This will be difficult as the results from ERT models will be smooth. On top of that, you may have a mixing sone, where water is brackish and not necessarily completely clean. Once you have identified conductive targets that may be related to saline water, you'll need to verify with another non-electrical method or drill.
VES is quite suitable than the ERT. Ina situation where fresh water zones exist over the saline water bearing zones VES can accurately delineate the boundary between the zones. For delineating the lateral boundary between saline and fresh zones profiling is good. Depending on the ground condition sometimes IP survey may be necessary. As the range of resistivity for saline water bearing sand, fresh water bearing clay overlap VESalone may be inadequate.
I am explaining as to how to differentiate salt & fresh water with some case studies where one 1D Resistivity profiling followed by 1D VES & in another case 2D ERT have been conducted. By mere resistivity surveys it can not be confirmed & it odd to be correlated with EC & TDS parameters of preferably of shallow aquifers. These parameters can be obtained by field kits or digital pens. The resistivity of saline water is less than 0.3 ohm.meters. IP sounding will be much helpful in delineating the saline water aquifers.
Hi all,
Like Dale said whether you use ERT or VES (the latter is more limited if you have 2D or 3D lateral variations), you will obtain a smoothed transition, which may or may not reflect the actual seawater freshwater transition. You can improve this by addint some constraint but those would require logging. You can aslo choose a threshold but it would be somewhat arbitrary if not checked by borehole. You can use TDEm which is better at delineating conductive targets, in particular at depths. You can have a look at our paper on the topic. Hope this will help.
Article Imaging artificial salt water infiltration using electrical ...
Article Characterization of seawater intrusion using 2D electrical imaging
Hi everyone
It is really difficult to differentiate saline and fresh water zones demarcated using JUST electrical resistivity surveys, because the presence of salinitty can affect any meassure, so It would be really tricky to differenciate, for example, fresh water in clays from saline water in sediments or even wheathered or fracturated rocks. So, if you have different or unknown lythological components in your survey zone, your interpretation of the electrical response could be wrong. Here in Chile we have carried out many different surveys especially over Salares (Salt flats) mainly occurring in volcano-sedimentary basins and most of the times it has been necesary drilling and profiling boreholes for electric conducivity meassures. After that, the calibration of your superficial geophysical survey will match better. In your case with alluvial fans the clay pressence becomes a relevant variable to interpretate.
Adding to that in costal zones there variations on the interface related to tidal variations; is theres is also a delta zone or close to big rivers other variation could be present related to the season or river regimen (cases we have worked on the south and central-south Chile).
Cheers!
This of course depends on many variables, including whether you expect lateral variation or layered horizontal variation, as well as the heterogeneity of the background soils / rock. One dimensional inverted VES data will identify a horizontal boundary, relatively easily assuming homogenous background soils, as will ERT. But if as Dr. Rucker above indicates there is is a significant mixing zone the boundary will be smooth.
One useful tool in addition to the ERT or VES data (and acquired at the same time), is induced polarization. By acquiring induced polarization data with your ERT data you can use the information to help discriminate between areas of increased conductivity due to increased salt distribution and areas of increased conductivity as a result of the presence of saturated clays. This is especially useful as saturated clays are likely the primary naturally occurring subsurface material (outside of materials with high metallic content) that would exhibit resistivity values similar to seawater.
As Dr Rucker & Mr. Glaser have mentioned, it is not easy. Using 2D Resistivity + TDIP tomography is a better option since "normalized chargeability" can provide more information.
It is true that resistivity survey alone is not much useful in deciphering the saline / fresh zones. It is necessary to have a calibration of the VES data with the hydrogeological profile obtained from some direct evidence like drilling and well construction data. It is very difficult to deferentiate between the saline water sand and the clay zones with slightly inferior water. In Odisha state in India through such exercise it was determined that resistivity of
Resistivity values will reduce with respect to salinity. Thus there will be change in resistivity values from freshwater to saline water. This resistivity variation between freshwater-saline water will indicate the interface. Though VES results indicate the interface, Self Potential method is expected to give better results.
The following papers related to studies conducted in an Island is enclosed for your reference. Hope this will help you.
Article Geophysical investigation for groundwater exploration in Lak...
Article Study on Saline Water Intrusion into the Shallow Coastal Aqu...
Article “Geophysical Investigations for Groundwater Exploration in A...
VES: Vertical Electrical Resistivity (1D);
ERT: Electrical Resistivity Tomography (2D).
In general saline water shows lower resistivity value than fresh water!!!
It may be difficult to differentiate between saline sand and saline clay.
From the practice, I believe the most important thing is the contrast between the saline and the fresh water. So, if the contrast is not high enough, it could even existe mix zones very hard to differientate only with the responde detected en surface geoleectrical ground survey. Also, if the ground functions as a background signal, with saline precipitacions or othe high conductivity minerals, all efforts could be useless. If the depth of the zone to be surveyed it is not too much you could probably install drive point piezometers and make profiles of electric conductivity, so you willbe able to have a control of the zone and interzones of fresh and saline water. You also would be able to "look" tha variations on the shape of It.
Regards
VES is not good to detect the variation of salinity lateraly. ERT is better to detect this variation laterally
For detecting lateral variation in salinity better to go for resistivity profiling.
Saline water gives low resistivity value where as fresh water give high resistivity value!
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