result. The goal is not to apply the threshold to filter the data for the random noise suppression. We tried to introduce the wavelet domain as a replacement for the time domain. Thus, changing the domain is so hazardous; because the kernel effects of each transform could change the first break, which means that an erroneous refraction interpretation but note that DWT approaches have divided into many types based on frames, filter Banks and dimensions. In this way, this study has found one of the suitable domains of DWT that could provide a reliable result. Another aspect of this study that should be mentioned is that there is no reconstruction performed after decomposition. Our finding shows that to provide a better decomposition for the first break, picking the wavelet type must be short enough to avoid the effect of the kernel selection. We are, therefore, test db2 as a mother wavelet. After the decomposition, the data will be provided to a number of scales. Each scale has its frequency content, which means that random noise concentration will be at particular scales. These scales that contain more random noise will be eliminated and the best scale with the lowest possible amount of random noise will be chosen by the statistical approaches. After this selection, the first break selection will proceed to provide accurate interpretations. Subsequently, the velocity of each layer will be estimated using these first arrival times. Another achievement in this study is to find a true scale in DWT domain to avoid redundancy aspects. To select the true scale, a statistical approaches proved that we can find best scale for analysis.

Various Approaches to Forward and Inverse Wide-Angle Seismic Modelling Tested on Data from DOBRE-4 Experiment

The interpretation of seismic refraction and wide angle reflection data usually involves the creation of a velocity model based on an inverse or forward modelling of the travel times of crustal and mantle phases using the ray theory approach. The modelling codes differ in terms of model parameterization, data used for modelling, regularization of the result, etc. It is helpful to know the capabilities, advantages and limitations of the code used compared to others.

Near-surface shear-wave velocity estimation based on surface-wave inversion

Rayleigh waves always coexist with reflection waves in land seismic exploration. Many previous works have shown the value of Rayleigh waves in obtaining near-surface shear velocity, however, few of them show successful applications from exploration geophysics. With an aim of practical application of Rayleigh waves in exploration data, we discuss how to obtain an accurate and high-resolution dispersion curve, the inversion algorithm, and the comparison of velocity estimated from Rayleigh-wave inversion with refraction interpretation and tomographic inversion. We first analyze the resolution difference of f-v spectra calculated through f-k, τ-p, and phase-shift approaches and propose a two-step spectrum normalization scheme for accurate picking of the dispersion curve. We then adopt the weighted damped least-squares method as the inversion algorithm and find through synthetic data that the estimated shear velocity based on the algorithm is quite accurate and the inversion algorithm is fast and robust. Finally, we select seismic data from one land exploration project in China as an example for test, and find the inverted velocity from Rayleigh-wave dispersion has a good correlation with refraction interpretation and tomographic inversion.

Controls on the location of compressional deformation on the NW European margin

Abstract The distribution of Cenozoic compressional structures along the NW European margin has been compared with maps of the thickness of the crystalline crust derived from a compilation of seismic refraction interpretations and gravity modelling, and with the distribution of high-velocity lower crust and/or partially serpentinized upper mantle detected by seismic experiments. Only a subset of the mapped compressional structures coincide with areas susceptible to lithospheric weakening as a result of crustal hyperextension and partial serpentinization of the upper mantle. Notably, partially serpentinized upper mantle is well documented beneath the central part of the southern Rockall Basin, but compressional features are sparse in that area. Where compressional structures have formed but the upper mantle is not serpentinized, simple rheological modelling suggests an alternative weakening mechanism involving ductile lower crust and lithospheric decoupling. The presence of pre-existing weak zones (associated with the properties of the gouge and overpressure in fault zones) and local stress magnitude and orientation are important contributing factors.

Study of iron deposit using seismic refraction and resistivity in Carajás Mineral Province, Brazil

This work comprises the acquisition, processing and interpretation of 2D seismic shallow refraction (P-wave) and resistivity profiles located in the iron ore deposit of N4WS, Carajás Mineral Province (CMP), northern Brazil. The geophysical methods were used to identify the boundaries of the iron ore deposit. Another objective was to evaluate the potentiality of these geophysical methods in that geological context. In order to validate the results, the geophysical lines were located to match a geological borehole line. For the seismic refraction, we used 120 channels, spaced by 10m, in a line of 1190m, with seven shot points. The resistivity method used in the acquisition was the electrical resistivity imaging, with pole-pole array, in order to reach greater depths. The resistivity line had a length of 1430m, with 10m spacing between electrodes. The seismic results produced a model with two distinct layers. Based on the velocities values, the first layer was interpreted as altered rocks, and the second layer as more preserved rocks. It was not possible to discriminate different lithologies with the seismic method inside each layer. From the resistivity results, a zone of higher resistivity (>3937Ω·m) was interpreted as iron ore, and a region of intermediate resistivity (from 816 to 2330Ω·m) as altered rocks. These two regions represent the first seismic layer. On the second seismic layer, an area with intermediated resistivity values (from 483 to 2330Ω·m) was interpreted as mafic rocks, and the area with lower resistivity (<483Ω·m) as jaspilite. Our results were compared with geological boreholes and show reasonable correlation, suggesting that the geophysical anomalies correspond to the main variations in composition and physical properties of rocks.

HYDROGEOPHYSICS TECHNIQUES TO AQUIFER LOCATING AND MONITORING FOR INDUSTRIAL ZONE NORTH THANGLONG AND QUANGMINH, HANOI

HYDROGEOPHYSICS TECHNIQUES TO AQUIFER LOCATING AND MONITORING FOR INDUSTRIAL ZONE NORTH THANGLONG AND QUANGMINH, HANOI

Detecting different water table levels in a shallow aquifer with combined P-, surface and SH-wave surveys: Insights from VP/VS or Poisson's ratios

When applied to hydrogeology, seismic methods are generally confined to the characterisation of aquifer geometry. The joint study of pressure- (P) and shear- (S) wave velocities (VP and VS) can provide supplementary information and improve the understanding of aquifer systems. This approach is proposed here with the estimation of VP/VS ratios in a stratified aquifer system characterised by tabular layers, well-delineated thanks to Electrical Resistivity Tomography, log and piezometer data. We carried out seismic surveys under two hydrological conditions (high and low flow regimes) to retrieve VS from both surface-wave dispersion inversion and SH-wave refraction interpretation, while VP were obtained from P-wave refraction interpretation. P-wave first arrivals provided 1D VP structures in very good agreement with the stratification and the water table level. Both VS models are similar and remain consistent with the stratification. The theoretical dispersion curves computed from both VS models present a good fit with the maxima of dispersion images, even in areas where dispersion curves could not be picked. Furthermore, VP/VS and Poisson's ratios computed with VS models obtained from both methods show a strong contrast for both flow regimes at depths consistent with the water table level, with distinct values corresponding to partially and fully saturated sediments.

A crustal model for northern Melville Bay, Baffin Bay

AbstractThe interpretation of seismic refraction and gravity data acquired in 2010 gives new insights into the crustal structure of the West Greenland coast and the adjacent deep central Baffin Bay basin. Underneath Melville Bay, the depth of the Moho varies between 26 and 17 km. Stretched continental crust with a thickness of 25 to 14 km and deep sedimentary basins are present in this area. The deep Melville Bay Graben contains an up to ~11 km thick infill of consolidated and unconsolidated sediments with velocities of 1.6 to 4.9 km/s. Seaward, at the ~60 km wide transition between oceanic and stretched continental crust, a mount‐shaped magmatic structure is observed, which most likely formed prior to the initial formation of oceanic crust. The up to 4 km high magmatic structure is underlain by a ~2 km thick and ~50 km wide high velocity lower crust. More to the west, in the oceanic part of the Baffin Bay basin, we identify a two‐layered, 3.5 to 6 km thin igneous oceanic crust with increasing thickness toward the shelf. Beneath the oceanic crust, the depth of the Moho ranges between 11.5 and 13.5 km. In the western part of the profile, oceanic layer 3 is unusually thin (~1.5 km) A possible explanation for the thin crust is accretion due to slow spreading, although the basement is notably smooth compared to the basement of other regions formed by ultra‐slow spreading. The oceanic crust is underlain by partly serpentinized upper mantle with velocities of 7.6 to 7.8 km/s.

DOWNHOLE SEISMIC REFRACTION SURVEY OF WEATHERED LAYER CHARACTERISTICS IN ESCRAVOS, NIGERIA

The problem of datum correction in seismic surveys and the failures associated with civil engineering structures with specific reference to roads and bui ldings has been attributed to lack of understanding of the soil structure among others. This is why this surve y on downhole seismic refraction was conducted from nine borehole locations in Escravos, Nigeria to det ermine the weathered layer characteristics of the a rea. In carrying out this study, the McSeis 160MX instrument was employed in recording data obtained from a se t of hydrophones arranged in a definite manner in eac h of the boreholes. The data were analysed graphica lly by using the seismic refraction interpretation tech niques. Results of interpretation indicate that the velocity of the weathered layer ranged between 454 and 533 m sec -1 with an average value of 495 m sec -1 . The results also revealed that the depth of the weather ed layer ranged between 2.91 and 4.41 m with an average value of 3.68 m. Analysis of the results revealed t hat the depth and velocity of the weathered layer i ncreased downward towards the southern part of the study are a. This is an indication that the weathered layer s loped downward to the south. It is recommended that shots for seismic survey should be located at a depth of 4.41 m to eliminate the effects associated with the low velocity layer. It is also recommended that constru ction of heavy buildings and roads especially bridges should have their foundations laid to a depth of 4.41 m t o enable the infrastructure sit on the consolidated l ayer.

Imaging Geological Structures Up to the Acquisition Surface Using a Hybrid Refraction-Reflection Seismic Method

The aim of seismic imaging is to reconstruct the reflectivity associated with subsurface structures. In standard imaging techniques, the reflectivity model usually starts a few meters below the surface, the actual depth being dependent on data acquisition parameters and the mute used to remove stretching of first arrivals after normal moveout correction. In this paper, we describe a method to image the reflectivity of near-surface structures starting from the acquisition surface. This is achieved by processing both the first arrivals and the reflected phases present in data collected for refraction surveys. The proposed imaging procedure works in three steps. First, we obtain a velocity model for the shallow region by combining the Plus-Minus method of refraction interpretation with tomographic inversion of first arrival times. Second, by processing reflection events present in the refraction data, we obtain a standard reflectivity section for the deeper region. Finally, we compute reflectivity for the shallow region using the velocity model estimated from first arrival information in step 1. This velocity model is used both to compute reflectivity and to convert it in time. The reflectivity obtained for the shallow region is associated with velocity contrasts. In order to merge it with the reflectivity section for the deeper region a scaling factor between the two sets of reflectivity sections must be computed and applied. The novelty of this contribution is the use the tomographic velocity model in evaluating reflectivity for the upper part of the section. This improves the continuity of information about all near-surface structures in comparison with previous works that were limited to reflection data. Three field examples illustrate the proposed procedure showing continuous information about reflectivity of structures starting from the acquisition surface.

UPHOLE SEISMIC REFRACTION SURVEY FOR LOW VELOCITY LAYER DETERMINATION OVER YOM FIELD, SOUTH EAST NIGER DELTA

The uphole refraction survey was carried out in ‘‘YOM’’ field 4-D prospect to evaluate the weathering thickness and velocity of the low velocity layer (LVL). Five uphole locations were occupied in the study area and the time – intercept technique of the seismic refraction interpretation was employed. For the different locations (A, B, C, D and E), the weathering thickness was found to vary from 0.9m to 1.9m with velocity from 250m/s to 800m/s. A sub weathering layer was also observed in all locations except A. Also, the velocity of the consolidated layer was found to vary from about 1750m/s to 1800m/s with a mean velocity of 1766m/s. Using this result to eliminate the effect of LVL on reflection data from the graphical approach, it was observed that the elevation and weathering correction (static correction) was found to be eliminated at a datum of 2m deep.

Understanding the 3D Structure of a Thermal Water Fissured Granite Aquifer by Use of Geophysical Studies

The thermal hot springs of Caldas de Moncao, located in Northern Portugal, are supplied by a thermal water aquifer that must be preserved from pollution and anthropic effects. Enhancement of knowledge about the aquifer is therefore needed, so sustainable management and water quality preservation can be conducted, namely by replacing the springs by boreholes, which are less vulnerable to pollution. Several investigations have been conducted with this purpose, to review the available geological and hydrogeological information from the area. For this reason, several geophysical surveys were reprocessed and others were acquired and planned, so that a 3D conceptual groundwater flow model of could be drawn. Three seismic reflection profiles were acquired with the objective of locating deep faults capable of transporting groundwater to the surface and to complement information about shallow features available before the seismic survey. This infor- mation included interpretation of a dipole-dipole survey conducted in 2001, that reached an investigation depth of approximately 40 m, and two shallow boreholes drilled, after a deeper geoelec- trical survey performed in the 1980s, to replace the springs. Besides the seismic reflection profiles, seven vertical electrical soundings were also acquired and refraction interpretation of the first reflec- tion data to arrive was also undertaken to complement the information. This paper reports integrated interpretation of the geophysical and geological data to obtain a 3D conceptual model to achieve the above mentioned objectives. After interpretation and lateral correlation of the structures found in the geophysical pro- files, the deep 3D conceptual model integrating the geological, hydrogeological, and geophysical information was constructed. This model was the basis for planning the location of an explora- tory borehole that was drilled afterwards. From two initial locations appearing as the strongest candidates for a successful thermal water production well, one was chosen to drill a third borehole to supply the spa facilities. The third borehole results were very positive— 250 m deep, temperature approximately 50C, and water flow 12.5 l/s. This example shows the utility of using geophysical methods to increase knowledge about groundwater circulation and detect favourable areas for thermal water borehole drilling to replace the hot springs with the objective of protecting the aquifer from pollution.

Spiral-staircase photonic structures of metallic nanorods

In this paper, we report a thorough investigation of the optical properties of a 3D spiral-staircase structure of metallic nanorods by means of rigorous full-electrodynamic calculations.Weanalyzethenatureofthephotoniceigenmodesofthis crystal in the light of group theory and explain their physical origin. Moreover, we study the change in the polarization state of a wave transmitted through a finite slab of the crystal and reveal the occurrence of strong optical activity effects. Finally, we provide a consistent interpretation of negative refraction,whichcanoccurwithincertainregionsoffrequency and angles of incidence on a specific surface of the crystal, by reference to relevant isofrequency contours obtained from detailed photonic band-structure calculations. It is worth noting that the structure under consideration does not support purely left- and right-circularly polarized (LCP and RCP, respectively) eigenmodes, in contrast to mostly studied chiral metamaterials with, e.g., C4 symmetry. 17‐23,34‐36,38‐40,42,43,45,46

Comments on: Palmer, D., 2010. Is visual interactive ray trace an efficacious strategy for refraction inversion? Exploration Geophysics, 41, 260–267

Palmer’s paper addresses visual interactive ray tracing (VIRT), specifically referring to Whiteley (2004) and Whiteley and Leung (undated). In these papers, field examples of seismic refraction data from Welcome Reef and Mt Bulga were re-interpreted using VIRTandwavepatheikonaltraveltime(WET)Tomography.These different approaches to near surface refraction interpretation assume very different earth models, from the discrete or ‘blocky’ modelsforVIRTtothecontinuousvelocitymodelsforWET.This enhanced flexibility permits more realistic seismic modelling in near-surface materials, spanning soils, weathered rock and fresh rocks. My comments and discussions address many of the technical deficiencies, inconsistencies, unsubstantiated statements and claims in Palmer’s paper. They are directed at improving the rigour with which near-surface refraction interpretations are undertaken and the quality of these interpretations. In the title, Palmer poses an incompletely defined question. It is not clear actually what ‘visual interactive ray trace’ is,

Uphole Seismic Refraction Survey for Low Velocity Layer Determination over Yom Field, South East Niger Delta

The uphole refraction survey was carried out in ‘‘YOM’’ field 4-D prospect to evaluate the weathering thickness and velocity of the low velocity layer (LVL). Five uphole locations were occupied in the study area and the time – intercept technique of the seismic refraction interpretation was employed. For the different locations (A, B, C, D and E), the weathering thickness was found to vary from 0.9m to 1.9m with velocity from 250m/s to 800m/s. A sub weathering layer was also observed in all locations except A. Also, the velocity of the consolidated layer was found to vary from about 1750m/s to 1800m/s with a mean velocity of 1766m/s. Using this result to eliminate the effect of LVL on reflection data from the graphical approach, it was observed that the elevation and weathering correction (static correction) was found to be eliminated at a datum of 2m deep.

Geophysical studies of Australia's remote eastern deep-water frontier: results from the Capel and Faust Basins

The Capel and Faust basins are located in a frontier part of offshore eastern Australia, about 800 km east of Brisbane in 1000 - 3000 m of water. These basins are being evaluated for their petroleum potential as part of the Australian Government?s Offshore Energy Security Program. This article outlines the current status of integrated interpretation of 2D seismic reflection, sonobuoy refraction and potential-field data acquired during Geoscience Australia marine survey GA-302 conducted between late 2006 and early 2007. This survey collected 5920 km of high-quality 106-fold seismic reflection data using an 8 km streamer to 12 s two-way time at 37.5 m shot interval and a line spacing of 20 - 50 km. A subsequent swath-bathymetry and geological sampling survey (GA-2436), completed in late 2007, also collected potential-field data in the north-west of the study area with a 3 - 4 km line spacing (Fig. 1). These data have been integrated in 3D to help constrain the geometry and thickness of sediment depocentres in the region.