Geological features such as lithological layers, faults, fractures, dykes and other geological structures are important for site characterization and understanding the nature of the subsurface for engineering and civil projects. The Electrical Resistivity Tomography (ERT) technique is highly suitable for geological site characterization as it is highly sensitive to lithological variations and the presence of water-bearing structures such as faults and fracture zones. It is also effective to map structures such as dykes and sills with fresh dolerite being a highly electrically resistive rock type.

Open Ground Resources owns three AGI Supersting R8 systems with 56 electrode capability, and cables up to 10m electrode separation to conduct ERT surveys with an effective penetration depth of up to 100 metres.

The seismic velocities of the near surface, especially average shear wave velocities up to 30m depth (Vs30) is an important parameter used to determine the response of a site in the event of an earthquake. Seismic velocities are also used to assist with the design of foundations and other underground structures and to obtain the seismic properties of the soil, bedrock or man-made material such as fill. These properties are essential to assist with the design, construction and maintenance of large engineering structures.

Open Ground Resources uses the MASW technique for shear wave velocity and Vs30 measurements and follows a 1D sounding or 2D approach to obtain accurate shear wave velocity measurements with depth. Various sources (sledgehammer, accelerated weight drop, excavator, ambient seismic energy) and surveying geometries (geophone spacing, shot offset spacing) are used to ensure that both high and low frequency surface wave energy is captured across a wide frequency range. MASW data is processed using SurfSeis v6 software.

Compressional wave velocities are measured using the Seismic Refraction technique utilizing 24-48 channel configurations utilizing sledgehammer or explosive sources.

Downhole and Cross-hole surveys are also used to obtain very accurate compressional and shear wave velocities to depths of up to 50 metres.

Geotechnical engineering investigations such for structures such as power stations, wind farms, tailings dams, and numerous other ground investigations require accurate knowledge of the engineering bedrock topography including possible weak zones in the bedrock profile as well as an indication of the weathered and fractured nature of the bedrock.

Geophysical techniques such as Seismic Refraction (SR), MASW and Electrical Resistivity Tomography (ERT) can provide a cost-effective investigation of the soil/bedrock profile to depths of tens of metres using non-invasive and environmentally friendly seismic and electrical energy sources.

Seismic velocity is reduced by the presence of fractures/joints and weathering of rocks and fractured/weathered bedrock is characterized by a lower seismic velocity compared to competent rock. Measurement of bedrock seismic velocity can therefore be used to obtain some qualitative indication of the degree of weathering or fracturing of the bedrock, especially when borehole information is available to assist in correlation of the seismic velocities with rock as interpreted from the borehole results.

The Seismic Refraction (SR) technique provides an accurate measurement of compressional (p-wave) seismic velocity of the various subsurface velocity layers and the bedrock topography as seismic waves travel slower in the soil/overburden layer and faster in the bedrock layer. Open Ground Resources utilizes a Geometrics 24-48 channel system with a weight accelerated ESS-100 source, 6.2kg sledgehammer and plate or explosives for imaging up to depths of 100 metres.

SeisImager, Rayfract, Geogiga Refractor and DW Tomo 2D software is used to process data using a tomography (smooth layering) or discreet layered models (GRM) depending on the requirements and the observed type of velocity model. The tomography models work well in areas with large lateral velocity changes, extreme topography and gradually weathering profiles. The GRM model provides accurate velocity models in areas with a layered/discreet velocity structure.

Multichannel Analysis of Surface Waves (MASW) can also be used to map depth to bedrock through the measurement of surface waves which can be used to infer the shear wave velocity profile of the subsurface. It provides high lateral resolution and good imaging of a weathered bedrock profile in saturated areas where p-wave velocities are typically ambiguous.

SR and MASW share similar data acquisition geometries and is often acquired simultaneously to produce compressional and shear wave velocities for site investigations.

Electrical Resistivity Tomography (ERT) surveys can also be performed to map a depth to bedrock profile although the 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. ERT is however useful to determine lateral variations in the bedrock profile and lithological variations. A combination of ERT and SR is often used to provide a comprehensive geophysical model of the subsurface.

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.

The presence of unknown subsurface voids poses a significant risk to infrastructure and human life when a sinkhole is formed, often with catastrophic consequences. These voids may vary from vary shallow cavities below concrete slabs to deeper more natural voids formed by sinkhole development in dolomite / limestone rock environments. Mining activities also may result in hazardous sinkholes which form due to shallow mining.

Open Ground Resources utilizes GPR, Gravity, ERT and the MASW techniques to map and delineate possible subsurface voids and/or low-density zones.

Electrical installations and substations for solar, wind and other similar installations require accurate values of the electrical resistivity of the subsurface for earthing design of the electrical system. The electrical resistivity of the soil depends on numerous factors such as the type of soil (sand, clays, etc.), moisture content, presence of minerals and salts, etc.

Open Ground Resources uses a Megger DET2/3 or AGI Supersting R8 system for soil electrical resistivity measurements. The 4-pin Wenner method is commonly employed to measure soil electrical resistivity to the required depth of interest with the ASTM G57 – 95a (2001) standard as a guideline. This method provides a 1D measurement at a specific location with electrical resistivity as a function of depth. Measured data is also inverted using EarthImager 1D inversion software to obtain an accurate electrical resistivity model of the subsurface.

Potential instabilities associated with the karst topography of dolomite and potential voids require a proactive approach in terms of development of structures in areas underlain by dolomite. Gravity is an extremely effective technique for characterization of the dolomite bedrock, and the presence of subsurface voids. ERT can also be used in conjunction with gravity to provide a complementary view of subsurface lithologies and potential voids.

Inorganic contamination of groundwater typically lowers the electrical resistivity of the groundwater and surrounding soils and the ERT method is used to map bedrock fractures and deeper groundwater contamination in the bedrock aquifer as well as shallow plumes in the weathered aquifer. The movement of pollution plumes from the source can be traced by a series of parallel traverses downstream of the source positions, and the optimum position for groundwater monitoring boreholes can be determined.

Maintenance and remediation of 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 the shallow road surface and to identify problems with the road pavement layers, presence and extent of voids, and water-saturated zones.

GPR can be used to map the thickness of concrete slabs as well as voids below these slabs as well as the presence and spacing of existing rebar. It is an ideal diagnostic tool to identify, analyse, and understand problems with roads, concrete slabs, tunnels and similar structures.

Geophysical methods are routine used to map the extent of old landfill sites and quarries which were used as landfill sites. The thickness of the waste material and the composition of the waste material can be mapped using ERT and/or Seismic Refraction. The gravity technique can also be employed to map the lateral extent of the landfill or waste material due to the relatively low-density of fill material.

EM and GPR can be used to map the presence of buried objects such as drums with GPR capable of mapping individual objects due to its high-resolution capabilities.