Multichannel Analysis Of Surface Waves Technique Research Articles

Application of seismic refraction and MASW methods for investigating the Spillway Fault trace along the western side of the Aswan High Dam, Egypt

AbstractAn earthquake of local magnitude ML = 4.6 occurred on November 7, 2010, 4.5 km northwest of the Aswan High Dam on the Spillway Fault. In the Aswan metropolitan region this earthquake was felt intensely. As no surface rupture was found, the focal mechanism and the distribution of seismic activity was one of the tools used for finding fault dimensions. The composite fault-plane solutions for the observed events on the Spillway Fault showed a left lateral strike-slip faulting with normal-fault component striking NNW-SSE. Also, remote sensing techniques were applied for the detection and identification of the geomorphology and geometry of the Spillway Fault. In this research, sub-surface layers and structures are delineated utilizing near-surface seismic techniques. Furthermore, the area’s supposed path and position of the Spillway Fault are also investigated. Two active seismic techniques, Seismic Refraction and Multi-Channel Analysis of Surface Waves (MASW), are utilized for recording near-surface seismic wave data at 9 sites. The seismic refraction profiles are conducted as a 2D cross-section on the trace of the detected Spillway Fault in the study area to evaluate the maximum depth of penetration of the P-wave for fault investigation. The constructed 2D seismic and structural sections from P-wave results show that the obtained average depth of about 30 m. In addition, the estimated P-wave velocities extend from 600 m/s to over 6500 m/s. Some lateral variation in the seismic wave velocities in all layers may represent fault zones. Moreover, the 1D MASW technique is conducted to estimate the velocities of the shear wave for the upper 30 m (Vs30) to provide the site classes and soil characteristics along both sides of the detected Spillway Fault trace in the study area. The calculated Vs30 values emphasized the idea of the existence of a normal dip-slip fault trace which divides the study area into two different lithological parts. The first part is located on the eastern side and characterized by almost class B (hard rock, according to NEHRP classification), while the other part is located to the west, and shows almost class type C (denoted as dense soil and soft rock soil).

The Combination of Hvsr and Masw Methods with Landsat 8 Imagery to Assess the Changing Shoreline along the Coastal Area of Central Bengkulu

The Central Bengkulu Regency possesses a coastline measuring 21.8 km, situated in direct proximity to the Indian Ocean. The heightened wave and ocean current dynamics possess the capacity to induce detrimental effects in the form of coastal abrasion along the coastline. This study conducted an examination of the rate of shoreline changes resulting from coastal abrasion in Central Bengkulu Regency, Bengkulu, Indonesia. Horizontal-to-vertical spectral ratio (HVSR) analysis, multichannel analysis of surface waves (MASW), and Landsat-8 remote sensing were all used in the current investigation. The project gets started with conducting field investigations and measurements of geophysical techniques, including the collection of Landsat-8 satellite imagery spanning the period from 1990 to 2020. Additionally, an examination was conducted on the rate at which the shoreline undergoes changes in velocity. In order to determine the increase in change that occurs along coastlines over a period of several decades, a study was carried out during which Landsat-8 satellite images were analyzed. This study investigates the application of the HVSR technique for assessing the seismic vulnerability of the ground and, additionally, the MASW technique for measuring the shear wave velocity of the coastline’s soil. Both of these methodologies are compared and contrasted with one another in this research. The findings indicated that the mean pace of coastline change in Central Bengkulu Regency was 1.5 m/yr, with the maximum velocity recorded at 4.1 m/yr. This high velocity of shoreline change is correlated with the Vs30 value of MASW measurement and Kg of HVSR measurement, where the subsurface soil structure along the coast of Central Bengkulu Regency from Vs30 measurement shows that it is dominated by stiff soil structure that is susceptible to erosion. Outcomes from the study can inform decision-making processes about safeguards and preventative measures to prevent further coastal degradation. HIGHLIGHTS In this paper, we present a framework using application passive and active seismic method with Landsat 8 for knowing the changing of shoreline coastal area in Central Bengkulu Indonesia This study demonstrated that the high velocity of the shifting shoreline is correlated with Vs30 and Kg values The subsurface soil structure is predominantly composed of stiff soil, and the changing shoreline starts at 1.5 mm/yr, which makes the coast of Central Bengkulu vulnerable to erosion GRAPHICAL ABSTRACT

Characterization of sub-surface structure, using seismic refraction and multi-channel analysis of surface waves methods in Ajere Ekori Yakurr LGA of cross river state

The study was to characterize the sub surface at Agere in Ekori, using seismic refraction method, multichannel analysis of surface waves technique and borehole intrusive technique. Data were collected using a 12channel seismograph and other accessories required for seismic refraction data collection. Software called seismicimager was used to examine the data. The primary wave velocity in the first layer varied from 690 m/s at 4.2 m to 96 m/s at 7.3 m. A Vp range of 315 m/s to 484 m/s at a depth of 2 m is present inside the layer and represents the organic soil constituents. A Vp range of 669 m/s to 1756 m/s represents loose sand (dry), loose made ground (rubble), landfill rubbish, disturbed soil, and clay landfill, all within a depth of 2.3 m to 12.1 m. In addition to the borehole intrusive method, multichannel analysis of surface wave (MASW) techniques was used to calculate the soil profile based on velocity. The source was a 7 kg sledge hammer, the detectors (receivers) were 24 units of 4.5 Hz geophones, and the recorder was a Terraloc Mark 8 ABEM. Seismicimager software was used for analysis. At Ajere 1 through 6, the MASW test configuration employed 5 m geophone spacing and a source offset distance of 5 m, while at Ajere 7, it used 1 m geophone spacing and a source offset distance of 2 m. Near the boreholes, all of the MASW test arrays were run. The trustworthy seismic data from Ajere 1 to 6 at depths of 0.7 m to 13.1 m and 4.7 m to 17 m. Based on SPT N values, the results showed that the shear wave velocities had been classified into three layers of soil: very soft, soft, and firm. The velocities below 164 m/s, between 164 and 190, and 190 m/s to 320 m/s were classified as these soil types. In the meantime, a drilling invasive technique based on SPT N value determines changes in the soil layer. Hard material shear wave velocity data was not provided. In conclusion, because of its non-destructive, non-invasive nature and relative speed of evaluation, the MASW technique has the potential to be adapted in soil study to complement intrusive technique.

Passive multichannel analysis of surface waves using 1D and 2D receiver arrays

We use the seismic multichannel analysis of surface waves (MASW) method, which originally used active sources (i.e., controlled by users), to present a new alternative technique for surface-wave analysis of passive data (i.e., seismic sources not controlled by users). Our passive MASW technique uses two types of receiver arrays, 1D and 2D, for two different purposes. The 2D square array is used for assessing the number and directionality of passive seismic sources to help identify optimal time window(s) when such sources are of sufficient quality and aligned with the 1D receiver spread. Then, these time windows are used to selectively extract passive seismic data recorded by the 1D receiver spread for subsequent MASW analysis. In such a manner, this hybrid 1D–2D passive MASW approach can approximate the higher quality advantages of the active MASW method. This technique is successfully applied at a test site in southcentral Kansas taking advantage of train energy as the passive sources monitoring and assessing the potential development of vertically migrating voids that could endanger groundwater and infrastructure. The 1D–2D passive MASW method has proven beneficial for optimizing passive-data dispersion patterns and accurately assessing relative velocities for investigations with reduced labor costs and minimal added processing time and can be considered as a contribution to the surface-wave analysis tools.

Reconstruction of a Reference Subsoil Model for the Seismic Microzonation of Gori (Georgia): A Procedure Based on Principal Component Analysis (PCA)

ABSTRACTThis article focuses on the full exploitation of geological and economically viable geophysical surveys for the seismic characterization of the shallow subsoil in the frame of microzonation studies in urban areas where economic resources for detailed seismic response analyses are scarce. In these conditions, the outcomes of inexpensive geophysical surveys (e.g., based on ambient vibration monitoring or surface-wave prospecting) must be fully exploited. To reduce the uncertainties related to these kinds of procedures, their joint interpretation in the light of geological evidence is mandatory. To this purpose, we propose the application of principal component analysis to combine the results of distributed single-station ambient vibration measurements (horizontal-to-vertical spectral ratio [HVSR] technique) to provide a preliminary zonation of the study area. The zones identified in this way are then characterized by considering the available geognostic boreholes, VS profiles deduced by the joint inversion of HVSR curves, and available Rayleigh-wave dispersion curves deduced from active seismic prospecting (multichannel analysis of surface-waves technique). The final outcome allows the definition of a preliminary seismic model of the study area, which is also constrained by the available geological data deduced from on-purpose surveys. The proposed approach has been applied to the city of Gori (Georgia). The proposed approach allowed a reliable assessment of buried geometries, geological domains, and the distribution of lithofacies, which can control the local seismic response. In detail, the major role of paleovalley infills and interfluve domains has been enlightened by adding in evidence concerning the peculiar stratigraphic relationships and buried morphologies, which may determine 1D and 2D resonance effects.

A critical insight into the influence of data acquisition parameters on the dispersion imaging in passive roadside MASW survey

Passive roadside method of multichannel analysis of surface wave (MASW) is a simple passive technique of subsurface exploration that can provide deeper subsoil investigation compared to conventional active MASW technique. However, attributed to the lack of thorough experimental validation with other techniques and establishment of various acquisition parameters, the number of real world applications of the method is scanty. The paper presents the results of extensive experimentation to highlight the importance of two primary acquisition parameters, namely the length of receiver array and acquisition time, on the dispersion imaging of roadside survey at sites comprising different traffic volume. The length of receiver array directly affects the resolution of dispersion image and the estimated depth of shear-wave velocity profile. Thicker energy band and poor resolution in the lower frequency band of dispersion image are found to be the outcome of utilizing smaller arrays. The span of acquisition time is a site dependent parameter and is governed by the volume of the traffic and number of existing sources on the road surface nearby the test site. Comparatively smaller recording time is found effective in producing dispersion images with sufficient resolution in a heavier traffic volume case. The paper also illustrates a detailed comparison of results obtained from roadside and active survey in terms of the dispersion imaging and shear wave velocity profiles at the specific sites.

Integrated geophysical techniques for subsurface imaging of active deformation across the Himalayan Frontal Thrust in Singhauli, Kala Amb, India

Non-invasive geophysical techniques employed across the Himalayan Frontal Thrust (HFT) at Singhauli in the north-western Frontal Himalaya to understand shallow subsurface geological structures and their nature in the context of active tectonics. The combination of high-resolution Ground Penetrating Radar (GPR), Electrical Resistivity Tomography (ERT) and Multi-channel Analysis of Surface Waves (MASW) techniques with the different acquisition parameters found to be beneficial in gaining high-resolution images of shallow subsurface geological structures. The contact zone of resistivity obtained between <134 and >134 Ωm suggests the juxtaposition of dissimilar lithological units explained as two faults dipping to the northwest related to the HFT. The average separation between Fault 0 and Fault I is 12m which in general agreement with the trench excavation survey. The GPR radargram over a length of 45m show offset reflectors at a horizontal distance of ~12–15m close to the active deformation zone. The horizontal reflectors abruptly truncate across two northerly dipping planes. The 2D Vs section (MASW1&2) exhibited dipping attitude of the litho units (with different velocity zones) in the northeast direction which are also consistent with the local geological setting. Prominent liquefaction feature identified in the ERT section specifically with the 1 and 1.5m electrode spacing show explicit expression of its existence due to contrasting resistivity ranging between 100 and 450 Ωm. A strong correlation has been established between the field based geological observations, carried out earlier in this area through a trench excavation survey, with the present work using geophysical techniques. The integrated approach found to be highly beneficial in those areas where contrasting sub lithological units exist in terms of their physical properties. This study argues for judicious use of the ERT, GPR and MASW techniques to delineate shallow subsurface geology across various active faults in the Himalayan terrain to comprehend the active deformation related to large magnitude paleoearthquakes and earthquake hazard assessment.

Delineation of shallow structures in the vicinity of Ulu Slim hot spring using seismic refraction and MASW techniques

ABSTRACT Ulu Slim hot spring possesses the highest surface temperature with a reported temperature of 104° C. Hence, the site may be suitable for electric power generation from the geothermal aquifer. In the present work, two seismic techniques are used for shallow structures investigation. The methods are the seismic refraction and the Multi-Channel Analysis of Surface Waves (MASW). The delineation of superficial structures will aid in developing the area without affecting the geothermal system. The instrumentation used for both is similar to a large extent. The main differences are the natural frequencies of geophones used. MASW requires a low natural frequency geophone, whereas seismic refraction uses high-frequency ones. Both techniques are applied at five locations distributed in the 5 km x 5km area. The results obtained from both methods are also tending to confirm with each other. From all profiles, three layers are delineated in the vicinity of the hot spring. The top layer is impermeable clay with an average Vp of 500 m/s and Vs <200 m/s. Underneath this surface layer, the possible aquifer unit characterised by sand, with Vp in the range of 1000 to 2800 m/s and Vs between 200 m/s and 300 m/s. The final layer is the bedrock characterised by a Vp higher than 3700 m/s and Vs greater than 300 m/s. A particularly striking feature of the bedrock structure obtained from our results shows that the bedrock is relatively close to the surface of the vicinity of the hot spring, with a depth of about 5 m. These results confirm the conceptual model proposed for the Ulu Slim hot springs as of granitic origin. Moreover, a plot of the Poisson ratio indicated regions of high water saturation that may represent the hot water pathway to the surface.

Surface wave surveys for imaging ground property changes due to a leaking water pipe

This study demonstrates the use of Multi-channel Analysis of Surface Waves (MASW) to measure changes in Rayleigh wave velocity relating to both the initial trench construction and subsequent simulated failures (water leaks) of a buried water-pipe. The MASW field trials were undertaken in conjunction with a wider suite of geophysical monitoring techniques at a site in South-west England, within an area of clayey sandy SILT. The Rayleigh wave velocity through a soil approximately equals the shear wave velocity, which in turn is predominantly dependant on the shear modulus of the soil (G) and this can be inferred to give a measure of the relative strength of a soil. It is proposed that the time-lapse measurement of Rayleigh wave velocity may be used to monitor ongoing changes in soil strength and therefore the MASW technique could perform a significant role in monitoring the initiation/progression of any internal processes within a geotechnical asset, before they would otherwise be identified through visual inspection alone.

Engineering geotechnical evaluation of soil for foundation purposes using shallow seismic refraction and MASW in 15th Mayo, Egypt

Despite the modernity of the 15th Mayo city, buildings suffer progressively from engineering problems which are increased over the last 10 years like cracks, fissures, cavities and tilting of building. The main target of this work is to delineate the dynamic engineering characteristics of subsurface rocks and soils through interpretation of P- and S-wave velocities, knowledge of probable causes of structural construction damages and calculate Vs30 for determining site class. Therefore, field work of shallow seismic refraction (SSR) and multichannel analysis of surface waves (MASW) were undertaken in this work to evaluate causes of these effects. Nineteen profiles for both SSR and one dimension (1D) MASW were performed to assess the geotechnical characteristics of subsurface soil for building purposes and calculating Vs30 for site class evaluation.SSR survey was implemented using a seismograph of 48-channel with vertical geophones of 40 Hz. The first breaks were analyzed employing SIPT2 program. S-waves were generated using 1D MASW technique with seismograph of 24-channel model Strata View with 2 m offset distance and two shooting. Field-work was undertaken employing vertical geophone of low-frequency (4.5 Hz). SeisImager/SW program package was employed for processing seismic results. Some important elastic moduli, engineering and petrophysical characteristics were evaluated. Findings of VP and VS revealed that the proposed site composed mainly of three geoseismic zones; the first zone is the surface layer of weathered limestone, fragments of gravels and fractured marl which characterizes by incompetent to slightly competent rock quality. The second zone is underlain the previous layer, its lithology is clayey marl of fairly to moderately quality of competent rock, which causes serious problems for the buildings due to presence of clay. The third zone is the hardest layer of limestone content and was employed as a bedrock layer which characterizes by good quality of competent rock.VS30 (averaging VS for the uppermost of 30 m) was calculated for near-surface geologic units applying NEHRP's rules. The proposed site was categorized into two classes: class C (very dense soil and soft rock), that characterizes majority of the sites, and class D (stiff soil). These parameters are principally used as key inputs for building design.

A state-of-the-art review of passive MASW survey for subsurface profiling

Passive multichannel analysis of surface wave (MASW) is a nondestructive technique capable of exploring soil subsurface to a great depth by utilizing ambient long-wavelength passive surface waves without requiring an active source. The method yields subsurface information in terms of shear wave velocity (Vs) profile of the surveyed area. Vs is a vital dynamic property of soil which has numerous applications in various geotechnical and earthquake engineering problems. The accuracy of final Vs profile obtained by MASW technique primarily depends upon the resolution of the dispersion curve which is plot of frequency-dependent phase velocity of the propagating Rayleigh wave. The present study critically reviews the effects and influences of various geometrical parameters (such as the shape and size of receiver arrays) and the recording and processing parameters (such as acquisition time, sampling frequency, and vertical stacking) that control the quality of raw wavefield data and subsequent dispersion imaging. The study reveals that there is no fixed recommendation for any of the mentioned parameters to be used to obtain a high-resolution dispersion image in case of passive MASW survey. Therefore, extensive experimentations are required to further understand and develop standard guidelines for field experimentation and data processing so as to reinforce the method as a stand-alone technique.

Vs30 mapping at selected sites within the Greater Accra Metropolitan Area

A large part of Accra is underlain by a complex distribution of shallow soft soils. Within seismically active zones, these soils hold the most potential to significantly amplify seismic waves and cause severe damage, especially to structures sited on soils lacking sufficient stiffness. This paper presents preliminary site classification for the Greater Accra Metropolitan Area of Ghana (GAMA), using experimental data from two-dimensional (2-D) Multichannel Analysis of Surface Wave (MASW) technique. The dispersive characteristics of fundamental mode Rayleigh type surface waves were utilized for imaging the shallow subsurface layers (approx. up to 30 m depth) by estimating the 1D (depth) and 2D (depth and surface location) shear wave velocities at 5 selected sites. The average shear wave velocity for 30 m depth (Vs30), which is critical in evaluating the site response of the upper 30 m, was estimated and used for the preliminary site classification of the GAM area, as per NEHRP (National Earthquake Hazards Reduction Program). Based on the Vs30 values obtained in the study, two common site types C, and D corresponding to shallow (>6 m < 30 m) weathered rock and deep (up 30 m thick) stiff soils respectively, have been identified within the study area. Lower velocity profiles are inferred for the residual soils (sandy to silty clays), derived from the Accraian Formation that lies mainly within Accra central. Stiffer soil sites lie to the north of Accra, and to the west near Nyanyano. The seismic response characteristics over the residual soils in the GAMA have become apparent using the MASW technique. An extensive site effect map and a more robust probabilistic seismic hazard analysis can now be efficiently built for the metropolis, by considering the site classes and design parameters obtained from this study.

Resolving phase wrapping by using sliding transform for generation of dispersion curves

The complexity involved with the phase unwrapping procedure, while performing the existing spectral analysis of surface waves (SASW) on the basis of two sensors, makes it difficult to automate and requires frequent manual judgment. As a result, this approach generally becomes tedious and may yield erroneous results. The multichannel analysis of surface waves (MASW) technique can resolve the problem of phase wrapping. However, the MASW technique normally requires a large number of closely spaced sensors, typically 24–48 or even more. We have developed a new method that is fast, accurate, and generally resolves the unwrapping of phase with the use of just two sensors, provided the signal-to-noise ratio remains high. In this approach, the unwrapping of the phase can be performed without any manual intervention and an automation of the process becomes feasible. A few examples, involving synthetic test data and surface-wave tests, have been tested to determine the efficacy of our approach. Comparisons of the results have been made with the corresponding solutions using existing SASW and MASW techniques.

Near-field effects on site characterization using MASW technique

The application of Multichannel Analysis of Surface Wave (MASW) technique is increasing in geotechnical engineering to characterize near surface materials. The dispersion property of Rayleigh wave is utilized in MASW method. MASW often suffers from near-field effects which may result in either underestimation or overestimation of Rayleigh wave phase velocity due to the body waves contamination near to the source. In this paper, a detailed numerical study has been carried out to examine the near-field effects considering three different types of typical S-wave velocity models with four different impedance scenarios in each case. The study shows that the impedance contrast between the half-space and overlying soil layer is having a considerable effect on the underestimation of phase velocity. These near-field effects are also found to be influenced by the type of the S-wave velocity model as well as far and near offset distances. With the increase of impedance contrast, the level of underestimation seems to increase at lower normalized array centre distance due to mode jump. However, such jump can not be observed with limited far offset distances generally used in usual practice due to poor resolution in the dispersion spectra. Significant near-field effect are observed for lower far offset distances and inversely dispersive S-wave velocity models. Underestimation of Rayleigh wave phase velocity is quantified in terms of two normalized parameters. Finally, a field study is also conducted to verify our numerical findings.

Multichannel analysis of surface waves (MASW) for seismic site characterization using 2D genetic algorithm at Bahrah area, Wadi Fatima, Saudi Arabia

The present study deals with the seismic site classification of Bahrah area, Wadi Fatima, to characterize the local site conditions. The dynamic behavior of sediments was studied by the application of surface wave inversion. The multichannel analysis of surface waves (MASW) shallow geophysical technique was utilized for site classification. MASW survey was carried out at 66 sites along with 13 seismic refraction profiles at suitable localities. MASW and seismic refraction profiles were processed and compared with the available borehole data. The integration of MASW techniques with seismic refraction and borehole data progressively enhanced the subsurface visualization and reliability of the shear wave velocity estimation in the subsurface in the study area. The subsurface shear-wave velocity model was achieved by the solution of an inverse problem-based dispersion of surface waves and propagation in a vertically heterogeneous medium. The 2D genetic algorithm was employed for the inversion of dispersion curves to obtain velocity and thickness of subsurface layers. The depth to engineering bedrock and velocity of shear waves in the first 30 m was deciphered and mapped. The depth of bedrock in study area varies from 4 to 30 m, and VS 30 ranges from 320 to 800 m/s. The most of study area falls in B and C class categories in addition to few sites of D class according to the NEHRP guidelines.

Near-surface site characterization at Quriyat City, Sultanate of Oman using HVSR and MASW techniques

It is well known that the effects of near-surface structures on ground shaking represent an important factor in earthquake ground motion amplification; in particular, soft sedimentary cover could strongly amplify the seismic motion. The parameters of the soft layer that controls the amplification most are the fundamental frequency and the shear-wave velocity. The frequency band of the ground motion that is affected by such factors depends on the thickness and the velocity of the sedimentary layers. In order to investigate and characterize the near-surface structures beneath Quriyat coastal city in the Sultanate of Oman, Nakamura’s technique which was characterized by the horizontal-to-vertical spectral ratio (HVSR) of the ambient noise measurements was used to estimate the fundamental frequency of the soft soil at 70 sites. The shear-wave velocity is an important tool in the design of buildings for site-specific conditions and is a much more diagnostic tool for engineering properties. The multichannel analysis of surface waves (MASW) was carried out at 40 sites at Quriyat City in order to evaluate the near-surface shear-wave velocities. The fundamental frequencies were found to be in the range of 2.0 Hz (occupying the coastal soft soil area) and 23.3 Hz (occupying the area close to the hard rock). The average shear-wave velocities down to 30 m depth (Vs30) ranged from 225 m/s (class D according to NEHRP standard for site classification and covering the coastal area) to 1058 m/s (class B covering the area in vicinity of hard rock). The results demonstrate that the HVSR of ambient vibrations were in a good agreement with the values of VS30. Areas of lower VS30 values exhibit lower fundamental frequency and cover areas with considerable soft soil thickness and vice versa.

Multichannel Analysis of Surface Waves and High-Resolution Electrical Resistivity Tomography in Detection of Subsurface Features in Northwest Himalaya

Geophysical studies using multichannel analysis of surface waves (MASW) and high-resolution electrical resistivity tomography (HERT) have been jointly carried out on an experimental basis in the field. The motive is to study shallow subsurface features (i.e. faults traces, cavities and palaeo-channels) in the foothill zone of Northwest Himalaya. These techniques have shown their potentiality in successfully identifying shallow (0-24 m) fault traces and dissolution features/palaeo-channels. Depending on the sensitivity of the MASW and HERT techniques, geophysical signatures of the subsurface features were recorded and further resolved with the help of synthetic simulation. The synthetic simulation of 2D electrical response has been carried out over the initial model for subsurface fault traces as well as palaeo-channels. The initial model has been refined iteratively to bring the synthetic response close to the field response and hence the final refined model is considered to be the true representation of the subsurface.

Numerical Investigation of 3D multichannel analysis of surface wave method

Multichannel analysis of surface wave (MASW) method is an efficient tool to obtain near-surface S-wave velocity, and it has gained popularity in engineering practice. Up to now, most examples of using the MASW technique are focused on 2D models or data from a 1D linear receiver spread. We propose a 3D MASW scheme. A finite-difference (FD) method is used to investigate the method using linear and fan-shaped receiver spreads. Results show that the 3D topography strongly affects propagation of Rayleigh waves. The energy concentration of dispersion image is distorted and bifurcated because of the influence of free-surface topography. These effects are reduced with the 3D MASW method. Lastly we investigate the relation between the array size and the resolution of dispersion measurement.

A geophysical strategy for measuring the thickness of the critical zone developed over basalt lavas

Estimates of the thickness variation in lateritic weathering profiles (LWPs) are important in tropical areas underlain by young basalt lavas like those found in Hawaii. Seismic shear-wave velocity data were obtained by a new application of multichannel analysis of surface waves (MASW) to map variations in the LWP and to derive the downward rate of advance of the weathering front in basaltic lavas. The MASW technique proved highly capable of imaging the internal structure and base of the critical zone, as confirmed by borehole data and direct field measurements. Profile thickness thus obtained, rapidly and without drilling, has applications to engineering and geochemical studies. The rate of advance of the weathering front derived from MASW in Oahu ranged from 0.010 m/ka to 0.026 m/ka in mesic zones (∼1500 mm/a rainfall), whereas an area with ∼800 mm/a revealed rates from 0.005 m/ka to 0.011 m/ka. These rates are comparable to those derived from recent solute-based mass balance studies of ground and surface water. Conventional P-wave seismic reflection did not perform as well for detecting boundaries due to a gradational seismic velocity structure within the weathering profile. Shear-wave velocity models showed internal variations that may be caused by textural differences in parental lava flows. Limitations in imaging depth were overcome by innovative experiment designs. Increasing source-receiver offsets and merging surface-wave dispersion curves allowed for a more objective derivation of velocity-frequency relations. Further improvements were made from a recently developed form of the combined active and passive source technique. These advances allowed for more detailed and deeper imaging of the subsurface with greater confidence. Velocity models derived from MASW can thus describe the LWP in terms of depth and variability in stiffness.

Estimation of Shear Wave Velocity for Near-surface Characterisation; Case Study: Ifako/Gbagada Area of Lagos State, S.W. Nigeria

Near-surface shear–wave velocity profiles were acquired at four locations; these at strategic stations in Ifako/Gbagada a sub-urban area in Lagos State. The geophysical surveyed obtained the shear-wave velocity data using Multichannel Analysis of Surface Waves (MASW) technique. These data were acquired with a view to delineating the existing or potential hazards relating to subsidence, distressing and weakening of structures above the earth which is important to public safety, mitigation of property damage and to see the effectiveness of MASW technique in engineering site investigation. The processing method was fully automated by software called SURFSEIS. The study showed that the entire profiles depicted a very low shear-wave velocity (~80m/s to 160m/s) region down to 15m, a signature of saturated peaty/clayey formation. Thus, subsidence, distressing and weakening of structures were inferred to probably resulted from the loose nature of the subsurface soil.