In offshore wind power projects, there are many cases where offshore CPT, which is easy to temporarily install and enable quick investigation, is carried out using Self Elevating Platform (SEP) as a method for measuring ground constants. However, in many cases, it is not possible to continue pushing to the required depth with CPT in the seafloor ground near Japan, and it is common to dig to a predetermined depth with a high-speed excavation method such as percussion boring and repeat CPT.

　In addition, in the offshore geotechnical investigation, it is important to understand the engineering base layer (Vs≧400 m/s). Generally, the acquisition of the S wave velocity is performed by a suspension PS logging (hereinafter referred to as susPS logging) using drilled boreholes, but problems such as being affected by the excavation diameter or the surface condition of the borehole wall have also been pointed out. This article shows the results of a new PS logging method using vibration during standard penetration test (SPT) in offshore boring and vibration during percussion drilling used in offshore CPT. This PS logging was named the submarine reverse PS logging because it is the reverse of the downhole PS logging performed on land (source position and receiving position are reversed). As a result of performing the offshore reverse PS logging in the same hole as the susPS logging or in the nearest hole and comparing the S wave velocities, it was found that the same results as the susPS logging can be obtained in the seafloor reverse PS logging. This method does not require a measurement sonde to be inserted into the borehole, and is effective for boreholes with collapse or large borehole diameters that are difficult to measure with susPS logging (usually the applicable diameter for susPS logging is approximately 200 mm or less).

　In this paper, firstly I introduce a definition of submarine landslides and the general topographical and basic geological features of submarine landslide morphology. Secondly I demonstrate case examples of submarine landslides on volcanic islands and coastal sediments, and discuss the occurrence age and mechanical balance as a problem of slope stability. The number of occurrences and scale of landslides in evaluating the scale determined from the topography are shown. Thirdly I show the occurrence and mechanism of submarine landslides in terms of slip surfaces and pore water pressure. Finally, we discuss the geological risks and problems of submarine landslides in the offshore wind power development project, which is one of the coastal development projects in recent years.

　Designing a 3D seismic survey is an important factor to minimize the risk of survey interruption in shallow water. In this paper, we will discuss the survey design of large seismic vessel "Tansa" for the National Program for Oil and Gas Prospecting as an example, that was introduced to Japan for investigating relatively un-explored offshore areas with seafloor deeper than 50 m. We will explain issues with shallow water survey that Tansa has, along with the background of the survey specification, equipment, and the limitation and challenges of the survey water depth. In order to minimize risk in shallow water survey, we emphasize three items for the pre-survey consideration;

　1) obtaining topographic maps or data to look for obstacles in the survey area, such as large construction not visible from sea surface. Collecting information of fishing gears that are installed on the seafloor is also necessary.

　2) Defining survey specification, such as cable length, is not only important for illuminating the target structure but also for risk analysis since seismic vessels are difficult to halt its operation when towing massive equipment.

　3) Setting the number of sources perpendicular to the shooting direction, and the number of cables and its interval has a direct link to horizontal resolution when designing an efficient data acquisition in limited survey period.

　In order to plan and carry out a 3D survey in shallow water using a large vessel, it is important to plan the survey specifications in consideration of the safety of the survey and the risk of survey interruption.

　In recent years, demand for submarine cables for both communication and electric power has been increasing. Before submarine cable installation work is carried out, desk-top studies and marine surveys are conducted. During the marine surveys, such detailed information as bathymetry, sub-bottom geology and obstructions is gathered and analyzed along the planned route. As the data is interpreted and mapped, the submarine cable route is rapidly optimized on-board. In particular, in coastal zones starting from the cable landing point to maximum water depths of around 1,500 m, the ability to bury cables up to 1 – 2 m deep below the seabed should be assessed in order to prevent cable damage due to such activities as trawling and anchoring. For these purposes, acoustic geophysical exploration techniques are mainly employed, and magnetic and electrical geophysical exploration techniques are also partially used.

　As for acoustic survey methods, MBES (Multi-Beam Echo-Sounder), SSS (Side-Scan Sonar) and SBP (Sub-Bottom Profiler) are employed. They are used to respectively display detailed seafloor topography; visualize the distribution of seafloor sediments and obstructions; and examine surface sedimentary layer thicknesses. MAG (Marine Magnetometer) is used to identify such anomalous objects along the route as unexploded ordnance and pre-existing cables. The electrical survey method is rarely employed to infer soil types of the sub-surface layer and/or to obtain a design parameter for cathodic protection.

　Geophysical survey results are interpreted for the purpose of cable burial assessment together with ground truthing as revealed by geotechnical surveys, e.g., by soil core sampling and/or mini-CPTs (Cone Penetration Test). However, mini-CPTs are not yet popular in Japan and soil samples recovered by core samplers are often disturbed. Therefore, the development and spread of geophysical survey techniques which can obtain such geophysical properties as the shear-wave velocity of the sub-bottom sediments may lead to much more reliable cable burial assessment surveys.

　In recent Japanese offshore wind farm projects, both investigations and construction work have often been conducted based on European technical specifications which strictly require high resolution instrumentation and the verification of measurement accuracy. This trend will likely spread and become the standard in the coming years. Taking this trend into account, geophysical survey techniques should also be applied to investigations related to sub-bottom sediments.

　As the law for promoting offshore wind power was implemented in April 2019, it is expanding the submarine ground survey market for the construction of offshore wind power generation. In general ground surveys, suspension PS velocity logging using boring holes has been used to measure the S-wave velocity required to grasp the distribution of the support layer of the ground or engineering foundation depth at sea. However, the offshore wind power generation (implantation type) targets sea areas with an average annual wind speed of 7 m/s or more and a water depth of about 10 to 40 m, and boring in harsh environments is not easy. Therefore, we have developed “Offshore Microtremor Array measurements” for the purpose of grasping S-wave velocity structure in short time without the need for boring survey. In this paper, after showing an overview of the development of this method, we then show the application examples and characteristics of offshore microtremor obtained from offshore microtremor array measurements conducted at various sites. The S-wave velocity structure estimated by microtremor array measurements is consistent with PS-velocity logging and multi-channel seismic reflection profiles, indicating the applicability of microtremor array measurements to offshore ground survey.

　3D high-resolution seismic (3D-HRS) survey was conducted to clearly reveal and map faults and fractures in a shallow-water region of the Beppu Bay, Japan, where is a pull-apart-basin developed with right-lateral strike-slip fault activities. 3D-HRS was conducted using a dense array of 6-short streamers combined with a single GI-gun (150 cu.in.). The system configuration can provide excellent illumination of geological features, even in shallow waters which is hard to image accurately with conventional methods. This system realizes high cost-efficient acquisition, which is light in weight, easy to deploy and retrieve cables from a small vessel. It is also capable of acquiring high resolution data. In the data processing, pre-stack noise attenuation, de-ghost, de-multiple, and acquisition footprints removal played an essential role to enhance seismic imaging quality. Compared to the existing 2D seismic section, post-stack time-migrated 3D-HRS seismic volume achieved much higher resolution. Furthermore, the 3D-HRS volume allowed to delineate highly detailed features of seafloor and subsurface. Following the seismic processing sequence, similarity and thinned fault likelihood (TFL) attribute workflows were applied to detect and visualize faults and fractures within the 3D-HRS volume. The results revealed a network of broadly distributed faults and fractures along an active fault system in the Beppu Bay.

　To confirm the effectiveness of the DC resistivity method using our developed DC resistivity survey system with a vertical electrode array loaded on a remotely operated vehicle, we carried out a data acquisition dive in the Hakurei field of the Izena Hole, the mid-Okinawa Trough, and estimated the resistivity structure using the data. Two distinct low resistivity zones were detected on the northern area of the survey line, which coincide well with known negative self-potential and vertical electric field anomalies. These low resistivity zones are interpreted as the occurrence of sulfides. By contrast, the southern area is imaged as an almost uniform structure. A relatively high resistivity zone was found between the two low resistivity zones, which corresponds to sedimentary layers of pumice identified in drilling cores. The data includes information on the sub-seafloor resistivity structure as well as the self-potential as a vertical electric field that can be interpreted without complicated analysis. The small footprint of the developed system near the seafloor is capable of surveys areas with rough bathymetry. To conclude, this instrument is a powerful tool for surveying seafloor hydrothermal deposits.

　Recently, blank zones in seismic survey have been expanding in shallow waters, such as the inner Tokyo Bay, due to the strict restrictions on the use of airguns for concerns about their potential impact on the marine ecosystem. As one of the techniques for breakthrough of the restrictions, we have used an underwater speaker as an environment-friendly seismic source, which only emits small sounds less than those that threaten fishes. As a result of seismic surveys using the underwater speaker since 2017, shallow gas layers have been detected 7-8 m below the seafloor in the inner Tokyo Bay, showing broad distribution in the western side of the bay and spatially limited distribution in the eastern side. According to composition analysis of the gases sampled from the seafloor, the gases contained 96.8% of methane. Considering the high flammability and the greenhouse effect of methane, disaster risks and global-warming effects can never be ignored. Further investigations on the gas layers in the inner Tokyo Bay must be important research to mitigate the potential risks in the metropolitan area.

　Acoustic mapping was conducted at different frequencies using different observation platforms where shallow methane hydrate was present. The results show similarities in topographic features but differences in the backscatter strength distribution patterns. In the case of topographic features, the difference in depth should have resulted from the difference in the frequencies used in acoustic mapping. However, the backscatter strength, which is a value based on the amplitude of the acoustic signals, may differ greatly depending on the frequency of the acoustic signals if there is a significant material boundary within the shallow subseafloor and if a low frequency can penetrate through the thin sediment covering the seafloor. In this case, it is possible to recognize the material boundaries present at the shallow subseafloor, which are usually difficult to observe visually because surface sediments cover the seafloor. Considering the subseafloor environment at the shallow methane hydrate-bearing areas, this may indicate either the presence of methane hydrate itself or the distribution of materials such as methane-derived authigenic carbonate rocks at the shallow subseafloor, which suggests a field of gas seepage.

　There are still many unknowns regarding the effects of environmental changes caused by ocean development and climate change on the marine environment, especially on the organisms and environments in sub-seafloor sediments, because methods for efficient monitoring in sediments have not yet been established. In this study, we aim to establish a technological basis for spatio-temporal measurement and assessment of benthic biota and environmental dynamics in sediments, and are developing new acoustic measurement systems for visualization of these sub-seafloor environments. In this paper, we introduce two acoustic measurement systems that have been developed in Japan for the purpose of detecting buried objects in the shallow sub-seafloor with high resolution: a 3D acoustic coring system (installed type, using sound waves with a center frequency of 100 kHz) and an acoustic mole (crawler type, using sound waves with a center frequency of 500 kHz). The results of field experiments using these systems, such as a survey of lotus root underwater in Izunuma, Miyagi Prefecture, and a buried object exploration test at the Shimoda Coastal Experimental Center of the University of Tsukuba, Shizuoka Prefecture, will also be introduced.

　Understanding and comprehension of vertical resolution of reflection seismic profiles are important for the interpretation of underground structures. When considering vertical resolution, we need to take account of separability which indicates the ability of the separation of two wavelets, and of visibility which comprehends thickness of the thin layer from amplitude. For the separability, the one-quarter wavelength law is applicable, and two wavelets are independently recognized. Visibility depends on the signal-to-noise (S/N) ratio, and the thickness of one thirties of wavelength can be detected in the outstanding S/N case. The theory of the separability can be formulated using Ricker or sinc wavelets, but the basic wavelet of the actual seismic section is not always simple. So, we have to manually interpret predominant frequency from the seismic section. Predominant frequency varies depending on the depth of the target. Regarding the source, smaller the size of source, higher the predominant frequency, but penetration of the energy degrades. Selection of the optimum source size for each target is essential. During the seismic interpretation, understanding of the geological setting is also important, which determines the difference of seismic anomaly and actual layer extents, for example. In the case of thin layer, we have to consider the apparent alteration of the wavelet shape and AVO effects. And when integrating core, log and seismic data, we need to consider the adjustment of the vertical resolution of them. When we interpret more complex actual seismic sections, consideration of specific underground condition and parameters of data acquisition/ processing is necessary in addition to the resolution basics discussed in this paper.

　Recently, a method of estimating the phase velocity of Love waves using array-derived rotation (ADR) of microtremors has been proposed. In this study, we applied this method to the horizontal component of microtremor array records observed in the Wakayama plain. The rotational motions were calculated using the improved method supporting a non-equilateral triangular array. Phase velocities were estimated from the rotational motions using a spatial autocorrelation (SPAC) method. The obtained phase velocities at the most array showed normal dispersion. The obtained phase velocities were close to those derived by using the f-k method in the overlapped frequencies. Compared to the phase velocities by the f-k method, the SPAC method of the ADR could estimate the phase velocities in the lower frequency. At the IMF array, theoretical phase velocities calculated from the velocity structure model based on the PS-logging and the 3D Wakayama velocity structure model agreed with the observed phase velocities. Power spectra of the rotation and divergence of the observed records indicated that the power of the Love wave was larger than those of the Rayleigh waves at the Wakayama arrays.

　To evaluate the effects of changes in the physical soil properties on seismic motions of a surface layer due to freezing, in this study, we attempted to extract the features of seasonal variation related to ground motion characteristics by data analysis of weight-drop measurements and borehole seismic observations in the near-surface, in Kitami, Hokkaido, Japan in a cold region. The phase velocities of the fundamental mode Rayleigh waves estimated by the weight-drop measurements were the fastest during the frozen period from the beginning of Dec. 2020 to the end of Mar. 2021, which was revealed from the 0 °C line (freezing line) in the soil layers. The phase velocities of the fundamental mode Rayleigh waves have the maximum coefficient of variation at a frequency of about 50 Hz. The surface-to-borehole spectral ratio of earthquake motions for horizontal components shows an attenuation during the frozen period and an amplification during the other periods. The attenuation of surface-to-borehole spectral ratio of earthquake motions may be caused by the small and/or reversed S-wave velocity contrasts between the surface layers and the basement due to the freezing. Furthermore, the horizontal-to-vertical spectral ratios of microtremors were small of less than 0.5 during the frozen period. The decrease of the horizontal-to-vertical spectral ratios of microtremors may be caused by the change of Poisson’s ratio of the surface layers due to the freezing as well as the reasons above. The attenuation of surface-to-borehole spectral ratio of earthquake motions and the decrease of the horizontal-to-vertical spectral ratios of microtremors during the frozen period were clearly demonstrated with the certain values. Finally, we quantitatively evaluated the seasonal variation of the phase velocity of Rayleigh waves, the surface-to-borehole spectral ratios of earthquake motions, and the horizontal-to-vertical spectral ratio of microtremors by frequent weight-drop measurements and continuous seismic observations.

　Subsurface resistivity structure is useful for discussion about distributions and movements of underground fluids; however, the effect by fluid viscosity on the resistivity of strata has not been sufficiently discussed. In this study, we infiltrated highly-viscous mud fluid into sediment samples in the laboratory, then investigated the relationship between mud fluid saturation and the resistivity of sample.

　River-bed sand, mixed with gravel and fine sand, was used in this study. As a result, the mud fluid samples indicate resistive feature at low saturated condition, higher than the samples filled by tap water, and have the larger saturation exponent. Based on qualitative discussion, the behavior of non-Newtonian fluid (Bingham fluid), which restricts the capillarity of mud fluid, seems to be the major cause. It is suggested that pre-survey investigations of characteristics of fluid and the relationship between saturation and resistivity through laboratory experiments are required, when high-viscosity fluid is the survey target.

　We studied an applicability of the electrical survey to model shallow subsurface properties used in evaluating site amplification characteristics in the freezing season. The electrical surveys were carried out repeatedly for 1 year in Kitami city, Hokkaido, to evaluate the effects of freezing soil on the electrical property of soil. Apparent resistivity values affected by the shallow subsurface increased as the freezing depth became deeper, and apparent resistivity curves consequently show the typical shapes at individual stages in the freezing process. The resistivity and the thickness of the surface layer also increased as the freezing depth became deeper. A good agreement of the seasonal variations in the electrical properties with the observed soil conditions, such as soil temperature and water moisture content, shows that the resistivity can be useful information to evaluate the influence of the freezing in constructing a soil model for the ground motion calculation. Furthermore, the depth to the lower-resistivity layer which is deeper than the freezing depth varied similarly with groundwater level. It indicates that the electrical survey is also useful to model a P wave velocity and a density of near-surface for an accurate estimation of an earthquake ground motion.

　In this study, to understand the seasonal variation of the result of surface wave and microtremor array explorations in areas where freezing occurs in the surface soil during the winter, we conducted both explorations throughout the year at the same site at Kitami Institute of Technology, located in Kitami City, Hokkaido, a cold region in northern Japan. The explorations were conducted at intervals of about 1 to 2 months from October 2020, when the ground is not frozen, through the severe winter season to October 2021 of the following year. First, the S-wave velocity structure was estimated using both phase velocities in October, the non-freezing period. The estimated structure shows a soft layer of less than 100 m/s from the ground surface to 1 m. The phase velocity of Rayleigh waves during the freezing period was higher than that during the non-freezing period in the frequency range of 10 to 50 Hz. The difference was especially clear above 15 Hz. The results of the small-radius microtremor array explorations conducted at the same time also showed that the phase velocity changed at over 10 Hz. On the other hand, there is almost no seasonal change below 10 Hz, and their standard error increases during the freezing period. The phase velocities from the microtremor array explorations can be connected with the results of the surface wave ones. Seasonal variations in phase velocities for each frequency corresponded with other observations.

　S-wave shallow seismic reflection survey and two-dimensional electrical resistivity survey targeting depths less than 100 m were applied to the strike-slip fault. This study was conducted in the Go-mura fault zone and the Yamada fault zone where surface rupture occurred during the 1927 Kita Tango earthquake. For the interpretation of the two-dimensional electrical resistivity survey results, we carried out in situ measurement of the electrical resistivity on outcrops around the survey area. It was confirmed that there is 10 times contrast in the electrical resistivity between the fresh or weathering granite and the fracturing part or clay part of granite. At the study area in the Go-mura fault zone, discontinuity and deformation of the reflection surface from the S-wave shallow seismic reflection profile corresponds to the location and displacement sense of the active fault estimated by boring and trench survey. As the result of applying two-dimensional electrical resistivity survey, the low resistivity zone continuing from deep underground to shallow was recognized near the active fault trace on the ground. This low resistivity zone indicates the fracture zone of host rock with fluid supply from fault activity. At the study area in the Yamada fault zone, the similar survey results as in the study area of the Go-mura fault zone were obtained. Discontinuity and deformation of the reflection surface from the S-wave shallow seismic reflection profile corresponds to the location and displacement sense of the active fault. In addition, as the result of two-dimensional electrical resistivity survey, the low resistivity zone continuing from deep underground to shallow was recognized near the active fault trace on the ground. The low resistivity zone was confirmed to be continuous down to the depth of 300 m or deeper by comparing with CSAMT survey result. Two-dimensional electrical resistivity survey was carried out with multiple measurement lines at another study area where the stepping of the active fault trace on the ground has been inferred from the comparison of the strike of the Yamada fault zone. As a result, the low resistivity zone was distributed almost linearly underground, and it could be shown that the distribution of active faults might be different between the surface and the underground.

　3D microtremor array measurement by dozens of receivers forming spatially unaliased arrays has been applied to obtain 3D S-wave velocity model. The receivers are placed on regular grid points in a survey area to keep wide coverage in the spacing and the azimuth of the receiver pairs. In town and residential area, however, the grid array geometry with small receiver spacing is difficult to achieve because access to personal properties is usually prohibited. Thus, we adopted several types of array geometries realized by only accessible roads such as linear, L-shape, T-shape, Cross-shape, and their combinations with small receiver spacing. Then, we combined the small arrays with grid arrays with large receiver spacing. The combined array was used for the data analysis as irregular array geometry. A field measurement by the irregular array geometry was conducted at 520 m by 640 m in Fukashiba district, Kamisu, Ibaraki, where houses and roads were severely damaged by the liquefaction that occurred at Tohoku Earthquake in 2011. The receiver spacing of the small arrays was 5m, while that of the large array was 40 m. The number of the receivers for each array measurement was 45 to 50. The 3D S-wave velocity model was estimated using the common midpoint spatial autocorrelation (CMP-SPAC) analysis method from 57 data measured by both array geometries. By comparing the dispersion curves calculated from the small, large, and the combined array, it was found that neither the small array nor the large array provides a wide enough frequency range for obtaining the S-wave velocity profile of shallow through deep subsurface. The small array was insufficient to calculate the accurate phase velocity in the lower frequency range, while the large array did not provide the phase velocity in the higher frequency range. On the other hand, the combination of both arrays provided a wide frequency range by supplementing the lack of the frequency by the small and the large arrays. As a result, a shallow to deep subsurface S-wave velocity profile was obtained from the combined array. The slice of the estimated 3D S-wave velocity model shows good agreement with the distribution of road damages and past excavated areas known by existing literatures. Therefore, we concluded that the irregular array geometry introduced in this study would be a valuable option for obtaining 3D S-wave velocity model when setting many receivers with grid array is difficult in town and residential area. The estimated S-wave velocity model can be used to create a geotechnical model for evaluating the liquefaction, and it eventually contributes to creating a hazard mitigation map in the future.

　In the magnetotelluric (MT) method, so as to identify the subsurface resistivity structure, the apparent resistivity and phase profiles are calculated by transforming time-series data into spectral data. The continuous wavelet transform (CWT) is well known as a new method of time-frequency analysis instead of the short-time Fourier transform. The CWT is superior in processing non-stationary wideband signals like the MT signal by adjusting the size of the wavelet according to the value of frequency. However, the calculation settings of the CWT, such as the type of basis function and the wavelet parameter, are often determined empirically because of the arbitrariness of the shape of the wavelet. Although there might be differences between the calculated MT responses and the true responses due to improper settings of the CWT, there are no detailed studies considering the effect of numerical errors derived from spectral transforms on MT data. In this study, focusing on the frequency band between 0.001 Hz and 1 Hz, we examined the optimum calculation settings of the CWT in processing MT data in terms of suppressing the numerical errors caused by the spectral transform of time-series data. First, we investigated the effect of change in the resolutions of time and frequency on the apparent resistivity and phase profiles by altering the shape of the wavelet. Next, we examined the type of basis function and the range of the wavelet parameter which would provide high-quality MT responses with high continuity that may accurately reflect the subsurface information. Through these experiments, this study proposed the complex Morlet function including a sine wave component with its wavelet parameter k set to 6 ≤ k < 10 as the optimum calculation settings of the CWT. We also show the validity of the proposed calculation settings by applying the CWT to MT survey data of different types. Superiority of the CWT with proposed settings is suggested especially when the signal-to-noise ratio of observed data is low. Consequently, the proposed calculation settings were confirmed to strike a balance between the resolutions of the time and frequency domains well and will therefore be effective in obtaining reliable MT responses.

　To establish an effective method for seismic exploration in investigating subsurface structures and physical properties beneath potentially active submarine volcanos in coastal area, we conducted a feasibility study using synthetic seismic data by numerical simulation. In this study, we assumed a 30-km-long seismic survey line across a shore line with a geological model of 10 km depth including virtual structures of past volcanic activities and present activity potential of a shallow submarine volcano. Because the virtual models contained only three low velocity bodies simulating magma chambers at multiple depths and no horizontally continuous reflectors in the crust, estimating velocities was difficult by general reflection-based velocity analyses. First, we generated synthetic data of first-arrival traveltime and waveform that simulated a seismic survey by numerical modeling of traveltime and elastic wave propagation respectively. Using the synthetic data, we estimated velocities by applying first-arrival traveltime tomography and full waveform inversion sequentially. Then, we applied reverse time migration for reflection imaging of subsurface structures using the inverted velocity models. We also tested the effects on the imaging results from different source-and-receiver layouts and shapes of the low velocity bodies. In our results, the general velocity structures to the bottom with two velocity bodies down to approximately 6 km were well inverted by the sequential inversion, and the geometry of the three low velocity bodies were clearly imaged by the reflection imaging in the depth domain. Although improvements with higher resolution and deeper targets should be necessary on the velocity estimation in further studies, our results suggest that the combination of velocity estimation by full waveform inversion and reflection imaging by reverse time migration is effective in complex geology where general reflection-based velocity analysis is challenging. The results also indicate the importance of pre-survey feasibility study using numerical simulation with the presumed models for designing seismic surveys, including optimal source-and-receiver layouts and effective data analysis methods.