4th Asia Pacific Meeting on Near Surface Geoscience & Engineering

The objective of this paper is to use high-density geophysical mapping in combination with borehole data and infiltration tests to produce a planning map showing the optimal locations for construction, SUDS and near-surface geothermal systems. The planning map will be conducted from the Nørrestrand area located north of Horsens, Denmark. The area is 150 Ha and has been mapped with DualEM-421S as well as detailed lithological soil descriptions from 25 boreholes and 25 infiltration tests.

Shale is formed rom clay particles. The classical Athy’s model on the exponential porosity reduction with the depth commonly uses based on localities only. Shale compaction curve affects the results of the basin modeling as it is important input data, thus the standard compaction curve of shales needs to be established. This study provided a newest Thailand shales data to propose the new empirical relationship among porosity, burial depth, and geological age. The numerical data of the geological time were estimated by the application from the relationship of velocity and depth in each geological ages. The variation of the Thailand shale data due to the different geological ages was found in this study. Finally, the standard curve and the three-dimensional (3D) model of shale compaction proposed by previous work was examined and confirmed by this research findings.

When building bottom-mounted offshore wind turbine in earthquake-prone Japan, it is important to estimate the depth of the engineering bedrock (S-wave velocity (Vs)≧400m/s) for seismic design. We have developed an offshore microtremor array measurements that can easily determine Vs profiles below seafloor. The measuring system consists of Ocean Bottom Seismometer Sensors (OBSS), a passive seismograph Atom, a buoy equipped with Atom, a RTK-GNSS system and an acoustic source OWS for specifying the position of the OBSS. In our system, four OBSS are installed on the seafloor at the apex and center of an equilateral triangle (base length is about 50m to 200m) and synchronization between each OBSS signal are performed by Atom's GNSS clock mounted on the buoy. Due to the small size and light weight of our measuring system, the above data acquisition can be performed by four workers using a fishing vessel of about 5 tons. We have applied it to many offshore wind farm site investigations. Comparison of each Vs profile at the site where array measurements and PS logging were performed were very consistent.

Using the example of the Novosibirsk hydroelectric power station, we demonstrate the possibility of using the elastic standing wave method to identify voids under the slopes of the dam embankment slabs and the levee hydraulic structures. The accumulation of amplitude spectra of acoustic noise recordings allows us to determine the frequencies of the first few modes of standing compression-tension waves generated by noise in concrete embankment slab. A twofold frequency increase of the lowest mode of standing waves is an indicator of the appearance of voids under the plates lying on a more rigid base. Areal noise measurements on the surface of the slabs make it possible to outline the voids formed under them.

Large 3-D geoelectrical surveys are widely used to map geological hazards and mineral deposits in areas with complex geology. The offset pole-dipole and dipole-dipole field setups are commonly used to map large areas rapidly. In order to increase the depth of investigation and reduce survey costs, attempts have been made to use larger offsets between the current electrode and potential electrodes. However, this sometimes result in negative apparent resistivity values, particularly in areas with large resistivity contrasts. The vector array configuration avoids this problem by making two measurements of the electric field at each potential station with the potential dipoles at approximately right angles. The measured potentials from the two dipoles are then combined together into amplitude and direction values. Previous work had used only the amplitude component. A new inversion scheme using both the amplitude and direction components is described. We show the improvements in the model resolution using a field data set from the French Pyrenees to map possible landslide zones.

The finite-element method used to calculate the potentials for 3-D resistivity models is usually the most time-consuming component in many inversion programs. Two approaches have been used to construct the mesh used by the finite-element method. One approach uses a rectangular mesh with hexahedral cells that is simple to implement. However, it uses an unnecessarily fine mesh in parts of the model that are far away from the electrodes where the potential changes slowly. A second approach uses an unstructured mesh with tetrahedral mesh cells that are created automatically in an iterative manner. Finer tetrahedral cells are used near the electrodes, and a coarser mesh is used in the more remote regions. This generates a mesh with a much smaller number of nodes. The disadvantage is the time taken by the automatic meshing routine and that an irregular model grid is used. This paper describes the use of a semi-structured mesh with a rectangular distribution of the model cells with a finer mesh near the surface and a coarser mesh at deeper regions using a combination of hexahedral and tetrahedral mesh cells. For large 3-D models, this method reduces the calculation time by 3 to 4 times.

This paper presents the results of a site inspection, vibration monitoring and multi-channel analysis of surface waves (MASW) undertaken at the Sentinel for Department of Transport and Main Roads (TMR). The purpose of this study was to assess the influence of traffic induced vibration on the stability of the Sentinel and to gain geological and geotechnical information that will contribute to the design of required long-term remediation works.

Over the past decades, it can be observed that several development of seismic methods utilizing the ambient noise is made. It is relatively important to have adequate information of the subsurface profile in geotechnical construction activities. A passive surface wave method, ambient noise tomography was used to delineate the geological profile for geotechnical engineering purpose at northern Singapore. Two linear array configuration measurement lines were deployed in geophone-interval spaced 5m, and the measurement was carried out at night time to avoid heavy traffic. Common midpoint spatial autocorrelation (CMP-SPAC) method was used to process ambient noise acquired. With the calibration to several borehole logs nearby, the subsurface stratigraphy along the proposed tunnel alignment was mapped and delineated. The Backfill and Kallang Formation modeled at the lowest S-wave velocity (Vs) ranges from 160m/s to 200m/s, residual soil and Bukit Timah Granite (GIV) ranges from 200m/s to 360m/s while Bukit Timah Granite (GIII) with Vs > 360m/s. The Vs-to-geological layer is almost similar in both measurement lines. This paper summarizes, the site of investigation, data acquisition, processing and investigation results.

In geothermal systems, heat is generated inside the Earth and reaches the surface; with the main parameters are thermal conductivity, as well as heat, and transport properties. A thorough understanding of the heat and mass of fluids in the natural state of the earth system is required. Here codes of 'Simulators for HEAT and MAss Transport' (SHEMAT) were used to simulate the flow of water and the distribution of heat. The hot spring system at Khlong Thom, Krabi Province, Southern Thailand, was chosen. First, a steady state model was simulated with parameters varied in a reasonable range. In the next step, water discharge from wells was simulated. When either the discharge rate or well depth was increased the temperature of the hot spring was reduced, and with an increase in both, discharge rate and depth of the well, the exit temperature of the hot spring area will drop significantly. That means that any development of the hot spring system that involves further discharge from wells has to be carefully planned and monitored. This needs to be understood by the relevant government agencies and stakeholders involved and affected by the development of the Khlong Thom hot spring.

No summary.