6th Asia Pacific Meeting on Near Surface Geoscience and Engineering

In seismic modeling, an illumination map visually represents the distribution and strength of seismic waves in a subsurface region. This vital tool aids geophysicists and seismic interpreters in assessing the effectiveness of illuminating a specific subsurface target. Challenges in seismic acquisition design, such as obstacles rendering areas inaccessible, necessitate adjustments to the shot point. A case study illustrates the use of the undershooting method to overcome silting obstacles in the transition zone seismic survey. This involved relocating the source line 1.1 km from the receiver line, effectively navigating around impediments.

Following the source line adjustment, simulations are crucial to gauge the impact on data fold coverage at each horizon. The Illumination Map becomes instrumental in precisely depicting fold coverage for each horizon within the undershooting method. This visualization offers valuable insights into acquisition efficiency, ensuring comprehensive coverage and overcoming obstacles, exemplifying the significance of the undershooting method in seismic exploration.

The position of an underwater target, as determined by the acoustic positioning system and topographic correlation, shows a discrepancy because of bubbles in the water column. Topographic data were obtained from an autonomous underwater vehicle (AUV) in the shallow methane hydrate–bearing area off Joetsu, Japan. Discrepancy between the two locations increased when bubbles existed between the AUV and the mothership. The positioning error trend reflected the positional relationship between the AUV and the ship that varied east–west with time, suggesting that bubbles play a role in reducing the velocity of acoustic signals in the water mass. A locational relationship that does not interpose bubbles between the underwater target and the mothership is required to obtain reliable positioning information for acoustic measurement.

Over the past decades, it can be observed that number of slope failures occurred has been increased across the global. Countries with tropical climate like Malaysia has more prone to the natural disaster due to its humid environment across the year. It is relatively important to have proper mitigation measures to prevent this geological hazard. A preliminary study at several slopes in Terengganu has been carried out, in order to delineate the soil profile and identify the potential weak zones in the investigation area. Passive surface wave method, so called ambient noise tomography, was used in this paper to meet the technical direction of the study. Five linear survey arrays deployed with geophone-interval of 3m was carried out. Several critical weak zones with S-wave velocity lower than 200m/s were identified across the survey arrays, especially at higher degree of slope angle. The S-wave velocity profiles are consistent with geological layers in all survey arrays, where greater thickness of regolith overlain the fresh rock. This paper summarizes, the site of investigation, data acquisition, processing and investigation results.

Addressing climate change and achieving sustainable energy solutions demands a significant scale-up in renewable energy installations, such as large wind farms consisting of multiple wind turbines, while minimizing their environmental impact. However, constructing wind farms requires substantial resources, leading to significant costs, particularly for foundations. Optimizing resource use and cost management becomes crucial to ensure environmentally responsible practices.

To promote effective wind energy projects, geophysical solutions are being explored. Understanding near-surface and near-seafloor characteristics (in marine environments) aids in identifying optimal turbine locations, considering factors like geology and sediment composition. Established geophysical methods, such as surface/guided wave analysis and inversion, provide valuable insights into near-surface and near-seabed (an)elastic properties. These parameters are vital for assessing foundation performance and predicting soil behavior under dynamic conditions, ensuring the safety and efficiency of wind turbines installations.

We proposed a new procedure ‘zoning modeling and linear transition’ to invert dispersion curves of surface wave and micro motion signal for near-surface S- velocity model. A set of three-dimensional near surface field dataset collected in a research area of southwestern China is used to illustrate the feasibility of our procedure, the subsurface S- wave velocity in this area is obtained and valid in further static correction step.

New single-channel high-resolution seismic reflection and multibeam bathymetry data allow a detailed study of the stratigraphy and tectonics of the underexplored Singapore Strait. The data collected aboard the R/V Galaxea during a dedicated 9-day survey (March 2023) reveal the seabed relief, sub-bottom morphologies, Late Quaternary sediment distribution, and underlying bedrock structure. Data show significant structural and stratigraphic contrasts between the western and eastern regions of the Singapore Strait. The folded and faulted bedrocks commonly observed in the west are associated with the Middle Triassic Jurong and Upper Triassic Sentosa groups. In the eastern region, seismic data reveals a network of buried incised channels that record late Quaternary glacial and interglacial environmental and sea-level changes at the core of the Sunda Shelf. Our results uncover unknown chapters in the evolution of the Singapore Strait and, more specifically, the stratigraphic development of Sundaland.

Fluid distribution mapping in the fluid injection process of oil field development is a significant challenge in the oil and gas industry. Therefore, a cost-efficient and accurate monitoring method is required. This paper describes the use of one of the geophysical methods, the electrical method, to map fluid movement in a more efficient cost compared to other methods. The laboratory experiment was conducted using soil as the cap rock and quartz sand as the reservoir rock. The Self Potential (SP) and Electrical Resistivity Tomography (ERT) methods show the presence of water fluid in the pool tank. Meanwhile, the measurement results of the ERT method using the Multigradient configuration when there is water injection with a flow rate of 1 L/m and a north-south direction obtained a resistivity value of about 0–30 Ωm. In general, the water injection system equipped with monitoring capabilities for controlling the injection flow rate improves overall experimental precision. The result of this study can be extended into a field scale study to directly monitor the subsurface fluid distribution.

We performed 18 linear arrays using 6 recorders simultaneously with high-sensitivity seismometer along the survey line which crosses an active fault, Nagao fault in Kagawa Prefecture, Japan. The survey line was set as the same line with that the seismic reflection survey have been conducted. We computed H/V, single-station cross-correlation and phase velocity using cross-correlation by zero-crossing method. The contour map of H/V shows significant variation of peak frequency at the expected fault location. The single-station cross-correlation and phase velocity also showed a variation at the fault location. It suggests a potential for the use of microtremors for the exploration of fault location with low-cost than seismic survey method.

Simulations of the fault rupture within the fault system were calculated using the parameters of the 2019 M6.1 Zambales earthquake and were compared to actual strong motion data. A fault model of the East Zambales Fault, which traverses the eastern side of the Zambales Range, was used in the simulation. By using the results of microtremor array measurements at different locations, we limit the structure velocity model applied in our calculations. In order to compare the results of actual strong motion data to the simulations, the coordinates of nine strong motion stations around the vicinity were selected and were used as coordinates for the peak ground motions of the simulations. The methodology used assesses the long-period ground motion that originates from a finite fault source model consisting of multiple asperities embedded in a velocity structure. The comparison of the actual strong motion data to the simulated earthquake scenario of the M6.1 earthquake in Zambales shows that the arrival time and peak of their velocity and acceleration response spectra are comparable. The results can be used as input data to evaluate the effects of deep underground soil layers and to calculate the soil response at different sites in Zambales during earthquakes.

Electrical Resistivity Tomography (ERT) has become widely used for engineering and environmental applications in the last couple of decades due to (1) the simplification and automating of resistivity meters and (2) the new generation of inversion software. Although the initial domain of application remain relevant today, these techniques are increasingly used for deep investigation and shallow environmental/geotechnical applications. These new applications involve working with complex resistivity, which can be measured in the time domain (TDIP: Time Domain Induce Polarization) or frequency domain (SIP: Spectral Induce Polarization).

The new algorithm could process data in time and frequency domain. The first step is to estimate the no linear self-potential in order to remove it. Second step is to compute resistivity and chargeability in time or frequency domain. The results show that the improvement of the processing is weak for the resistivity and for no noisy data but we have a better estimation of chargeability. The correlation highlight a good correlation between calculated out-phasing (in frequency domain) and chargeability (in time domain).