Refraction Velocity 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.

Geophysical and geochemical characterization reveals topography controls on critical zone structure in a low hilly region

AbstractCharacterizing the structure of the critical zone (CZ), which is often tens of metres deep, is challenging and crucial to understand earth surface processes. Existing findings on the effect of topography on CZ thickness focused exclusively on mountainous regions with steep slopes, not considering low hilly regions with gentle slopes. Seismic refraction and borehole surveys were conducted to characterize the effects of topography on a red soil CZ structure in the low hilly region of Jiangxi Province, subtropical China. This CZ is derived from Quaternary red clay (QRC) overlying Cretaceous sandstone bedrock. Regolith with a thickness of 8.2–9.5 m consisted of an upper layer of QRC (4.8–6.2 m thick) and a lower layer of weathered sandstone (2.6–4.0 m thick). The boundary between weathered and unweathered sandstone was identified by a Ca depletion profile and calcite weathering front. The boundary between the QRC and weathered sandstone was identified by morphological and textural features and a feldspar weathering front. The QRC had seismic refraction velocities &lt;1.3 km s−1, the weathered sandstone had velocities of 1.3–1.7 km s−1, while the sandstone bedrock had velocities &gt;1.7 km s−1. Following the profile sequence downslope, regolith thickness decreased linearly with decreasing ground elevation. The linear fitted regression slope was &lt;1. This was predominantly determined by QRC thickness, which in turn was mainly controlled by the difference between slopes of ground and bedrock surface and by surface erosion. Following the profile sequence across the slope, regolith thickness decreased linearly with decreasing ground elevation, with a slope of ~1. This was controlled by QRC thickness, which in turn was mainly controlled by surface erosion only, since the bedrock surface was nearly level. These results suggest that topography can control CZ structure in hilly regions with gentle slopes.

New Empirical Approach for the Estimation of Soil Cohesion and Friction Angle in 2D Form for Site Investigations

This paper presents the multiple linear regression (MLR) models developed from electrical resistivity and seismic refraction surveys for quick prediction of subsurface soil’s shear strength parameters. A total of four parameters have been considered with electrical resistivity and seismic refraction velocity as the independent variables: and soil cohesion and internal friction angle as the dependent variables. In order to mitigate the effects of nonlinearity of resistivity and velocity, both datasets were initially log-transformed to conform with the fundamental assumptions of regression analysis. Two models were therefore built based on the strong multiple linear relationships between explanatory and response variables, with coefficient of determination (R2), 0.777, p-values, &lt; 0.050, Durbin-Watson value, 1.787 and multicollinearity, 1.185. The obtained models’ coefficients were transferred and used for the estimation of 2D models soil cohesion and internal angle of friction for validation. Thereafter, the developed models demonstrated good performance, having subjected to accuracy assessment with results at &lt; 5%, and &lt; 10% for the root mean square error (RMSE) and weighted mean absolute percentage error (MAPE) respectively. Therefore, the new developed soil’s shear strength MLR models have provided continual description of soil properties in two-dimensional form, enhancing the subsurface information for site investigations as compared, to one-dimensional information from the invasive method.

What Do P-Wave Velocities Tell Us About the Critical Zone?

Fractures in Earth's critical zone influence groundwater flow and storage and promote chemical weathering. Fractured materials are difficult to characterize on large spatial scales because they contain fractures that span a range of sizes, have complex spatial distributions, and are often inaccessible. Therefore, geophysical characterizations of the critical zone depend on the scale of measurements and on the response of the medium to impulses at that scale. Using P-wave velocities collected at two scales, we show that seismic velocities in the fractured bedrock layer of the critical zone are scale-dependent. The smaller-scale velocities, derived from sonic logs with a dominant wavelength of ~0.3 m, show substantial vertical and lateral heterogeneity in the fractured rock, with sonic velocities varying by 2,000 m/s over short lateral distances (~20 m), indicating strong spatial variations in fracture density. In contrast, the larger-scale velocities, derived from seismic refraction surveys with a dominant wavelength of ~50 m, are notably slower than the sonic velocities (a difference of ~3,000 m/s) and lack lateral heterogeneity. We show that this discrepancy is a consequence of contrasting measurement scales between the two methods; in other words, the contrast is not an artifact but rather information—the signature of a fractured medium (weathered/fractured bedrock) when probed at vastly different scales. We explore the sample volumes of each measurement and show that surface refraction velocities provide reliable estimates of critical zone thickness but are relatively insensitive to lateral changes in fracture density at scales of a few tens of meters. At depth, converging refraction and sonic velocities likely indicate the top of unweathered bedrock, indicative of material with similar fracture density across scales.

Delineation of Weathered Layer Using Uphole and Surface Seismic Refraction Methods in Parts of Niger Delta, Nigeria

Uphole and surface seismic refraction surveys were carried out in parts of the Niger Delta, Nigeria, to delineate weathering thickness and velocity associated with aweathered layer. A total of twelve uphole and surface seismic refraction surveyswere shot, computed and analyzed. The velocity of the uphole seismic refraction ranged from 344.8 to 680.3 m/s with a thickness of 5.45 to 13.35 m. Surface seismic refraction ranged from 326.6 to 670.2 m/s and 4.30 to 12.0 m, respectively. The average velocity and thickness ranged from 559.6 to 548.0 m/s and 9.43 to 8.63m with differences of 11.6 m/s and 0.83 m respectively. The VW/VS ratios ranged from 0.955 to 1.059. This indicates that the uphole velocity is higher than the surface refraction velocity leading to low VW/VS values. This is a direct experimental proof of a low velocity zone, confirming the weathered nature of the area. The results of both refraction methods are reliable; the differences in surface refraction values are due to shot point offsets. Based on these findings, it is recommended that shots for seismic surveys should be located above 15.0 m in the area to delineate the effects associated with weathered layers to ensure that will be competent to withstand engineering structures.&#x0D;

Geotechnical Engineering Evaluation of Superficial Deposits Utilizing Seismic Methods at Al al-Bayt University, Jordan

Integration of seismic refraction tomography and multi-channel analyses shear wave methods were used at Al al-Bayt university campus in Mafraq, NE Jordan to delineate the shallow subsurface layering and determine their geotechnical properties, in addition, to estimate the geotechnical engineering parameters of the superficial deposits for detailed purposes. The P-wave refraction velocities were found to range from 180 to 2300 m/s for the three identified layers which indicating superficial soil deposits with an increasing degree of compaction with depth. The S-wave velocities were found to lie in a narrow velocity range (250 to 600 m/s), which badly resolved the subsurface layering. Moreover, four soil samples were taken from the sites of seismic survey points for density calculations. The average calculated density was found to be 1.85 g/cm3. Two soil samples from nearby seismic profile-1 and seismic profile-4 were tested for geotechnical properties. The results showed that the seismic method is an effective tool to estimate the different elastic moduli; Poisson's ratio, Young's modulus, Bulk modulus, Shear modulus, Lame's constants, and geotechnical engineering parameters; N-value, bearing capacity, material index, concentration index, and stress ratio of the superficial soil deposits. The study recommends conducting a detailed geophysical study before starting any type of geoengineering construction in the area.

Seismic velocity structure of the Queen Charlotte terrace off western Canada in the region of the 2012 Haida Gwaii Mw 7.8 thrust earthquake

AbstractA well-recorded Mw 7.8 megathrust earthquake occurred on 27 October 2012 under the Queen Charlotte terrace off the west coast of Haida Gwaii, western Canada. In this study, we supplement limited earlier seismic refraction work on the offshore velocity structure off Haida Gwaii with data from ocean bottom seismometers (OBS) operating between 6 December 2012 and 5 January 2013. The OBS recorded a portion of the aftershock sequence, and an active-source seismic survey was conducted in January 2013 to acquire seismic refraction data in the region of the Haida Gwaii earthquake across the Queen Charlotte terrace. P-wave velocity analyses using first-arrival tomography showed relatively shallow (2.0–3.0 km below seafloor) high-velocity material with values up to 4.0 km/s beneath the terrace. At the one OBS station seaward of the deformation front on the abyssal plain, refraction velocities of ∼4.5 km/s indicated the top of the oceanic plate at ∼1–2 km below the seafloor. At several OBS stations, converted shear-wave velocities were determined within the sediment cover using reflected arrivals. The S-wave velocities ranged from 0.5 to 1.5 km/s, and the corresponding P/S velocity ratio was between 3.0 and 4.2. The new refraction data confirm earlier interpretations of high-velocity material in the shallow terrace that may indicate fractured oceanic crustal material lies significantly above the location where a subducted slab is thought to occur under the terrace. Transpressive deformation of the Pacific plate may explain these observations.

Seismic refraction tracks porosity generation and possible CO2 production at depth under a headwater catchment

In weathered bedrock aquifers, groundwater is stored in pores and fractures that open as rocks are exhumed and minerals interact with meteoric fluids. Little is known about this storage because geochemical and geophysical observations are limited to pits, boreholes, or outcrops or to inferences based on indirect measurements between these sites. We trained a rock physics model to borehole observations in a well-constrained ridge and valley landscape and then interpreted spatial variations in seismic refraction velocities. We discovered that P-wave velocities track where a porosity-generating reaction initiates in shale in three boreholes across the landscape. Specifically, velocities of 2.7 ± 0.2 km/s correspond with growth of porosity from dissolution of chlorite, the most reactive of the abundant minerals in the shale. In addition, sonic velocities are consistent with the presence of gas bubbles beneath the water table under valley and ridge. We attribute this gas largely to CO2 produced by 1) microbial respiration in soils as meteoric waters recharge into the subsurface and 2) the coupled carbonate dissolution and pyrite oxidation at depth in the rock. Bubbles may nucleate below the water table because waters depressurize as they flow from ridge to valley and because pores have dilated as the deep rock has been exhumed by erosion. Many of these observations are likely to also describe the weathering and flow path patterns in other headwater landscapes. Such combined geophysical and geochemical observations will help constrain models predicting flow, storage, and reaction of groundwater in bedrock systems.

Acoustic global–local full‐waveform inversion for P‐wave velocity estimation of near‐surface seismic data acquired in Luni, Italy

ABSTRACTIn this work, we describe an experiment concerning global–local full‐waveform inversion, carried out on a P‐wave seismic reflection profile that was acquired at Luni, an archaeological site in Italy. The global full‐waveform inversion makes use of a two‐grid genetic algorithm scheme and recorded refraction and diving waves, to build an initial velocity model of the subsurface. Two important pieces of a priori information which help to better constrain the inversion results are the refraction velocity model and the Dix‐converted semblance velocity field obtained from time processing. A good match between observed and predicted data allows us to use the estimated velocity field as the starting point for a local, gradient‐based full‐waveform inversion that inverts the recorded data (except the surface waves). The final estimated velocity field shows two main discontinuities: one is very shallow and related to the refractor velocity model used and the other corresponds to the strongest reflection event observed in the pre‐stack depth‐migrated section, at a depth of 100 m. The pre‐stack depth‐migrated common image gathers provide evidence of a good horizontal alignment of this reflection, indicating an accurate velocity estimation down to 100 m depth that corresponds to the maximum offset used in the acquisition.

Constraints on the shallow deformation around the Main Frontal Thrust in central Nepal from refraction velocities

The youngest fault system in the Himalayan orogeny is the Main Frontal Thrust (MFT), the frontal ramp of the Main Himalayan Thrust, which is expected to host the largest and most damaging earthquakes in Nepal. We characterize the upper few hundred meters below the surface across two MFT fault strands using ten high-resolution seismic profiles that we acquired in 2014 and 2015 with a 6-tonne Vibroseis source. We use first arrival picks from 625,416 seismic traces to derive P-wave seismic velocity models using a wavepath eikonal traveltime inversion method, and derive estimates of alluvium thickness and water table depth across these faults (the Patu and Bardibas thrusts), allowing us to constrain the subsurface geometry of the MFT.Our results show that 1) seismic velocities range from 255 to 3660 m/s, consistent with dry and saturated alluvium, and Siwalik bedrock; 2) low-velocity alluvium varies between ~20–50 and ~80–120 m thick in the hanging wall and footwall of the Bardibas thrust, respectively, corresponding to ~60–70 m of uplift of the hanging wall since deposition; 3) the two thrusts are soft-linked, and the western tip of the Bardibas thrust lies ~6 km west of its surface expression; 4) during the dry season, the water table is ~25–100 m higher in the hanging walls of the faults than in their footwalls, due to the larger thickness of permeable alluvium in the footwalls, and the water table shallows towards the east in the hanging wall of the Bardibas thrust; and 5) consistent with previous studies, the Patu thrust breaches the surface, while the Bardibas thrust is blind at Ratu Khola. These results also demonstrate that it should be possible to constrain the rate of uplift above the Bardibas thrust by drilling and dating sediments on both sides, which would complement existing measurements from terrace uplift.

Décollements, Detachments, and Rafts in the Extended Crust of Dangerous Ground, South China Sea: The Role of Inherited Contacts

AbstractWe investigate the crustal structure of the Dangerous Ground (South China Sea) through processing and interpretation of coincident wide‐angle reflection and refraction seismic data. Continental crust of Dangerous Ground has been moderately thinned, down to 15 km, so that most of the structures accompanying the early opening of the South China Sea from Cretaceous to Miocene have been preserved. Subbasement reflectors as well as refraction velocities image an interpreted dismantled Mesozoic metamorphic unit in the southernmost section of our study area. A rollover structure indicates that the reflective base of the unit was used as a décollement where low‐angle normal faults root and blocks rafted. The metamorphic unit is discontinued in a nearby basin located immediately to the north, where the refraction velocity model shows thinning of the crust from 20 to 15 km, with the presence of a 5‐km‐high mantle dome. In this deeper basin, mass transport deposits are found lying on a strong amplitude basement reflector interpreted as the footwall of an ~15 km offset crustal detachment surface that we link down to the mantle dome. We infer that the detachment reactivated an inherited low‐angle contact most probably related to the Yanshanian belt. In map view, the reactivated structure forms a half‐graben basin oriented NNE‐SSW oblique to the generally accepted direction of extension. This orientation follows the general trend of a granitic belt that spanned the South China margin prior to extension, related to the subduction of the Paleo‐Pacific.

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.

Analysis of reduced traveltime creates nonlinear delay-time solution

This case study utilizes time-arrival picks and a two-layer depth/velocity model to enhance the refraction solution using nonlinear refractor velocities. The near surface has rapidly changing topography and thickness of the low-velocity layer. The refractor velocity is varying due to outcropping limestone horizons. The acquisition of the 3D survey is hampered due to inaccessible terrain with cliffs up to 150 m in height. There are two large areas with source omissions due to irrigated cultivations. For this 3D seismic survey, multiple tests applying tomographic modeling fail to resolve the near-surface anomaly. The starting model for this analysis is a linear reciprocal-path delay-time model. The proposed method redetermines the delay time, taking into account the nonlinear refractor velocity. The enhancement produced near-surface static corrections that improved the imaging of the 3D seismic volume and created an analytical tool of the solutions uncertainty.

Application of geophysical techniques to support geological mapping projects

Better and more detailed information on the bedrock and near-surface geology is needed to assist geological risk assessments, efficient use of water resources or management of rapidly growing regions. Since 2007, the Cartographic and Geological Institute of Catalonia (ICGC) has developed a geological mapping programme, which includes information related to hydrogeology, urban geology or geological risks among others. Within this framework, geophysical techniques make an important contribution refining geological mapping by adding subsoil information to traditional geological data. In this work, we present two studies carried out by ICGC in order to map bedrock geometry or to characterize near-surface sediments in areas with scarce borehole information. The first example of this support focuses on the combination of passive and active geophysical techniques in order to image the Quaternary/Neogene boundary, the bedrock top and to locate faults or the geological map of the Girona urban area (NE of Spain). The second case study shows the potential of reprocessing vintage seismic oil datasets to increase knowledge of the near-surface geological structure in a Neogene Basin located north of Girona (Emporda Basin). The reprocessed seismic reflection image of the first km depth has been interpreted using the refraction velocity model and passive seismic information as constraints. The main objectives are to recover bedrock geometry and structure as well as fault imaging, which are critical for geological mapping projects. The common pattern of the methodologies applied to each case is the integration of different geophysical datasets and the use of both active and passive techniques.

Joint inversions of three types of electromagnetic data explicitly constrained by seismic observations: results from the central Okavango Delta, Botswana

The Okavango Delta of northern Botswana is one of the world's largest inland deltas or megafans. To obtain information on the character of sediments and basement depths, audiomagnetotelluric (AMT), controlled-source audiomagnetotelluric (CSAMT) and central-loop transient electromagnetic (TEM) data were collected on the largest island within the delta. The data were inverted individually and jointly for 1-D models of electric resistivity. Distortion effects in the AMT and CSAMT data were accounted for by including galvanic distortion tensors as free parameters in the inversions. By employing Marquardt–Levenberg inversion, we found that a 3-layer model comprising a resistive layer overlying sequentially a conductive layer and a deeper resistive layer was sufficient to explain all of the electromagnetic data. However, the top of the basal resistive layer from electromagnetic-only inversions was much shallower than the well-determined basement depth observed in high-quality seismic reflection images and seismic refraction velocity tomograms. To resolve this discrepancy, we jointly inverted the electromagnetic data for 4-layer models by including seismic depths to an interface between sedimentary units and to basement as explicit a priori constraints. We have also estimated the interconnected porosities, clay contents and pore-fluid resistivities of the sedimentary units from their electrical resistivities and seismic P-wave velocities using appropriate petrophysical models. In the interpretation of our preferred model, a shallow ∼40 m thick freshwater sandy aquifer with 85–100 Ωm resistivity, 10–32 per cent interconnected porosity and <13 per cent clay content overlies a 105–115 m thick conductive sequence of clay and intercalated salt-water-saturated sands with 15–20 Ωm total resistivity, 1−27 per cent interconnected porosity and 15–60 per cent clay content. A third ∼60 m thick sandy layer with 40–50 Ωm resistivity, 10–33 per cent interconnected porosity and <15 per cent clay content is underlain by the basement with 3200–4000 Ωm total resistivity. According to an interpretation of helicopter TEM data that cover the entire Okavango Delta and borehole logs, the second and third layers may represent lacustrine sediments from Paleo Lake Makgadikgadi and a moderately resistive freshwater aquifer comprising sediments of the recently proposed Paleo Okavango Megafan, respectively.

Accurate water‐table depth estimation using seismic refraction in areas of rapidly varying subsurface conditions

ABSTRACTA common approach for estimating water‐table depth simply and reliably is using seismic refraction. Typical layered solutions from seismic refraction are successful in areas where the water table does not fluctuate rapidly. However, there are many areas around the world where water tables rise and fall rapidly in response to intense rainfall events, especially in tropical and semi‐arid regions. These areas are frequently heavily populated, and the shallow unconfined aquifers, used for drinking water and irrigation, are under increasing threat of overexploitation and pollution. To help mitigate these concerns, a reliable method to accurately determine water‐table depth in these environments is required. We present a unique analysis method based on a calculated reference refraction velocity that gives not only accurate predictions of water‐table depth for cases of rapidly fluctuating water tables but also accurate predictions of largest historical recorded depth to water.Data used came from a series of 60 refraction surveys carried out at 15 locations with monitoring bores in a tropical area. In a time‐lapse approach, four surveys were performed at each of the bore locations over a nine‐week period during both wet and dry conditions. Using a typical forward/reverse profile approach, we first calculate forward and reverse velocities. These layer velocities are then averaged using a scheme based on moisture conditions to give a reference velocity. Finally, the predicted depth for the water table is determined by identifying the depth at which this reference velocity occurs on a separately calculated refraction velocity tomogram. Results for predicting current water‐table depth for the 60 surveys gave water‐table depth predictions between 0.38 m above and 0.13 m below the measured water table using a 99% confidence interval. Conventional two‐layer solution predictions resulted in only 28% of predictions lying within 1 m of the measured water table. Using a modification to this approach, we are also able to accurately determine the maximum historical recorded depth to water table using information on soil lithology/texture, records for the greatest recorded depth to water table, and refraction velocity tomograms at known locations. This method can then be applied to locations with similar but known lithology but without monitoring bores.

Integration of seismic velocity measurements in the context of the CO2 Storage project in the Bonaparte Basin, offshore NW Australia

SUMMARY An accurate seismic velocity model is essential for depth conversion and rock property determination in the context of fluid flow modelling for secure storage of carbon dioxide in the study area. Three types of seismic velocity measurements are available within the study area: velocities derived from stacking of multi-channel reflection seismic data; velocities determined in the process of ray tracing modelling of large offset refraction data acquired by ocean bottom seismographs (OBSs) along the coincident reflection/refraction transect, and velocities from well log measurements. Generally, the OBS derived velocities are faster than the stacking derived velocities in the shallow section down to 2 km depth, with particularly large (exceeding 15 %) discrepancies in the NE part of the study area. High velocities in the OBS model are consistent with well log measurements in the Newby 1 and Flat Top 1 exploration wells. Integration of OBS refraction velocities with stacking velocities through the calibration of both of them against well logs is the best way to construct a reliable velocity model for the whole section. Refraction seismic data coverage offshore Australia is expected to significantly improve as in 2013 Australia will obtain a National Pool of OBSs suitable for multi-scale experiments.

Investigation of groundwater potentiality in the eastern part of Qena, central Eastern Desert, Egypt, using 2D homogeneous function method

The present study represents an application of the 2D homogeneous function method to investigate the near-surface groundwater potentiality to the east of Qena town, Egypt. The homogeneous function automatically inverts first arrival refractions to derive a 2D velocity distribution, which involves seismic boundaries. A complex set of observed refracted traveltimes along four seismic profiles (12 spreads) is used to construct refraction velocity field section and structural section. Such sections allow viewing the complex layered structure and delineating its boundaries and faults. The interpreted geoseismic layers were traced, according to their own gradients and velocity ranges. The resultant models were successful in fulfilling the proposed objective, as they provide: (1) the required details on the delineation of the subsurface layers, where three geoseismic layers with different ranges of velocities were detected; (2) determination of the thickness and the inversion boundary of water-bearing layer; and (3) detection of normal and reverse faults with displacement of about 10–40 m and variable dipping directions. The obtained result was found to be completely compatible with the information gained from identified boundaries of the nearby borehole.

A comparison of seismic and radar methods to establish the thickness and density of glacier snow cover

AbstractWe show that geophysical methods offer an effective means of quantifying snow thickness and density. Opportunistic (efficient but non-optimized) seismic refraction and ground-penetrating radar (GPR) surveys were performed on Storglaciären, Sweden, co-located with a snow pit that shows the snowpack to be 1.73 m thick, with density increasing from ∼120 to ∼500 kg m–3 (with a +50 kg m–3 anomaly between 0.73 and 0.83 m depth). Depths estimated for two detectable GPR reflectors, 0.76 ±0.02 and 1.71 ± 0.03 m, correlate extremely well with ground-truth observations. Refraction seismic predicts an interface at 1.90 ± 0.31 m depth, with a refraction velocity (3730 ± 190 ms–1) indicative of underlying glacier ice. For density estimates, several standard velocity-density relationships are trialled. In the best case, GPR delivers an excellent density estimate for the upper snow layer (observed = 321 ± 74 kg m–3, estimated = 319 ± 10 kgm–3) but overestimates the density of the lower layer by 20%. Refraction seismic delivers a bulk density of 404 ±22 kgm–3 compared with a ground-truth average of 356 ± 22 kg m–3. We suggest that geophysical surveys are an effective complement to mass-balance measurements (particularly for controlling estimates of snow thickness between pits) but should always be validated against ground-truth observations.

Allowable Bearing Capacity for Shallow Foundation in Eket Local Government Area, Akwa Ibom State, Southern Nigeria

P-wave and S-wave velocities were obtained from seismic refraction survey in the foundation layer of Eket, the study area. The Tezcan’s approach discussed extensively in the work was used in conjunction with the existing mathematical relations between elastic parameters and seismic refraction velocities for the study of foundation layers in the study area. Based on the results, the elastic constants, allowable bearing pressure/capacity, ultimate bearing capacity and other parameters in Table 1 were determined. The result shows that allowable bearing pressure increases with increase in shear modulus and shear wave velocity. The empirical relation between allowable bearing capacity and shear modulus shows that the allowable bearing capacity increases with depth. Comparing our findings with some ranges of safe allowable bearing capacities of similar non cohesive/granular soils in literatures, the second layer with allowable bearing capacity range of 72.56 - 206.63 kN·m-2 (average = 154.78 kN·m-2) has been considered to be the safe shallow engineering foundation in the study area. The empirical relations between allowable bearing capacities shear modulus and shear wave velocity, in conjunction with the inferred maps, which serve as our findings, will be used as guide in the location of foundations. The inferred ultimate and allowable capacities correlate maximally for the two shallow foundations penetrated by the seismic waves. This perfect correlation reflects the uniqueness of the method.