Correlation of Seismic P-Wave Velocities with Engineering Parameters (N Value and Rock Quality) for Tropical Environmental Study

The physical parameters of the subsurface from the environmental site investigation are important for geoscientists and engineers to understand and very low cost-effective method, especially when combined with geophysical (seismic) and geotechnical (borehole) surveys. These parameters can be estimated from other obtained parameters. In this study, P-wave velocities of materials (soils and rocks) are studied both in the laboratory and field measurement. The obtained P-wave velocities are then compared with the engineering parameters such N values, rock quality, friction angle, relative density, velocity index, density and penetration strength from boreholes. The empirical correlations were also found in this study for selected parameters. The estimation of engineering parameters from P-wave seismic velocity values is applicable for tropical environmental study. It is found that, the ratio (VFIELD/VLAB) when squared, was numerically close to the value of percentage RQD. We found that the empirical correlation for tropical environmental study is VP = 23.605(N) - 160.43 and the regression found is 0.9315 (93.15%). Meanwhile, the empirical correlation between P-wave velocities and RQD values is found as VP = 21.951(RQD) + 0.1368 and the regression found is 0.8377 (83.77%). The correlation between apparent P-wave velocities with penetration strength for both study sites are found as and the regression coefficient is found as 0.9756. Thus, this study helps for the estimation and prediction the properties of the subsurface material (soils and rocks) especially in reducing the cost of investigation and increase the understanding of the Earth’s subsurface characterizations physical parameters.

High Resolution in Seismic Refraction Tomography for Environmental Study

Seismic refraction tomography (SRT) involves more complex mathematic algorithms to fit more flexible model. In the field procedure SRT in generally needs more shot points than standard seismic refraction survey to obtain high resolution profile. In this seismic refraction study, we have used 9 shot-points for inline and 10 shot-points for offset in purpose to obtaine high resolution of seismic refraction tomography. During a recent geophysical test site, the subsurface material was mapped along survey line using seismic refraction method. Analyses of the site investigation data revealed that the studied site was made up of two layers of the subsurface. The upper layer has velocity values with range of 500 m/s to1500 m/s which can be classified as unconsolidated surface deposits and mixtures of unsaturated sands and gravels. Meanwhile the lower layer has velocity values with range of2000 m/s to5500 m/s which is classified as compacted fine’s soil due to high pressure of the overburden. Analysis of seismic refraction data demonstrated that refraction tomography software systems are able to reveal subsurface material which represented by their seismic velocity value. Furthermore, the velocity model obtained in this study is agreed with its synthetic modelling result as initial model. This validity and reasonable results was able to assist in interpretation of the seismic refraction method for the environmental study.

Geoelectrical Survey for Engineering Investigation

Water seepage erosion has been and remains one of the major engineering problems. However, most engineers will much depend on borehole data and soil test for designing and problem detection. By considering of the cost and destructive method, selection of geoelectrical prospecting would be appropriate. Therefore, two electrical geophysical surveys were carried out in Sekolah Menengah Kebangsaan Dato’ Haji Mohd Nor, Gelugor, Pulau Pinang to map the presence of the unknown underground water sources (saturated zones) and its movement. With the total of seven resistivity lines parallel to each other and self potential (SP) with 5 m × 5 m gridding survey were successfully done. The resistivity result from line 1 (R1) to line 6 (R6) shows the subsurface consist of saturated zones with range between 3 m up to 10 m depth, before it reaches the R7 which is believed as the accumulation zone. Meanwhile, as for SP result it shows the water flow from higher value (north-east) towards the lower value which is mostly at the south area. In conclusion, geoelectrical survey could assist in detecting and solving engineering problems as it proven by the result from each method.

Electrical Resistivity Survey in Bukit Bunuh, Malaysia for Subsurface Structure of Meteorite Impact Study

An electrical resistivity tomography (ERT) study was conducted at Bukit Bunuh, Lenggong Perak (Malaysia). The study is to justify the features and environmental subsurface geological structure which is due to the meteorite impact. The ERT survey used resistivity equipments comprises of 4 survey lines with 5 m electrode intervals, covering an area of approximately 64 km2. The survey lines were carried out using ‘roll-along’ technique. The data were processed and analysed using RES2DINV, Excel and Surfer software in order to produce electrical resistivity tomography for qualitative interpretations. The results show the variation of resistivity value and faults. The 2-D resistivity results generally show the study area was divided into two main zones, alluvium with resistivity value of 10 - 800 Ωm, and bedrock with resistivity value of > 1500 Ωm and depth of 20 - 40 m. There are lots of fractured zones which are different than normal, identified along the survey lines. The South-North line shows the fractured zones were identified at 760 - 3800 m and 4700 - 5900 m. The West-East line, the fractured zone was identified at 1545 - 6570 m and North-West to South-East line, the fractured zone was identified at 740 - 5850 m. Meanwhile, South-West to North-East line, the fractured zone was identified at 720 - 1520 m. These interesting results were indicated at position of longitude;100.965 - 100.978 and latitude;5.056 - 5.066 where the bedrock depth is 40 - 60 m MSL and highly fractured. The area was surrounded by high elevated bedrock. Integration of 2-D resistivity results with boreholes is successful give valid and reliable results. The results of the study indicate that these geophysical approaches have a capability of retrieving the meteorite impact subsurface of the studied area.