Comparison of Geological Profiles from Borehole Data Based 3D Geological Model and Geophysical Testing

Abstract

ABSTRACT Three-dimensional (3D) geological modeling has been used for design and construction works to overcome the complex nature of underground geological conditions. The accuracy and reliability of geological information reflected by the 3D model are critical for engineers to make sound judgement. The 3D model is usually established using borehole data. The data gaps between boreholes affect the accuracy of the 3D geological model. Instead of getting additional borehole data, geophysical survey is a promisingly cost-effective method to fill the data gaps and improve the assessment of the underground uncertainties. In this study, a case of using the seismic surface wave methods to determine the engineering rockhead and thus fill the data gap in the 3D geological model is presented. In relation to a subway project in Singapore, the combination of geophysical methods and 3D geological model for rockhead detection at this site was explored. Seismic geophysical survey was carried out to map the rockhead which was calibrated and verified using borehole data. This improves the accuracy of the rockhead obtained from 3D geological model. Through this study, a procedure of using geophysical methods to obtain additional data to verify and update the existing 3D geological model has been established. INTRODUCTION Three-dimensional (3D) geological modeling has been widely used for underground design and construction works to overcome the complex nature of underground geological conditions (Dong et al., 2015; Pan et al., 2018; Pan et al., 2020; He et al., 2020; Zhu et al., 2021). The accuracy and reliability of the geological information indicated by the 3D model are critical for engineering practitioners to comprehend and visualize the subsurface conditions and make a sound judgement (Wu et al., 2022; Qi et al., 2021). It is considerably significant and valuable in practice to locate the top surface of moderately weathered rock (Weathering Grade III or better) in the 3D geological map since the geo-materials and mechanical behaviors start to vary apparently from this interface. The engineering rockhead is conventionally identified via the traditional geotechnical investigation which involves drilling boreholes and the subsequent interpretation of the borehole logs and coring samples. The accuracy of the interpreted rockhead or geological profile largely depends on the borehole quantity, location and spacing, as well as the interpretation of geologist. When the boreholes are sparse, the uncertainties become large and thus additional boreholes are required to reduce them. Unfortunately, getting more borehole data is expensive. It is also challenging to carry out drilling works within restricted areas such as urbanized cities or inaccessible terrains such as natural reserve or lakes.