Geo-Info Ltd. - Specialist Surveyors

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Resistivity Survey

Resistivity imaging is a widely used practice for mapping geological structure within the shallow subsurface. This method measures the electrical current dispersion within the ground, through a set of up to 72 steel rods (electrodes). Each electrode is put into the ground in a specific array to the project, through careful acquisition planning. Electrical current is injected into the ground via two electrodes and created voltage difference is read by two or more potential electrodes. A set of predefined, automated measurements will provide a two-dimensional distribution of the subsurface resistivity. The results of resistivity imaging can be related to the ground composition including the porosity, water saturation and concentration of dissolved minerals in the ground, which link to specific rock or soil types. Resistivity imaging yields the best results in clayey ground, whereas other techniques (eg. GPR) are less effective.

Geo-Info use the latest resistivity methods and equipment to provide our clients with detailed insight into the subsurface. The extent of the survey depends on the investigation target. Our most common 2m spaced test takes approximately two hours and reaches depths of between 15m – 20m depending on the ground composition. This non-invasive approach gives Geo-Info the advantage of providing continuous and accurate information about the subsurface composition, while keeping it cost effective in comparison with traditional invasive methods.

Deliverables

Acquired data is processed using specialist inversion software to produce a high-resolution 2D model of the subsurface apparent resistivity. Subsequently Geo-Info engineers can perform a detailed interpretation. To ensure maximum accuracy at interpretation stage, resistivity will be tied with any available bore-hole data.

Applications

Resistivity imaging is successfully applied to identify and characterize:

Limitations

local ground conditions:

space:

interference:


Seismic refraction

Seismic refraction is commonly used for the detailed study of the shallow subsurface (less than 30 meters). This method has a proven track record of working along roads and railways. It utilizes the refraction of seismic waves arriving and propagating along geological boundaries where the seismic velocities of the lower layer are greater than the velocity of the overlying layer. Near surface investigation benefits from this method as usually a loose soil overlies more compacted material or bedrock. A source and a geophone array are placed on the ground's surface (after a calculated acquisition plan depending on required depth). Once the source is activated, waves are recorded using a seismogram. Geo-Info can calculate the seismic velocities of the layers and identify depths of individual refracting interfaces.

Depending on the required depth of investigation, horizontal resolution and location site, the wave is generated by striking a metallic plate with a sledge hammer, dropping an accelerated weight, a vibrator battery powered source or explosive charge. The depth of penetration is approximately 1/5 of the geophone spread length. The basic useful components of a seismic record are the direct wave, the refracted wave and the critically refracted wave. From these, Geo-Info can acquire data on the velocity and density of the underlying strata. Further investigation can provide information on rock strength and rippability by using Poisons ratio and Young’s moduli.

Deliverables

Through the interpretation procedure, the first arrivals from the time records are accurately picked at each geophone location, which subsequently then produces a layered velocity model of the subsurface. To convert it into an actual geological model, individual refracting interfaces are correlated with borehole data and tied with real physical boundaries in the ground, such as the soil-bedrock interface and other lithological boundaries.

Applications

Seismic method using refraction technique may be used to identify and describe:

Limitations

The four main limitations are listed below:

  1. sledgehammer and plate can reach up to 20m in good local conditions,
  2. accelerated weight drop needs to be transported on a vehicle (unsuitable for limited access sites, eg. railways)
  3. explosives require additional permits, can’t be used in urban/protected areas,
  4. battery powered source (eg. ElViS) does not have sufficient output acquisition parameters for useful refraction and MASW studies

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