Incorporating geotechnical and geophysical investigations for underground obstruction detection:A case study

Determining the location and boundary of underground obstructions and/or anomalies is a common problem and often a great chal-lenge for tunneling and underground construction.In this study,geotechnical investigations(penetration tests and borehole drilling/sam-pling)and geophysical investigations(surface wave method and cross-hole seismic method)were conducted with the aim of identifying the location and boundary of rock obstructions in Changi East,Singapore.The surface wave method is frequently used in the sites with lateral homogeneity in previous studies,but its application in the sites with rock obstructions is rare.The experimental results of this study indicate that the surface wave method is also able to determine the upper surface of rock obstructions,but difficult to identify the lateral and bottom boundaries of rock obstructions.To improve the precision of detection,the full waveform inversion(FWI)method was used to process the data from the cross-hole seismic survey.The results indicate that the inversion precision of P-wave is higher than that of S-wave.The horizontal and vertical ranges of rock obstructions in the P-wave inversion results are 14–26 m and 7.5–11.0 m respectively,roughly consistent with the results of penetration tests(about 15–25 m)and borehole log(8.85–10.80 m).This result proves that the sequential application of first-arrival time analysis and FWI can effectively delineate the boundary of rock obstruc-tions.Finally,the results of various detection methods were analyzed and compared in this study.Considering the advantages of various methods,we propose a cost-effective and high-precision workflow containing both geotechnical and geophysical investigations.

Metallogenic model of the Shuangjianzishan Ag-Pb-Zn district,Northeast China:Revealed from integrated geophysical investigation

The Shuangjianzishan deposit in Inner Mongolia is a typical Ag-Pb-Zn deposit of the southern Great Xing’an Range.Proven reserves of Ag,Pb,and Zn in this deposit have reached the scale of super-large deposits,with favorable metallogenic conditions,strong prospecting signs,and high metallogenic potential.This paper reports a study involving integrated geophysical methods,including controlled-source audio-frequency magnetotelluric,gravity,magnetic,and shallow-seismic-reflection methods,to determine the spatial distribution of ore-controlling structures and subsurface intrusive rock for a depth range of<2000 m in the Shuangjianzishan ore district.The objective of this study is to construct a metallogenic model of the ore district and provide a scientific basis for the exploration of similar deposits in the deep and surrounding regions.We used three-dimensional inversion for controlled-source audio-frequency magnetotelluric data based on the limited memory quasi-Newton algorithm,and three-dimensional physical-property inversion for the gravity and magnetic data to obtain information about the subsurface distribution of ore-controlling structures and intrusive rocks.Under seismic reflection results,regional geology,petrophysical properties,and borehole information,the geophysical investigation shows that the Dashizhai group,which contains the main ore-bearing strata in the ore district,is distributed within a depth range of<1239 m,and is thick in the Xinglongshan ore block and the eastern part of the Shuangjianzishan ore block.The mineralization is spatially associated with a fault system characterized by NE-,NW-,and N-trending faults.The magnetic and electrical models identify large,deep bodies of intrusive rock that are inferred to have been involved in mineralization,with local shallow emplacement of some intrusions.Combining the subsurface spatial distributions of ore-bearing strata,ore-controlling faults,and intrusive rock,we propose two different metallogenic models for the Shuangjianzishan ore district,which provide a scientific basis for further prospecting in the deep regions of the ore district and surrounding areas.

Geophysical Investigation of Groundwater Potentials of the Nanka Sand in Obosi and Its Environs, Anambra State Nigeria

Geophysical investigation for Groundwater which entails the use of vertical electrical sounding (VES) methods was carried out in parts of the Idemili area of Anambra State, Nigeria. Geologically the study area falls within the Nanka Sand, belonging to the Ameki Group in the Tertiary Niger Delta Basin. The investigation intends amongst others to assess the groundwater potential of the area, find out the existence of possible aquifer(s) within the area, and estimate the possible depth of a borehole. 15 VES stations were established at the site using Ohmega System Tetrameter and Schlumberger electrode configuration with current electrode spacing (AB/2) of 350 meters. The geologic parameters, such as the number of geo-electric layers, the thickness of the geo-electric units, and their resistivities were interpreted from the VES curves. A qualitative assessment of the curves shows that a model with 5 layers would sufficiently model the field curves. The thickness value ranged from 0.1 to 86 m, the resistivity ranged from 258 to 13,258 Ωm, and the depth ranged between 1.2 and 109 m. Quantitative interpretation of field curves involved inversion with appropriate parameters using Zond Res1D software.