Scholte Wave Dispersion Modeling and Subsequent Application in Seabed Shear-Wave Velocity Profile Inversion

Recent studies have illustrated that the Multichannel Analysis of Surface Waves (MASW) method is an effective geoacoustic parameter inversion tool. This particular tool employs the dispersion property of broadband Scholte-type surface wave signals, which propagate along the interface between the sea water and seafloor. It is of critical importance to establish the theoretical Scholte wave dispersion curve computation model. In this typical study, the stiffness matrix method is introduced to compute the phase speed of the Scholte wave in a layered ocean environment with an elastic bottom. By computing the phase velocity in environments with a typical complexly varying seabed, it is observed that the coupling phenomenon occurs among Scholte waves corresponding to the fundamental mode and the first higher-order mode for the model with a low shear-velocity layer. Afterwards, few differences are highlighted, which should be taken into consideration while applying the MASW method in the seabed. Finally, based on the ingeniously developed nonlinear Bayesian inversion theory, the seafloor shear wave velocity profile in the southern Yellow Sea of China is inverted by employing multi-order Scholte wave dispersion curves. These inversion results illustrate that the shear wave speed is below 700 m/s in the upper layers of bottom sediments. Due to the alternation of argillaceous layers and sandy layers in the experimental area, there are several low-shear-wave-velocity layers in the inversion profile.

The assessment of local site effects and dynamic behaviour in Nicosia, Cyprus

Single-station microtremor measurements were conducted to investigate earthquake and soil behaviour for the first time in Nicosia, Cyprus. Cyprus is located in a tectonically complex area in the Eastern Mediterranean where three plates meet. The study area was chosen to cover the areas to be opened for new development. Nicosia, the capital of Cyprus, is also the island's most important cultural, industrial, commercial, and transportation centre. The study creates base maps for the soil to assess earthquake resistance crucial for construction. Microtremor Method was applied at 100 stations and the Multi-Channel Analysis of Surface Waves (MASW) method was used at 52 stations. Also, RefractionMicrotremor (Re-Mi) and L-Shaped Spatial Autocorrelation (L-SPAC) methods were carried out at 17 stations to substantiate the research. The results of the microtremor method indicate that the predominant soil period values have an average of 1 second and pre-dominant peak period values are generally found between 0.1 to 5 s at the study area. Peak amplitude values are observed between 1 and 2.4. The Vulnerability Index Parameter (Kg) exceeded 20 at the central and the southern stations, and Kg values change between 7 and 54 units. The Kg values were found to be higher than 20 in soils where shear wave velocity is lower than 760 m/s. At the same time, the values of the predominant peak period were greater than 1 second. Cyprus is located in the Alpine Himalayan earthquake zone. The Cyprus Arc is known as the main seismic source of the island, It constitutes the tectonic border among African and Eurasian lithospheric plates in the region. During an earthquake in Nicosia, seismic waves will be amplified by an average of 1.5 times and soil deformation will occur due to the exceeding elastic limits. The results provided important insight into soil behaviour and indicated its reactions in a potential earthquake.

Mapping the liquefaction potential index (LPI) in Ratu Agung subdistrict, Bengkulu City, Indonesia using the shear wave velocity approach

On the 4th of June, 2000 and 12th of September, 2007, Ratu Agung Sub-district, Indonesia experienced significant damage due to liquefaction after the earthquakes. Therefore, this study aims to determine the Liquefaction Potential Index in the area. Data of shear wave velocity (Vs) was collected using the Multichannel Analysis of Surface Wave (MASW) method. The measurement location was set up on a grid of 32 observations points with field investigations. Furthermore, Simplified Procedure and LPI methods were used to measure the soil liquefaction potential and vulnerability level. The results showed that the value of shear wave velocity in the Ratu Agung Subdistrict ranged from 102 m/s to 442 m/s. Also, the liquefaction vulnerability levels varied from high to very high categories due to the maximum soil acceleration and conditions dominated by loose sand, as well as the influence of different geological formations in the zone. In conclusion, an empirical equation was successfully proposed to analyze the liquefaction vulnerability.

The Optimum Field Configuration for Active MASW Survey on Peat Soil

The application of the MASW method on engineering investigation required optimization of the field configuration to ensure high quality dispersion image for reasonable shear wave velocity profile estimation. The limited investigation with respect to peat soil condition has motivated the study to determine the optimum field configuration for peat soil. The challenging characteristics of peat soil including high void ratio, compressibility, water content and low shear strength further complicates the determination of optimum field configurations. The study focused on the determination of optimum field configurations for active MASW method which includes the receiver spacing, source offset, sensor frequency and sampling interval. The results obtained shows that, the optimum receiver spacing to obtain high signal to noise ratio dispersion image was 1 meter. Smaller receiver spacing causes domination of higher modes and wide bandwidth, while longer receiver spacing causes significant drop in signal to noise ratio governed by rapid energy dissipation with distance. For the source offset, the distance of half the total spread length (X1 = L/2) provides the best resolution and minimised near-field and far-field effect. While, 4.5 Hz sensor frequency and sampling interval between 100 to 250 s provides sufficient low frequencies for deeper depth investigation and denser data. Overall, the influence of receiver spacing, source offset and sensor frequency on the dispersion image resolution was significant.

Struktur Bawah Permukaan pada Lokasi Rencana Pembangunan Rumah Sakit Muhammadiyah Kota Bengkulu dengan Metode seismik MASW Secara 2 Dimensi

A study on the subsurface structure has been conducted using the multichannel analysis of surface wave (MASW) method near Tugu Hiu, Bengkulu City, the location where Muhammadiyah hospital will be built. We aimed to determine the subsurface structure in 2-Dimension (2D) to classify soil type on the location. The data were measured with seismograph PASI 16S24-P using 24 geophones for a track of 72 m of total length and 1.5 m of intervals. The seismic data were processed by using seismic ParkSeis software. The results show that shear wave velocity (Vs) is 800-1200 m/s at a depth of 1-5 m, indicating this layer is filled by rock type (unsaturated). At a depth of 6-8 m below surface, Vs value decreases to 400-500 m/s which may indicate that the soft rock type fills the layer. The results of this study provide a high level of confidence, about 80-100%, in the Vs value. Thus, we suggest for the foundation of hospital construction, the pillars for construction must be for more than 10 m in depth with a solid concrete foundation. This to ensure the foundation can withstand the building in case of shake during an earthquake.

Field Investigation on the Effects of Base Plate Material on Experimental Surface Wave Data with MASW

Source characteristics significantly affect the signals generated during seismic testing. Proper source selection plays a major role in data quality and can potentially improve investigation outcomes. This is particularly true for surface wave testing where the goal is to establish the frequency variation of phase velocity. Little research has compared the input energy caused by different base plates when impact sources are used. Consequently, data were collected using the Multichannel Analysis of Surface Waves (MASW) method with three base plate configurations (aluminum, aluminum over a rubber mat, and polyethylene) and two different sledgehammers. The variations in signal frequency content and amplitude spectra, energy, and dispersion images were systematically analyzed. There were appreciable differences in the energy introduced by different base plate configurations. Both the aluminum/rubber and the polyethylene base plates led to power spectra increases relative to the aluminum base plate. Subsequently, the aluminum/rubber and polyethylene base plates typically transferred more energy into the subsurface. This was not necessarily the case when the base plates were used on soft surficial soils. The variations in predominant frequency content were also less substantial, though the aluminum/rubber and polyethylene base plates developed slightly lower-frequency content at the expense of higher-frequency components in the dispersion curves. Despite the noticeable differences in energy transfer and frequency content, the base plate materials did not appreciable alter interpretation of the dispersion behavior at the sites given the uncertainty present in the dispersion images. This highlights that the selection of MASW base plate materials can be correspondingly driven by practical considerations such as noise, portability, and durability. Consequently, base plate materials with viscoelastic characteristics are a promising alternative to conventional metallic plates for coupling impact sources in surface wave testing, though care should be exercised when employing them at sites with soft ground conditions.