3rd Asia Pacific Meeting on Near Surface Geoscience & Engineering

In Bangkok many important infrastructures such as high-rise buildings, tollways, sky trains and bridges, MRT lines etc. are founded in Bangkok clays to a depth of 30 m or more so. Site investigation for construction projects is conventionally done using geotechnical laboratory and field testing, however, there is a trend for geotechnical engineers and researchers to become more interested in applying near-geophysical methods in site investigation of clay terrains due to its low cost, wide and fast coverage. In this study, it is the first time the passive seismic method (SPAC) was successfully conducted in integration with electric imaging surveys to provide with a detailed geotechnical-geophysical characterization of the shallow Bangkok subsoil. All types of Bangkok clay, i.e., weathered, soft, medium and stiff layers could be identified and matched well with layering by geotechnical properties.

This paper presents results of a landslide investigation undertaken at the old landslide site along a major highway in South East Queensland, Australia. The purpose of this investigation was to understand the extent and mechanism of slope failures and to gain geological and geotechnical information that will support the design of required long-term landslide remediation works.

Geoscience Australia has undertaken a number of regional drilling projects in collaboration with state and territory geological surveys. To reduce uncertainty associated with intersecting the targeted stratigraphy, we have actively used a range of geophysical methods to estimate cover thickness to identify prospective sites for drilling.

To solve the problem of large amount of survey data and low calculation efficiency of time-domain airborne electromagnetic three-dimensional forward modeling, we combine the compressed sensing theory with 3D modeling to develop a rapid and stable forward method. The synthetic test shows that we can take small computational cost to obtain the EM responses of all survey stations in the entire survey area with high accuracy. This saves the computational cost largely and thus is very promising.

The simulated annealing algorithm (SA) is a global optimization algorithm. It has the advantages of not relying on the initial model and being easy to jump out of the local optimum. It has many applications. However, excessively long runtimes have prevented it from being used in more complex and wide areas. Scholars have tried many acceleration methods to improve the practicality of sa algorithm. Based on research of Aarts and Korst (1986) , LEE (1996) propsed the asynchronous Multiple Markov chains Parallel SA (asynchronous MMC PSA) algorithm. Due to the high parallelism of this method, SA can be significantly speeded up. Asynchronous MMC PSA has been used to solve many optimization problems. However, no related studies on the inversion of asynchronous MMC PSA in electromagnetic data have been found. In this paper, the asynchronous MMC PSA algorithm is applied to invert a synthetic model, and we analyzes the detailed performance of the algorithm and the solution quality.

Three-dimensional resistivity and I.P. surveys are widely used to map mineral deposits in areas with complex geology. Field survey methods such as the offset pole-dipole and dipole-dipole arrays have been designed to map large areas efficiently. However, it has been found that negative apparent resistivity values are sometimes encountered in areas with large resistivity contrasts with large offsets between the current electrode and potential dipoles. One method to avoid this problem is to use the vector array configuration where two measurements are made of the electric field at each station with two potential dipoles at approximately right angles. The combined potential is independent of the orientation of the two potential dipoles with respect to the current electrode, and the apparent resistivity value is always positive. A common arrangement used is with the current and potential electrodes at alternating parallel lines. We examine the use of different offsets between the current and potential dipoles to improve the survey resolution and depth of investigation. A new inversion method using the amplitude of the combined potentials is also described, and results from a field survey are shown.

Rare earth elements (REE) are a group of 15 elements that range in atomic number from 57 (lanthanum) to 71 (lutetium) with addition of scandium and yttrium. The unique physical and chemical properties of these elements are used for high technology industry. The study area is in the Mesozoic formations (that situated in Singkawang Basin, West Borneo). Geochemical and geostatistical analysis are used in this paper. Kriging results of REE in Banan Formation shows high concentration in the southwest part of the basin and low concentration in northeast part of the basin suggesting that the REE are more enriched near the source of the sediment and in the terrestrial unit. Moreover, kriging results of REE and trace element in Pedawan Formation also shows high concentration in the southwest part of the basin and low concentration in northeast part of the basin suggesting that the REE are enriched near the source and in the terrestrial unit. In addition, REE immobility characteristic makes this approach is powerful and reliable. This study shows that REE is more enriched near the source of the sediment or toward terrestrial unit. This study can help the planning for deciding exploration target interval.

A brittle region subjected to the same hydraulic pressure will produce fractures more easily and frequently as compared to a ductile region. The analysis of shale brittleness, in principle, gives a better understanding of its ability to hydraulic fracking, an important technique in shale gas production. The brittleness index (BI) depends on the brittle mineral content, such as quartz, dolomite and calcite. In this research, the brittleness index was calculated using modified Wang and Gale’s (2009) method. This method is based on the ratio of brittle mineral content (quartz, feldspar, calcite and dolomite) to the bulk mineral composition of shale. The mineralogical results are based on data obtained through X-ray diffraction (XRD) technique. Based on brittleness index log modeling, the brittleness index of shale in the study area ranges from 0.3–0.85 representing fair to good quality shale for hydraulic fracturing.

A three-dimensional ambient noise tomography was applied to bedrock depth mapping at northern Singapore Island. Measurements were carried out over a survey footprint of approximately 700 x 400 m. Eighty cableless (nodal) seismograph units with vertical component 2 Hz geophones were deployed with a receiver spacing of 7 m. Total number of receiver locations are 3474. The GPS clock permits units to be synchronized over any distance without cables. Seismic ambient noise was recorded using a receiver spacing small enough to avoid spatial aliasing. A common midpoint spatial autocorrelation (CMP-SPAC) method was used to process ambient noise and 3D S-wave velocity (Vs) model to a depth of 60 m was obtained. We compared the resultant Vs model with 38 boring logs and summarized a relationship between Vs and depth at the top of the bedrock (GII). The Vs at the top of the bedrock linearly increased with depth and we defined the Vs of the bedrock as a function of depth. A depth to the bedrock ranges between 10 m and 60 m. The resultant Vs model and extracted bedrock depth is consistent with borehole logs. This paper summarizes, the site of investigation, data acquisition, processing and investigation results.

Shear wave velocity and site amplification study play vital role to reduce the effect of triggered and natural earthquakes disasters. Our study area comes in moderate to low seismic zone of India and low to moderate size earthquakes comes due to mine activity and complex geological settings. To reduce the effect of these calamities we have acquired microtremor data in a linear array of 2 Km profile length at the spacing of 250 m. HVSR curves which is computed by the Fourier transform of microtremor data, is used to estimate the shear wave velocity structure. The shear wave velocity is estimated by the inversion of HVSR curve using Monte-Carlo (MC) inversion technique. We have used guided Monte-Carlo inversion technique which is very stable. The best model of 1D shear wave velocity is interpolated into 2D shear wave velocity profile, which gives the three different layers. First layer having thickness 3–12 m with velocity range 400∼700 m/s. Second layer thickness is about 18 m having velocity range 700∼1150 m/s, this demonstrates the engineering bedrock. Third layer has velocity >1150 m/s. The 2D shear wave velocity and HVSR profile have quite good agreement in terms of impedance contrast. Peak HVSR amplitude range varying in the range 2.77–6.25 and predominant frequency having range of 4.61–10.4Hz.