Identification of soil characteristics using Vp & Vs tomograms for hazard assessment: case study from Ain Sokhna area, Egypt

The sphere survey concerned 18 seismic refraction profiles (Fig. 2) protecting a complete size of roughly 3150 m alongside the Cairo-Suez Highway within the Ain Sokhna space. As well as, wireline logging information from the Suez well-3, drilled by the Egyptian Geological Survey and Mining Authority (EGSMA), have been included as a geological reference to boost the reliability of interpretations. This mixed methodology enabled the development of detailed velocity tomograms, the estimation of key dynamic geotechnical properties, and the evaluation of subsurface competency for engineering purposes.

Location map of the research space.

The goals of this research are threefold:

1. 1.

   To research compressional-wave (Vp) and shear-wave (Vs) velocities utilizing SRT and MASW strategies with a view to delineate subsurface lithology and layering.

2. 2.

   To calculate a complete suite of dynamic geotechnical and elastic parameters for the recognized soil and rock items.

3. 3.

   To guage the competency of subsurface supplies and assess their implications for hazard mitigation and future engineering developments in Ain Sokhna.

4. 4.

   Calculating Vs30 to find out NEHRP website class.

Format of the acquired seismic profiles throughout the research space.

Geologic setting

The Ain Sokhna area is located alongside the western margin of the Gulf of Suez throughout the Suez Governorate, Egypt. This space occupies a transitional zone between the coastal plain bordering the gulf and the elevated mountainous terrain of the Northern Galala and Gabal Ataqa ranges to the west. Owing to its strategic place, Ain Sokhna has change into a focus for industrial and infrastructural growth, which necessitates detailed geotechnical and geophysical investigations³¹.

Geomorphologically, the research space could be divided into 4 principal items:

1. 1.

   Coastal Plain:

This low-lying zone extends from the shoreline of the Gulf of Suez westward towards the highlands. It’s predominantly lined by Quaternary clastic sediments, consisting of intensive sandy and gravelly deposits that type a gently eastward-dipping plain. These unconsolidated supplies symbolize the youngest geomorphological options within the area.

1. 2.

   Highlands:

The Northern Galala Plateau and Gabal Ataqa type probably the most distinguished highland items. Gabal Ataqa reveals steep escarpments, with elevations exceeding 900 m above sea degree. The highlands are structurally managed, reflecting the affect of main fault methods related to the rifting of the Gulf of Suez.

1. 3.

   Low-Elevation Hills:

A lot of this unit consists of sedimentary rocks spanning from the Higher Eocene by means of the Miocene. These embody limestones, sandstones, and different clastic deposits, which type rolling hills and ridges between the highlands and the coastal plain.

1. 4.

   Drainage Community:

The area hosts a number of important drainage basins that transport sediments from the highlands to the Gulf of Suez. Probably the most distinguished are Wadi Hagoul, South Wadi Hagoul, Wadi Akheider Bada, and Wadi Ghweiba. These wadis strongly affect the sedimentary structure of the world by supplying coarse-grained deposits to the coastal plain.

Geological map of the western aspect of Gulf of Suez after (Conco, 1987).

From a geological perspective (Fig. 3), the stratigraphy of Ain Sokhna data a protracted and complicated historical past spanning the Eocene to Quaternary. The Eocene succession is represented by the Mokattam Formation (Center Eocene) and the Maadi Formation (Higher Eocene). These are overlain unconformably by Oligocene deposits, which embody each volcanic items (basaltic flows and dolerite intrusions) and sedimentary sequences reminiscent of sandstones and conglomerates. The Miocene succession is characterised by the Sadat Formation (Decrease Miocene) and the Hagul Formation (Higher Miocene), each consisting primarily of sandstones and marls. Youthful Pliocene deposits comprise gravels, sands, and flint pebbles, whereas the Quaternary sediments are dominated by unconsolidated alluvial and terrace deposits, together with gravels, cobbles, boulders, and sands^27,28.

Structurally, the area lies throughout the Gulf of Suez rift system and shows a particular extensional tectonic fashion. Quite a few regular faults dissect the world, forming half-graben and domino-style fault blocks. Fault orientations are numerous however three main traits dominate: NNW-SSE, E-W, and WNW-ESE. The NW-SE faults are probably the most important, controlling the event of main wadis reminiscent of Hagoul, Badaa, and Ghweiba. The E-W faults are thought to have initiated throughout the Late Eocene to Oligocene, whereas the WNW–ESE faults are distinguished within the despair between Gabal Ataqa and the Northern Galala Plateau. These structural options collectively govern the subsurface topography, sediment distribution, and groundwater potential of the area^34,35.

In abstract, the Ain Sokhna space presents a posh geological and geomorphological framework formed by interactions between sedimentary processes and extensional tectonics linked to the Purple Sea and Gulf of Suez rifting. This mixture of lithological variability and fault-controlled constructions underscores the significance of integrating geophysical strategies to precisely characterize the subsurface for engineering and hazard evaluation functions.

Information acquisition

To be able to characterize the subsurface circumstances at Ain Sokhna, each compressional (P-wave) and shear (S-wave) seismic surveys have been performed utilizing survey tomography and surface-wave recording. The seismic structure consisted of a number of profiles with lengths ranging between 120 and 240 m, oriented in each NE-SW and NW-SE instructions to supply satisfactory protection of the positioning. Geophones have been deployed at 10 m intervals alongside every profile, using spreads of 12 and 24 channels. For the P-wave survey, a 15 kg sledgehammer hanging a metal plate served because the seismic supply, whereas for the S-wave survey, horizontal strikes towards a wood plank have been used to generate polarized shear waves appropriate for MASW evaluation.

Every unfold was recorded utilizing a Geometrix 24-channel seismograph (mannequin 1125E) and a McSEIS-SX acquisition system, with a number of shot factors positioned in ahead, reverse, and offset geometries to make sure dependable first arrival choosing and dispersion imaging. In complete, 18 seismic traces have been collected, producing an built-in survey size of roughly 3.15 km. Ten shot gathers per unfold have been acquired to maximise information redundancy and enhance the signal-to-noise ratio (Desk 1).

The P-wave dataset offered high-resolution first-arrival journey occasions for refraction tomography, whereas the S-wave survey was primarily used to extract dispersion curves and generate shear-wave velocity profiles by means of MASW inversion for every profile. Collectively, these complementary strategies supplied a strong dataset for imaging the speed construction of the close to floor, enabling correct delineation of lithological boundaries and the derivation of key elastic parameters important for geotechnical and seismic hazard evaluation.

Information processing

All seismic information acquired within the Ain Sokhna survey have been processed utilizing the SeisImager/2D and Zond software program, which includes complete modules for refraction and tomography evaluation based mostly on delay-time and ray-tracing strategies. For the P-wave data, the primary arrivals have been manually picked from every shot collect (Fig. 4). These arrivals have been then used to assemble time-distance curves after assigning the suitable subject geometry parameters, together with unfold size, geophone spacing, and supply receiver offsets. Then the time-distance curves generated for every profile have been analyzed. This step produced dependable seismic velocity distributions for compressional (Vp) and wave throughout the survey traces^36,37. For the S-wave data the dispersion curves are generated and the section velocities are picked to assemble the section velocity curve (Fig. 5).

Examples of the picked seismic refraction information for Profile 1, illustrating the comparability between the noticed and modeled journey time-distance curves derived from the survey.

P-wave velocity fashions have been obtained by means of tomographic inversion of first-arrival journey occasions utilizing a least-squares method mixed with ray-tracing ahead modeling. A 3-layer preliminary mannequin was adopted, consisting of unconsolidated wadi deposits (Vp = 600 m/s), consolidated sediments (Vp = 1,100 m/s), and sandstone bedrock (Vp = 2,500 m/s). To enhance mannequin stability and cut back inversion artifacts, L2-norm regularization, velocity damping, and boundary smoothing have been utilized. The subsurface was discretized utilizing a progressively refined block-based parameterization, and ray protection was evaluated by means of hit-count evaluation. The inversion converged after roughly 9 iterations, yielding a median RMS travel-time misfit of three.93 ± 0.58% throughout the eighteen profiles.

Generated dispersion curves for the analyzed profile, highlighting the fundamental-mode section velocities used to assemble the corresponding section velocity curve.

Shear-wave velocity (Vs) fashions have been derived from MASW information acquired utilizing a 15-kg sledgehammer supply. Basic-mode dispersion curves have been extracted utilizing phase-shift evaluation and inverted by means of a Gauss-Newton optimization scheme constrained by the corresponding P-wave fashions. Preliminary Vs estimates have been guided by assumed Vp/Vs ratios of 1.90, 1.75, and 1.65 for the three recognized layers. The inversion produced a median dispersion-curve RMS misfit of 6.36 ± 1.26%, with phase-velocity uncertainties of 28.8 ± 8.5 m/s.

The common depth of investigation reached roughly 40 m, whereas dependable characterization of the higher 30 m was achieved alongside the longer profiles, enabling strong Vs30 estimation. The ensuing velocity fashions resolved three main subsurface items with distinct geophysical traits, in good settlement with lithological data from Suez Nicely-3.

Wireline log information from the higher interval of Suez Nicely-3 situated adjoining to the research space.

The ultimate P-wave and S-wave velocity fashions have been built-in into two-dimensional subsurface sections for every profile, offering an in depth characterization of lateral and vertical variations in seismic velocities. The P-wave tomograms delineated soil stratification and bedrock geometry, whereas the S-wave fashions resolved shear-wave velocity distributions important for estimating elastic moduli and different dynamic geotechnical parameters. Hydrogeological circumstances, notably the regional groundwater desk at roughly 14 m depth, have been inherently included into the speed inversions and Gardner-derived density estimations, guaranteeing that the calculated elastic properties, together with Poisson’s ratios (0.21–0.28 throughout the bedrock), precisely symbolize in-situ subsurface circumstances. The built-in interpretation recognized three principal subsurface items with distinct geophysical traits, in shut settlement with lithological data obtained from close by boreholes.

Engineering and geotechnical parameters

This part particulars the secondary geotechnical indicators derived from main seismic parameters compressional wave velocity (Vp), shear wave velocity (Vs), and bulk density (ρ). These derived values present a quantitative framework for assessing materials stiffness, compressibility, and load-bearing efficiency, that are important for evaluating basis stability and seismic hazard potential³⁸.

Dynamic elastic moduli

Elastic moduli describe how subsurface supplies reply to stress and pressure. These constants provide complementary insights into soil and rock habits beneath each static and dynamic loading circumstances³⁹.

Probably the most utilized elastic constants in geotechnical and seismic research embody Younger’s modulus, shear modulus, bulk modulus, Poisson’s ratio, and Lame’s parameters. Every supplies complementary insights into soil and rock habits beneath static and dynamic loading circumstances²¹.

Firstly, we are going to use a Gardener equation to calculate the density to make use of it for elastic moduli⁴⁰:

V_p in m/s, a = 0.31 when the density is given in g/cm³.

This empirical relationship was calibrated utilizing wireline logging information from Suez well-3 to make sure exact density assignments for the area’s particular lithological items.

Younger’s modulus (E)

Younger’s modulus Measures axial stiffness and is used to estimate settlement habits beneath basis hundreds. Excessive values point out dense, lithified supplies^41,42,43.

the place: V_p and V_s are main and shear wave velocities, (:rho::)is rock density.

Shear modulus (G)

The shear modulus, also called the rigidity modulus, it quantifies resistance to shear stress and displays the inherent energy of the soil skeleton or rock framework⁴⁴.

the place: V_s are shear wave velocities, (:rho::)is rock density.

Bulk modulus (Okay)

The majority modulus measures resistance to uniform compression, representing how a fabric responds to adjustments in quantity beneath hydrostatic stress. A low bulk modulus displays extremely compressible soils, whereas greater values characterize extra consolidated or cemented supplies with decrease compressibility^45,46.

the place: V_p and V_s are main and shear wave velocities, (:rho::)is rock density.

Poisson’s ratio (σ)

Poisson’s ratio relates lateral pressure to longitudinal pressure beneath axial loading. It’s extremely delicate to lithology, porosity, diploma of saturation, and clay content material. Values approaching 0.5 signify saturated or weakly consolidated supplies, whereas very low or near-zero values point out robust, brittle, or extremely competent rocks. Unfavorable values, although uncommon, might happen in anisotropic or extremely fractured formations^47,48.

the place: σ is Poisson Ration, V_p and V_s are main and shear wave velocities.

Lame’s parameter (λ)

Lame’s parameter (λ), at the side of the shear modulus, is utilized in elasticity concept to explain stress-strain relationships in isotropic supplies. This parameter is essential in superior geophysical modeling, notably for seismic wave propagation research and rock physics purposes^48,49.

the place: σ is Poisson Ration.

Integrating these moduli permits for the identification of weak zones and the delineation of soil-rock interfaces.

Utilized engineering indices and efficiency parameters

These parameters bridge the hole between geophysical imaging and geotechnical design by serving as proxies for subsurface competence.

Materials competence indices

Stress Ratio (S_i ). The stress ratio expresses the stability between vertical and lateral pressure in soils beneath load. Bigger values are usually related to free, water-rich, or fine-grained soils, whereas smaller values point out denser and extra competent floor^50,51.

the place: σ is Poisson’s Ratio.

Focus Index (C_i ). C_i is a velocity-ratio based mostly competence parameter usually used to evaluate the diploma of fabric consolidation. Larger values point out stiffer and extra competent deposits, whereas decrease values mark weak or compressible horizons^50,52.

the place: σ is Poisson’s Ratio.

Materials Index (V). V is a qualitative measure of the “materials high quality” in relation to basis stability, derived from elastic constants and velocity ratios. Larger V values usually correspond to stronger, extra compact supplies, whereas unfavourable or low values point out weak or fractured deposits^42,53.

the place: λ is Lame’s Fixed. G is the Shear Modulus, σ is Poisson’s Ratio.

Basis efficiency and stability

The estimated final bearing capability (qult) and settlement (δ) values have been derived from dynamic elastic moduli obtained from seismic velocity profiles and may due to this fact be considered preliminary geotechnical indicators relatively than design parameters. These outcomes present a dependable, spatially steady evaluation of subsurface stiffness and competence, supporting website screening and geohazard analysis. Nonetheless, they’re supposed to enhance not change site-specific geotechnical investigations, together with borehole information and static load testing, which stay important for last basis design.

Final Bearing Capability (q_ult ). The utmost contact stress that the soil or rock can maintain earlier than failure. Bearing capability will increase with density and shear-wave velocity, reflecting improved basis competence^41,54.

the place: Vs is shear wave Velocity, (:rho::)is rock density.

Settlement (δ). Settlement is the downward deformation of soil beneath structural hundreds, predominantly elastic at small strains. Bigger moduli correspond to smaller settlements, whereas tender or compressible deposits yield greater values^41,55.

the place: q_ult for the load at unit space is the stress worth relying on the depth z, δ_z is the settlement worth for the soil column with the depth z.

Commonplace Penetration Resistance (N-value). The Commonplace Penetration Check (SPT) blow rely (N) is a main index for evaluating the density of cohesionless soils and their liquefaction potential the tendency of saturated soils to lose energy beneath cyclic loading. On this research, N-values are estimated for the surficial unconsolidated wadi sediments (Layer 1) utilizing the velocity-based correlation^56,57,58.

the place: Vs is shear wave Velocity.

This empirical relation is particularly calibrated for shallow, cohesionless supplies the place low Vs and N-values point out greater liquefaction susceptibility. For the deeper, competent sandstone bedrock, N-values are reported as ‘equal resistance’ to supply a constant scale of fabric stiffness, though liquefaction terminology will not be relevant to those lithified items.

Vs30

The common shear-wave velocity to a depth of 30 m ((:{V}_{s30})) is the usual metric for seismic hazard evaluation. It’s calculated by accounting for the thickness and velocity of every particular person layer throughout the prime 30-meter interval²⁵.

the place (:{d}_{i}) is the thickness (in meters) of the ith soil/rock layer throughout the prime 30 m, and Vsi is the corresponding shear-wave velocity (in m/s). This formulation accounts for the contribution of every layer to the full journey time of shear waves over the depth interval.

This index is used to categorize the world into NEHRP Website Lessons (A by means of E), starting from arduous rock to tender soil  (Desk 2). These classifications straight decide the ground-motion amplification components and design spectra required for protected engineering in seismic codes^59,60.

Uncertainty evaluation and error propagation

The geotechnical parameters derived on this research are computed from P-wave velocity, S-wave velocity, and bulk density, every of which carries measurement uncertainty inherited from their respective tomographic and MASW inversions. Based mostly on the uncertainty evaluation and error propagation parameters, the imply relative uncertainties for the seismic and derived geotechnical properties could be summarized in Desk 3.

Uncertainty and reliability analyses demonstrated that the MASW inversion produced strong and constant subsurface fashions. Constrained by corresponding P-wave velocity (Vp) fashions to reduce inversion non-uniqueness, the inversion achieved a median dispersion-curve RMS misfit of 6.36 ± 1.26%, with a phase-velocity uncertainty of 28.8 ± 8.5 m/s. The imply relative uncertainty of the derived S-wave velocity (Vs) fashions throughout the eighteen profiles was 4.51%, indicating excessive mannequin stability. The reliability of the inversion outcomes was additional validated by means of robust settlement with unbiased lithological and wireline log information from Suez Nicely-3, confirming the recognized three-layer subsurface construction. Though the inversion outcomes stay influenced by the assumed preliminary Vp/Vs ratios (1.65–1.90) and the inherent trade-off between decision and the utmost investigation depth (~ 40 m), the derived Vs profiles present high-confidence estimates throughout the higher 30 m, guaranteeing dependable Vs30 willpower and NEHRP website classification.