Can I combine resistivity and GPR for bedrock mapping?

Yes, resistivity and GPR make an excellent pairing and together will often provide a more complete visualization of the bedrock depth and/or topography across a wide range of geological environments. 

The obvious assumption is that you would use both investigation methods along the entirety of every profile; the GPR giving rapid detail of shallower bedrock and resistivity completing the picture in deeper areas (shallow layer information from GPR can even be used to improve the inversion of the resistivity data). Alternatively, it can be efficient to use them individually, choosing the most suitable method for the underlying geological conditions. For instance, resistivity is the most suitable choice in clayey areas or where the bedrock is found at greater depths, whereas GPR could be used on, for example, till and sandy ground. For this type of survey planning, it is good to obtain information on the terrain (from site visits or online mapping) and soil types (often available in large-scale maps from national geological survey agencies).  


Whilst in the planning stage, you should consider the following points regarding resistivity measurements: 

  • Investigations can be made to great depth (around 500 meters).  
  • Works in any geological conditions. 
  • Gives excellent results for bedrock levels and topography below conductive sediments. 
  • Gives additional information on fractures and fault zone identification. 
  • Field work would be carried out by at least two persons and approximately 800-1000 m profile / day can be investigated using a close electrode spacing to provide good detail.  
  • Requires good grounding of electrodes, which can be time consuming in urban or particularly arid areas. 
  • The results are often non-unique, the inversion process can provide multiple models that would be equally valid based on the data collected and the resistivity span of various geological materials overlap

For GPR measurements the following should be considered:  

  • Investigations are very fast, from walking pace up to highway speed. 
  • Measurements are carried out by one person.  
  • Gives high resolution results, where shallow bedrock as well as very undulating topography can be resolved.  
  • Investigations are limited in depth compared to resistivity. 
  • Interpretation can be hard if the bedrock is overlain by boulders or if the bedrock surface is fractured.  
  • Can not be used in all geological conditions, as conductive material affects the GPR energy.  

For both methods, measurements can be carried out in 1D, 2D and 3D. The measurements can normally be carried out even in rough terrain, without extensive clearing of for instance shrubbery. 

Both methods will deliver data in the form of depth to bedrock from ground surface, in simple text-files or dxf-files which can be imported into CAD or GIS software, where most of the further planning of road, railway, building, bridge projects are carried out.  

Note! You should always confirm your interpretation results of both GPR and resistivity investigations with some other method, such as soil-rock probing or weight soundings. 

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