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OEMG Digital Geophysics for Land & Freshwater


OEMG global utilises our Digital Geophysics System for land and freshwater. OEMG Digital Geophysics is a high resolution sub-bottom profiling system, modified from the proven principles and methods of land based geoelectrical survey. OEMG Digital Geophysics for Land offers the same operational and processing benefits as our marine systems. This includes enhanced acquisition and processing techniques that provide high resolution quantitative (i.e. depths and thicknesses) and qualitative (i.e. quality or resistivity) data about sub-bottom structural geology. It has brought resistivity to the forefront of land and freshwater sub-bottom investigations. This page explains why the OEMG Digital Geophysics is the superior choice for shallow sub-bottom surveys in most circumstances, over conventional acoustic systems and classical geo-electric techniques.

Key Benefits of the OEMG Digital Geophysics

If the land data is acquired on a suitable grid, it may be stitched together utilising proprietary krieging algorithms to create a 3-Dimensional map of the sub-bottom. This is an extremely useful tool for understanding the geological setting, and targeting geotechnical and/or environmental sampling.

Undertaking OEMG Digital Geophysics is one of the most important aspects of study for any major infrastructure project. It forms the base data for initial project approvals, design and cost estimates and allows clients to capture and own opportunity risk. Getting Geophysics right however is a difficult task and the risks of a poorly thought out or executed campaign are often underestimated. A detailed OEMG IDGM will provide the end user with a simplified actionable view of the geological setting that will add value to all phases of a project.


Operational Benefits

OEMG Digital Geophysics has many features that distinguish it from traditional acoustic methods, most notable are the ability to:

  • Accurately map thickness and density of calcarenite (caprock)
  • Accurately map hard or soft layers below a hard capping
  • Accurately distinguish between gravels, sands muds and clays
  • Accurately distinguish between hard and soft, weathered and un-weathered rock types
  • See no degradation of results due to the presence of gas in the sediment
  • Operate on land and in water depths from 0.1m to 180m

Key outcomes

OEMG Digital Geophysics results significantly improve the understanding of complex geology by providing:

  • Accurate Qualitative and Quantitative data
  • Three dimensional acquisition
  • Four dimensional modelling
  • Significant reduction in the number of required boreholes and increased confidence that all geological structures have been sampled
  • Enhanced understanding of the geological setting and an associated reduction in overall project risk.

 Principles of Operation

Both the acquisition and processing methods of OEMG Digital Geophysics have been modified from traditional land based resistivity methods. In traditional land-based resistivity surveys, an electrical current is injected into the sub-surface by means of two current electrodes or a Current Electrode Pair (CEP). The voltage gradient associated with the electric field of the injected current is measured by a Voltage Electrode Pair (VEP) placed between the CEP (Figure 3, left). Based then on the values of current and voltage measured between the VEP, the average or Calculated Resistivity (CR – measured as Ohmmeter (Ohmm)) for the volume of subsurface between the VEP is determined to the limit of penetration. Penetration is largely determined by the distance between the CEP. Therefore, multiple CEPs are placed at increasing distances about the VEP to cover a range of depths. The result is a field curve (Figure 3, right) that reflects the changing CR from each CEP. In this example, the CR for CEP 1 (Point 1) has returned a relatively low CR that rises quickly (points 2 to 3, etc). This is typical of a thin layer of soft sediments overlying rock. The above described field curves are then the basis for a qualitative assessment of the sub-surface geology.

Principles of Vertical Electrical Sounding – On land. Left: a typical land base setup for a resistivity survey. The VEPs are placed between the CEP. Current and voltage measured at the VEP is used to derive the Calculated Resistivity (CR). Right: a typical resistivity curve for a given injection of current as a function of CEP (X axis) and CR (Y axis).

The CR of a geological structure depends on its porosity, water saturation and the water resistivity. Gravel usually has a lower porosity than sand and its resistivity thus is higher. Clay with generally very high porosities shows very low resistivities. Conversely, solid rock has a low porosity resulting in very high resistivities. Every geological structure therefore has a unique resistivity.

Acquisition is then tailored to the requirements of the surveys, for example foundation surveys that require an understanding of the deeper geology, more electrodes are be placed for each shot. For a shallow survey of an aquifer or utilities or road alignment, less electrodes can be placed for each shot, meaning data can be acquired at a faster rate.

Opportunity Benefits

A considered deployment of OEMG Digital Geophysics technology as part of an IDGM offers clients many opportunities over traditional ground modelling methods:

  • Early identification and ownership of upside opportunity risk
  • The ability to target geotechnical and environmental studies to avoid the uncertainties of random or grid studies
  • The ability to tailor designs and construction techniques at or before tender
  • The ability to engage in modern collaborative tendering process that equitably shares risk for mutual benefit
  • The ability to create sustainable design

OEMG 4D Processing