Seismic Refraction Measurements Research Articles

A geological and geotechnical information system (GEO-GIS) for urban planning and its application to the Acıpayam Basin (Denizli, Turkey) with some observations from a M w 5.5 earthquake on 20 March 2019

There are many site investigation projects used for the urban planning of a city. They are archived in hard copy or scanned in image files stored on computers. In this study, geological, geotechnical and geophysical data obtained from different site investigation projects in the Acıpayam district were evaluated in the same coordinate system using a geological and geotechnical information system (GEO-GIS) computer program. The Acıpayam GEO-GIS database includes field tests in 82 boreholes, 85 observation pits, laboratory test results of soil samples, 99 seismic refraction measurements and 70 microtremor measurements. Required information from the database can be displayed at its spatial coordinates on a base map and new maps can also be drawn using the GEO-GIS database system. Resultant maps drawn using the Acıpayam GEO-GIS database show that in the northern and eastern parts of the Acıpayam district, where the Oğuz, Kırca, Yassıhöyük, Apa, Ovayurt, Karahüyük, Ucarı and Gedikli villages are located, weak soil layers exist. This means that some damage may be caused to adobe and masonry buildings in the villages during shaking associated with strong earthquakes, as observed after the 5.5 (M w ) earthquake that occurred on 20 March 2019. The Acıpayam GEO-GIS database system is a useful tool for engineers and planners that can assist in drawing GIS-produced maps for geotechnical evaluations of the basin. These maps can be used in decision-making projects for urban planning. Thematic collection: This article is part of the Leading to Innovative Engineering Geology Practices collection available at: https://www.lyellcollection.org/topic/collections/leading-to-innovative-engineering-geology-practices

Identification of Shallow Fault with Seismic Refraction Modelling in Gondang Area, Bojonegoro

There are various considerations in determining whether a field is said to have geothermal potential or not. One of them is the appearance of shallow faults in the area. This shallow fault can function as a permeable layer that is used as a fluid flow path in the geothermal system. Due to the emergence of manifestations around Mount Pandan in the Gondang area, Bojonegoro, seismic refraction measurements are also needed that can support and validate the geothermal potential in the area with the hope of shallow faults that are visible through the interpretation of seismic wave propagation velocity. This measurement produces an anomaly that is thought to be a shallow fault on the track that passes through the main fault, namely on tracks 1, 2, and 5, not far from the manifestation of the Banyukuning hot spring. This is also reinforced by direct observation of the surface conditions as a result of the appearance of shallow faults.

Application of Combined Local and Global Optimization Algorithms in Joint Interpretation of Direct Current Resistivity and Seismic Refraction Data: A Case Study of Dammam Dome, Eastern Saudi Arabia.

The main geological structures in the Dammam Dome are defined by integrating geophysical measurements and applying new methodological approaches. Dammam Dome is characterized by a well-developed fracture/joints system; thus, high complexity of the subsurface is expected. Direct Current Resistivity (DCR) and Seismic Refraction (SR) geophysical survey aimed to map the Dammam Dome's near-surface features. The geophysical data were acquired along two profiles in the northern part of Dammam Dome. To maximize the results from conducting DCR and SR measurements over a complex area, a combined local and global optimization algorithm was used to obtain high-resolution near-surface images in resistivity and velocity models. The local optimization technique involves individual and joint inversion of the DCR and SR data incorporating appropriate regularization parameters, while the global optimization uses single and multi-objective genetic algorithms in model parameter estimation. The combined algorithm uses the output from the local optimization method to define a search space for the global optimization algorithm. The results show that the local optimization produces satisfactory inverted models, and that the global optimization algorithm improves the local optimization results. The joint inversion and processing of the acquired data identified two major faults and a deformed zone with an almost N-S direction that corresponds with an outcrop were mapped in profile one, while profile two shows similar anomalies in both the resistivity and velocity models with the main E-W direction. This study not only demonstrates the capability of using the combined local and global optimization multi-objectives techniques to estimate model parameters of large datasets (i.e., 2D DCR and SR data), but also provides high-resolution subsurface images that can be used to study structural features of the Dammam Dome.

Improving moisture content estimation from field resistivity measurements with subsurface structure information

• Smoothness constraint in regularized resistivity inversion is removed at structural boundaries. • Structural unit-specific petrophysical models are used for moisture interpretation. • Moisture content is better estimated from resistivity with the new method. In Earth sciences, the measurement of soil and rock moisture content is essential in improving our understanding of various hydrologic processes. Recently, the electrical resistivity method has been frequently used to estimate the moisture content in the field. The uncertainty associated with resistivity-estimated moisture content is mainly from two sources: regularized inversion and petrophysical interpretation. In this study, to reduce the uncertainty, we propose (1) to use subsurface structural information from seismic refraction measurements to relax the smoothness-based regularization at structural boundaries and (2) to use structural unit-specific petrophysical relationships to translate resistivity into moisture content. The proposed methods are tested on a synthetic subsurface model featuring three distinct layers of a granitic critical zone (CZ). The results of the synthetic example show that both the spatial pattern and the moisture content values estimated with the new method are very close to the true model with low uncertainty. Compared to the traditional method, the estimation is significantly improved, particularly at the CZ boundaries, such as the regolith-fractured bedrock interface. We also apply the new method to a granitic hillslope to estimate the moisture content distribution from field resistivity measurements. Although no ground truth is available for validation, the estimated moisture content distributions exhibit some typical hydrological features in hillslopes, such as the perched water at the soil/rock interface and preferential flow path in fractured rocks. Therefore, it is concluded that incorporating structural information in resistivity inversion and using structural unit-specific petrophysical models can improve moisture content estimation from field resistivity measurements.

Assessment of seismic vulnerability using the horizontal-to-vertical spectral ratio (HVSR) method in Haenam, Korea

The town of Haenam in southwest Korea lies partially on reclaimed coastal land and experienced an unprecedented earthquake swarm during April and May 2020. Construction of a new town in the area means that there is demand to evaluate the seismic hazard caused by site-specific amplification of seismic ground motions by artificial unconsolidated cover. We used a microtremor horizontal-to-vertical spectral ratio (HVSR) method to identify resonance frequencies, image depths to bedrock, and assess seismic ground vulnerability across the epicentral area of the recent earthquake swarm. Microtremor measurements were taken at 144 sites across an 18.5 km × 12.5 km study area. The HVSR analysis shows resonance frequencies as low as 1.1 Hz. Using mean shear-wave velocities obtained by Multi-Channel Analyses of Surface Waves (MASW), we find that resonance frequencies correspond to depths to bedrock of between 3 m and 56 m. There are clear relationships between the distribution of resonance frequencies, depths to bedrock, surface geology, and areas of development in recent land reclamation projects. Higher resonance frequencies characterize areas of thin sedimentary cover or hard rock sites, while relatively low resonance frequencies characterize reclaimed land or greater depths to bedrock. We quantify the seismic hazard potential across the study area using the seismic vulnerability of ground index (Kg), and observe a clear association between zones of high Kg and areas of reclaimed land. This study demonstrates the value of the HVSR technique in characterizing the subsurface and assessing seismic vulnerability in areas of reclaimed land, where it can be used as a viable and low-cost alternative to large-scale active seismic refraction or MASW measurements. The results can also be used to inform and guide urban planning projects.

Improved use of seismic profiles to estimate allowable pressure in granular soils: Application to quaternary deposits with different gradings

Herein, a new approach for widening the applicability of seismic refraction profiles in shallow foundation designs is presented. To provide quantitative geotechnical information, a relationship was established to estimate the allowable bearing pressure of cohesionless soils according to the results of seismic refraction measurements, without the conventional uncertainty. The developed procedure was employed in the geomechanical characterization survey for the construction of a helipad at the centre of the Iberian Peninsula; here, seismic refraction tomography was used to compute sections of the velocity–depth curves using the wavepath eikonal traveltime inversion method. When the shear- and compressional-wave one-off velocities are transformed into dynamic moduli through conventional elastic equations, dynamic moduli images that provide quasi-continuous information on soil deformation properties can be generated. These results were combined with the data from dynamic probing super-heavy tests to evaluate the stress–strain moduli relationships, and the dynamic moduli were used to determine the bearing capacity. Furthermore, a general equation for the allowable bearing pressure in granular soils is proposed; this equation contains a new variable that depends on the grain size grading of the zone. Using this, the distribution of the allowable bearing pressure can be used as a criterion to detect spatial variability and problematic zones. The proposed methodology provides more deformation properties of subsoil; thus, it reduces the costs of ground-testing techniques as the boreholes can be better located according to the project foundation requirements.

2D inversion of refraction travel-time curves using homogeneous functions: a case study from Wadi El-Nakheil, Eastern Desert, Egypt

The area of study is located at Wadi El-Nakheil to the west of Quseir district along the Egyptian Red Sea Coast. The present work is a case study of applying the 2D homogeneous function method. This function automatically inverts the first arrival refractions to derive a 2D velocity distribution involving the seismic boundaries. The present study aims at investigating the near-surface geological conditions of Wadi El-Nakheil. These conditions include the following: the different stratigraphic units, structural configuration, and the groundwater occurrence. Authors achieved this aim by carrying out shallow seismic refraction measurements along three profiles using direct and reverse shooting. A newly developed field methodology is used in the data acquisition. The interpretation of the seismic data is performed using GODOGRAF software which represents a 2D refraction tomographic inversion algorithm. The interpretation revealed the presence of seven geo-seismic zones, overlain by low-velocity surface layer, with a velocity range of less than 800 to 6800 m/s. The detected zones are dissected by several normal faults with NE and SW dipping directions and maximum throw of about 20 m. An angular unconformity is noticed between the overlying horizontal layers and the dipping (towards SW) underlying layers. One of the interpreted zones is identified as the water-bearing zone where the velocity is between 2200–2400 and 2600 m/s. This was emphasized by the correlation with the nearest borehole in the area. The water-bearing zone is interpreted as the Nakheil Formation. The thickness of the water-bearing zone ranges from 20 to 55 m, while its depth ranges from 35 to 65 m.

Resistivity and Seismic Refraction Studies on Kısıklı Landslide (Antalya, Turkey)

When natural slope is disturbed by human activity such as road construction and infrastructure, continuous landslide monitoring is important to prevent loss of material and life. Therefore, this study aims to determine the landslide material, the possible sliding surface and the influence of groundwater on the landslide occurrence. Low cost monitoring landslide is performed which is vertical electrical sounding (VES) and seismic refraction methods. Case study in Kisikli district, Antalya Province, Mediterranean Region of Turkey. VES survey was performed using Schlumberger electrode array at six locations. VES results interpretation leads to detect of maximum five geo-electrical layers. First, second and third layers represent saturated and permeable layer, while fourth and fifth layers correspond to an impermeable layer. Seismic refraction measurements were carried out on three profile layers. Low velocity and elastic parameters relatively correspond to the permeable materials in near surface with thickness about 4-5 m higher porosity. The integrated of VES and seismic survey allows mapping the weathered material at depth and providing depth information of the sliding surface which occurs at a depth between approximately 5 m and 20 m.

The investigation of soil–structure resonance of historical buildings using seismic refraction and ambient vibrations HVSR measurements: a case study from Trabzon in Turkey

In this study, two different historical structures built in Trabzon have been processed by ambient vibrations and seismic refraction measurements. One of the investigated historical structures is the Ataturk Pavilion built in the nineteenth century, and the other one is Hagia Sophia which was built in the thirteenth century. These two buildings are among the most important historical buildings in Trabzon and are very important for the tourism of the city. In order to determine peak/s frequency and amplitude from the horizontal-to-vertical spectral ratios (HVSRs), we have performed several measurements of ambient vibrations both inside (at different floors) and outside (on the ground) of structures. We have also conducted seismic prospecting to evaluate the vertical 1D and 2D profile of longitudinal and shear seismic waves, Vp and Vs, respectively. To this purpose, we have performed seismic refraction tomography and MASW. Ambient vibrations and seismic measurements were compared with each other. The results show that average predominant frequencies and HVSR amplitudes of inside and outside of Ataturk Pavilion are 4.0 Hz, 7.8 Hz and 2.6, 2.3, respectively. The Vp values vary from 300 to 2070 m/s, and the Vs for maximum effective depth is up to 790 m/s in Ataturk Pavilion. On the other hand, average predominant frequencies and HVSR amplitudes of inside and outside of Hagia Sophia and its tower are 4.7, 4.4 and 2.4 Hz and 1.6, 1.8 and 6.9, respectively. Vp values range from 450 to 2200 m/s, and Vs for maximum effective depth is also up to 1000 m/s in Hagia Sophia. The frequency values (F0 = Vs/4 h) calculated from the velocities up to the maximum effective depth for Ataturk Pavilion are in good agreement with the predominant frequency values determined from ambient vibrations. Ataturk Pavilion and Hagia Sophia soils have been classed according to Eurocode 8 by using VS30 values. The class was defined as “B.” Moreover, the bedrock in studied area is basalt. The high Vp and Vs values are also compatible with the lithology. The HVSR curves measured at the Hagia Sophia show the presence of clear peaks when compared to the Ataturk Pavilion. At the same time, there are marked velocity changes in the Vs sections calculated in both areas. As a result, in both areas there are significant impedance contrasts in the subsoil. However, this impedance contrast is more evident in Hagia Sophia. This could be also compatible with a lithological transition. The possible soil–structure interaction was investigated by using all the results and evaluated in terms of resonance risk. It is thought that the probability of resonance risk at Ataturk Pavilion is low according to the ambient vibrations measurements. However, resonance risk should be taken into consideration at Hagia Sophia site since the predominant frequency values are very close to each other. Finally, this site should be investigated in detail and necessary precautions should be taken against the risk of resonance.

Applications of Shallow Seismic Refraction Measurements in the Western Carpathians (Slovakia): Case Studies

Abstract Shallow seismic measurement, specifically seismic refraction tomography, is an effective geophysical method that has applications in various sectors. It enables the search for and determination of the course of the interfaces, thus helping to resolve geological, environmental, hydrogeological, engineering, geotechnical and other problems. The paper demonstrates the possibilities of using these methods through examples of shallow seismic measurements that have been performed at various four locations in the Western Carpathian Mountains. The first case study describes Monastery Pond at Katarínka. It was found that, the basement of the Monastery pond is at a depth of 2-3 m below the surface and the results were also confirmed by electrical resistivity tomography (ERT). The next measurement through the thermal power station waste storage showed that the storage area base runs at a depth of about 20 m under the measured profile. The third case study addresses the depth of groundwater depth in the area of Borská nízina. The measurement confirmed the assumed depth of ground water level at 3.35 m below the surface. In the last case study, border fault between the Turiec Basin and the Malá Fatra Mts. was mapped by application of shallow refraction methods. The results show that shallow seismic methods shed light on the problem and in combination with other geophysical methods are an effective tool with great potential. They provide very useful data for shallow mapping applications.

MASW Seismic Method in Brebu Landslide Area, Romania

This paper is focused on assessing the possibility of enhancing the geotechnical information in perimeters with landslides, especially through applications of the Multichannel Analysis of Surface Waves (MASW) method. The technology enables the determination of the phase velocities of Rayleigh waves and, recursively, the evaluation of shear wave velocities (Vs) related to depth. Finally, using longitudinal wave velocities (Vp), derived from the seismic refraction measurements, in situ dynamic elastic properties in a shallow section can be obtained. The investigation was carried out in the Brebu landslide (3-5 m depth of bedrock), located on the southern flank of the Slanic Syncline (110 km North of Bucharest) and included a drilling program and geotechnical laboratory observations. The seismic refraction records (seismic sources placed at the centre, ends and outside of the geophone spread) have been undertaken on two lines, 23 m and 46 m long respectively) approximately perpendicular to the downslope direction of the landslide and on different local morpho-structures. A Geode Geometrics seismograph was set for 1 ms sampling rate and pulse summations in real-time for five blows. Twenty-four vertical Geometrics SpaceTech geophones (14 Hz resonance frequency) were disposed at 1 m spacing. The seismic source was represented by the impact of an 8kg weight sledge hammer on a metal plate. Regarding seismic data processing, the distinctive feature is related to performing more detailed analyses of MASW records. The proposed procedure consists of the spread split in groups with fewer receivers and several interval-geophones superposed. 2D Fourier analysis, f-k (frequency-wave number) spectrum, for each of these groups assures the information continuity and, all the more, accuracy to pick out the amplitude maximums of the f-k spectra. Finally, combining both values VS (calculated from 2D spectral analyses of Rayleigh waves) and VP (obtained from seismic refraction records) plots of mean geodynamic parameter evolution related to depth were constructed. Parameter value differentiations referring to slope stability are revealed. Lowest values of VS and both shear and longitudinal elastic moduli are defined for the area with landslide rockmass, in opposition with stable land for which the biggest values for same parameters are revealed. Intermediate values are signalized above the main plane of sliding, zone classified unstable.

Determination of engineering properties of soil on railway track routes (An example of Turkey between the cities of Sivas and Erzincan)

Subsurface structure and engineering properties of the Sivas and Erzincan railway route were investigated by using velocities of seismic wave, electrical resistivity, standard penetration test (SPT) data and laboratory results, which were collected from survey sites referred to in this study. For this reason, 62 seismic refraction and vertical electrical resistivity (VES) measurements at 59 points were done along the survey route. Moreover, 11 mechanical boreholes with SPT were drilled. Laboratory tests were applied on soil samples taken from boreholes for geotechnical features. Longitudinal, shear wave velocities and elastic parameters were determined by seismic refraction method, and underground resistivity distribution was calculated by VES and geotechnical data and SPT results were evulated for the subsurface integrity. Engineering properties of a 6.8 km stretch of planned railway alignment in southeastern Sivas were calculated in this study. According to these results, unsuitable segments of the high-speed alignment which have low groundwater level and low bearing capasities which depend on dynamic properties were examined.

Study on the applicability of the microtremor HVSR method to support seismic microzonation in the town of Idrija (W Slovenia)

Abstract. The town of Idrija is located in an area with an increased seismic hazard in W Slovenia and is partly built on alluvial sediments or artificial mining and smelting deposits which can amplify seismic ground motion. There is a need to prepare a comprehensive seismic microzonation in the near future to support seismic hazard and risk assessment. To study the applicability of the microtremor horizontal-to-vertical spectral ratio (HVSR) method for this purpose, 70 free-field microtremor measurements were performed in a town area of 0.8 km2 with 50–200 m spacing between the points. The HVSR analysis has shown that it is possible to derive the sediments' resonance frequency at 48 points. With the remaining one third of the measurements, nearly flat HVSR curves were obtained, indicating a small or negligible impedance contrast with the seismological bedrock. The isofrequency (a range of 2.5–19.5 Hz) and the HVSR peak amplitude (a range of 3–6, with a few larger values) maps were prepared using the natural neighbor interpolation algorithm and compared with the geological map and the map of artificial deposits. Surprisingly no clear correlation was found between the distribution of resonance frequencies or peak amplitudes and the known extent of the supposed soft sediments or deposits. This can be explained by relatively well-compacted and rather stiff deposits and the complex geometry of sedimentary bodies. However, at several individual locations it was possible to correlate the shape and amplitude of the HVSR curve with the known geological structure and prominent site effects were established in different places. In given conditions (very limited free space and a high level of noise) it would be difficult to perform an active seismic refraction or MASW measurements to investigate the S-wave velocity profiles and the thickness of sediments in detail, which would be representative enough for microzonation purposes. The importance of the microtremor method is therefore even greater, because it enables a direct estimation of the resonance frequency without knowing the internal structure and physical properties of the shallow subsurface. The results of this study can be directly used in analyses of the possible occurrence of soil–structure resonance of individual buildings, including important cultural heritage mining and other structures protected by UNESCO. Another application of the derived free-field isofrequency map is to support soil classification according to the recent trends in building codes and to calibrate Vs profiles obtained from the microtremor array or geophysical measurements.

On corner frequencies, attenuation, and low‐frequency earthquakes

AbstractWe have recently suggested that the nearly constant duration of low‐frequency earthquakes (LFEs) (and, equivalently, the band limitation of tectonic tremor) manifests a moment‐duration scaling that is fundamentally different from regular earthquakes and is most easily explained as rupture on asperities of roughly constant dimension. In that work, we employed qualitative arguments against potential bias by attenuation. Here we examine the role of attenuation more quantitatively through an analysis that avoids specification of particular source (e.g., Brune) models and relies on the particle velocity spectral maximum as the definition of apparent corner frequency. The analysis leads to the formal definition of a saturation frequency as the limiting value of apparent corner frequency as the true corner frequency tends infinity. The saturation frequency, a formal equivalent to fmax, can be used to set bounds on path‐averaged quality factor Q. We apply these relations to deep crustal and intraslab earthquakes beneath Vancouver Island to estimate bulk crustal attenuation parameters that are subsequently used to correct apparent corner frequency measurements of LFEs reported in our earlier work. The attenuation bias due to bulk crustal structure is shown to be small, with negligible effect on the principal conclusions of that study. However, a review of laboratory and seismic refraction measurements of attenuation in oceanic basalts and evidence for high P‐to‐S LFE corner frequency ratios raises the possibility that strong, highly localized, near‐source attenuation accompanying high pore‐fluid pressures could cause the bandlimited nature of LFEs through the depletion of high frequencies.

Characterization of the barquisimeto subsoil –state of lara, venezuela-, from applying geophyisical and geothecnical methods

Between 1998 and 2002, ambient noise measurements, geotechnical evaluations, seismic refraction and gravity measurements were carried out in Barquisimeto –Lara State, in order to obtain information about soil conditions in the city. The predominant periods derived from H/V analyses vary between 0.3 and 1.2 s, with values ranging between 0.3 and 0.6 s in the east of the city. Further to the west, predominant periods increase to average values between 0.6 and 0.9 s, corresponding to a thickness increase of the sedimentary cover. The Bouguer gravity anomaly values vary between -30.6 y -13.4 mGal, being the minimum values associated to an increasing thickness of the sedimentary cover in the Barquisimeto terrace. The seismic velocities of the terrace sediments, derived from seismic refraction measurements, range between 900 and 1200 m/s for P-wave and between 400 and 600 m/s for S-wave velocities. For deeper layers the P-wave velocities vary between 2200 and 2500 m/s, and the S-wave velocities reach 1000 m/s. From the analysis of 32 geotechnical drillings, we have obtained an image of the thickness of the terrace in the western part of city. From the joint interpretation of ambient noise, seismic refraction measurements and geotechnical drillings, we infer that the thickness of the terrace is between 30 to 50 m in the center of the city, increasing in thickness to the west and decreasing to the east.

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

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

Compressive and Shear Wave Velocity Profiles using Seismic Refraction Technique

Seismic refraction measurement is one of the geophysics exploration techniques to determine soil profile. Meanwhile, the borehole technique is an established way to identify the changes of soil layer based on number of blows penetrating the soil. Both techniques are commonly adopted for subsurface investigation. The seismic refraction test is a non-destructive and relatively fast assessment compared to borehole technique. The soil velocities of compressive wave and shear wave derived from the seismic refraction measurements can be directly utilised to calculate soil parameters such as soil modulus and Poisson’s ratio. This study investigates the seismic refraction techniques to obtain compressive and shear wave velocity profile. Using the vertical and horizontal geophones as well as vertical and horizontal strike directions of the transient seismic source, the propagation of compressive wave and shear wave can be examined, respectively. The study was conducted at Sejagung Sri Medan. The seismic velocity profile was obtained at a depth of 20 m. The velocity of the shear wave is about half of the velocity of the compression wave. The soil profiles of compressive and shear wave velocities were verified using the borehole data and showed good agreement with the borehole data.

Rayleigh‐wave Resonance Analysis: a Methodological Test on a Viking Age Pit House

AbstractSeismic surface‐waves may show amplitude resonances at certain frequencies depending on the thickness and elastic parameters of near‐surface layers. We investigate if resonance frequencies of Rayleigh‐waves, (seismic surface‐waves polarized in the vertical plane) can be used to prospect archaeological remains of small‐scale buildings such as pit houses. Our test site is a newly detected Viking age village on the island of Föhr (north Germany) where we concentrated on one typical pit house. The results from resonance analysis are compared with magnetic data, ground penetrating radar (GPR) and classical seismic refraction measurements. The method of Rayleigh‐wave resonance mapping used in this paper is based on the idea that Rayleigh‐wave oscillations on top of anthropogenic structures will show different resonances than on undisturbed soil. We perform spectral analysis of these oscillations to provide information related to the seismic site response. We process single vertical component recordings and map the change in resonance frequency that can be related to the archaeological objects. The test showed that the pit house can be mapped by Rayleigh‐wave resonance analysis with a horizontal resolution of ~0.6 m. Corresponding computations of the depth of the pit house agree with the results from GPR, magnetic modelling and refraction seismics. A modelling study helped to understand the connection between subsoil shear‐wave velocity model and the signal generated by the pit house. The progress of seismic field measurement is slow compared to GPR and magnetometry. However, since seismic methods are based on elastic subsoil parameters, it can be applied in cases where magnetic contrasts are low or GPR fails because of high electromagnetic wave absorption. Copyright © 2015 John Wiley & Sons, Ltd.

Backarc basin oceanic crust and uppermost mantle seismic velocity structure of the Shikoku Basin, south of Japan

Seismic refraction and reflection measurements were made along three profiles across the Shikoku Basin, one of the three backarc basins on the Philippine Sea plate. The P-wave velocity models show the presence of a very large horizontal irregularity related to the seamounts on the extinct spreading center, transition zones from the Shikoku Basin to Kyushu-Palau Ridge, and from the Shikoku Basin to the Izu-Ogasawara Island arc. However, the other areas were found to have features similar to those of a normal oceanic crustal model composed of oceanic Layer 2 with a high velocity gradient and Layer 3 with a small velocity gradient. The Shikoku backarc basin oceanic crust was found to be characterized by a thinner igneous crust, especially in Layer 3, and the azimuthal anisotropy in the uppermost mantle expected from the seafloor spreading was not detected.

Crustal velocity structure of the Lucky Strike segment of the Mid‐Atlantic Ridge at 37°N from seismic refraction measurements

We estimate the seismic structure of the slow spreading Lucky Strike segment of the Mid‐Atlantic Ridge, located approximately 300 km south of the Azores platform, using seismic reflection and seismic refraction data acquired in June 2005 as a part of the Seismic Study for Monitoring of the Mid‐Atlantic Ridge (SISMOMAR) survey. The three‐dimensional velocity model shows an upper crustal low‐velocity anomaly running parallel to the ridge axis, which is limited by the median valley bounding faults. The velocity models also show a low‐velocity anomaly underlying the axial melt lens reflector located at the segment center below the Lucky Strike volcano. This lower crustal low‐velocity region can be explained by elevated temperatures and possibly small amounts of melt. The lower crustal low‐velocity anomaly and the axial melt lens reflector constrain the geometry of the magma chamber responsible for the construction of the Lucky Strike volcano. The presence of this magma chamber and thick crust at the segment center are consistent with a focused melt supply to the segment center.