Shear Wave Velocity Measurements

The measurement of shear wave velocities is important for the design of large structures where the calculation of the site stiffness (elastic moduli) needs to be calculated. Global Geophysical utilizes the MASW (Multi-channel Analysis of Surface Waves) technique to determine site shear wave velocities.

Bedrock Topography And Weathering

The foundation design for large structures and dam sites require accurate knowledge of bedrock depth and the quality of the bedrock. The Seismic Refraction technique provides an accurate measurement of compressional (p-wave) seismic velocity and bedrock topography. Global Geophysical utilizes a Geometrics 24 channel system with a hammer and plate source for imaging up to depths of 50 metres below surface on average (note that penetration is a function of the ambient noise and site conditions). SeisOpt2D software is used to determine the actual seismic velocities of the subsurface based on measured arrival times, this method does not require any pre-selection of different layers, and works well in areas of large lateral velocity changes, and gradual depth of weathering.

Resistivity surveys can also be performed to map the general weathering profile being faster and less expensive than Seismic Refraction. However, Seismic Refraction allows for a more accurate definition of the bedrock depth compared to resistivity which suffers from a large range of bedrock resistivities making it difficult to accurately determine the bedrock depth. Resistivity is however useful to determine lateral variations in the bedrock topography and general stratigraphy which can be quantified using borehole drilling at selected resistivity variations. Global Geophysical recommend that large areas be initially surveyed with resistivity, with Seismic Refraction traverses in selected areas based on the resistivity results. This allow for a more cost-effective application of geophysics.

Gravity can also be used to map general variations in bedrock topography but is poor at resolving actual depths. It is very useful to determine relative bedrock variations, especially when the bedrock is dolomite, and to determine areas of very deep and shallow weathering which is of interest for geotechnical and groundwater applications.

Dolomite Gravimetric Surveys

Potential instabilities associated with the karst topography of dolomite and potential voids require a proactive approach in terms of development of housing and other structures. Gravity is an extremely effective technique for mapping of the dolomite bedrock, and the presence of possible voids which are invisible from surface. Global Geophysical uses a CG-5 gravimeter with a Trimble RTK GNSS R8 system to provide very accurate gravimetric measurements which are typically performed on a grid of between 5 and 30 metres. Smaller grids are required to image smaller targets, while larger grids can be used to map larger variations of the target. Typical surveys include gravity surveys for housing developments, golf estates, tailings dam geotechnical investigations, mapping of voids associated with undermining, etc.

Pipes, Services, Void And Buried Objects

The mapping of pipes and services are routine conducted in investigations requiring accurate layout of existing services. The actual position of old services are usually not known due to lost information and contractors carry a high risk of cutting or trenching into water and sewer mains, electrical cables, fibre-optic cabling and other services. GPR is a high-resolution technique capable of mapping these features and Open Ground Resources uses the GSSI SIR-3000 cart system with a 400 MHz antenna to map services. Real-time site information can be marked on site, or can be surveyed using sub-cm accurate GPS system in complex areas, and the actual positions of services can be put on a site plan in electronic format. Subsurface voids can be mapped using various techniques such as resistivity, gravity and GPR, with GPR being a very quick and effective method for mapping of voids in build-up areas and roads. Buried objects such as fuel tanks, drums, and metal debris can be mapped using a combination of GPR, Magnetics and/or EM-31.

Resistivity For Cathodic Protection

Resistivity surveys are commonly performed at new steel pipeline locations for cathodic protection for which the subsurface resistivity must be known. Resistivity of the subsurface determines the rate of corrosion of an object and a proper cathodic system needs to be designed to minimise corrosion. Design of power station and substation earthing systems also require that the soil resistivity must be accurately determined. Global Geophysical uses the AGI Supersting system to map soil resistivity with EarthImager2D being used to calculate truth soil resistivity by inversion of the apparent soil resistivity data.

Roads, Pavement And Concrete Investigations

Maintenance and Remediation on roads and pavements require an assessment and understanding of the nature and extent of the problem before designing a comprehensive and effective maintenance programme. GPR is a rapid and accurate technique for mapping of the shallow road surface and to identify problems with the road layers, presence and extent of voids, and zones of water-saturated areas. The proactive identification of these areas can focus a remediation programme which is designed for the problematic areas only. GPR can be used to map the thickness of concrete slabs as well as voids below these slabs.

Roads, Sewer And Water Pipeline, Manhole, Contour And Borehole Surveys

Open Ground Resources uses a Trimble RTK GNSS R8 system and Total Station to accurate map features such as roads and pavements, existing pipelines and manholes, fences, surface structures and buildings, general contour measurements, boreholes (existing and new positions), etc. Benchmarks can also be created according to the needs of the CLIENT which are referenced and tied to national trig beacons.