You have to check out the geology , aquifer, and salt water body (it could be salt water intrusion or up conning of brine water ...)

Carbonates could be rich on hematite and Limonite (iron oxides). Such minerals are very poor in matrix but could be concentrated in fractures fills and bed surfaces (Hard ground) both could created current short cuts showing low resistivity. If this is true the resistivity changes if you change the direction of acquisition as fractures in natures are rarely isotropic.

Actually Riadh, this area falls in Indo-Gangetic Alluvium where the alluvium column is about 600 to 800 feet. In this situation, the possibility of fractures/ hematite presence seems very little............

OK, I see, have you then any idea about lithology and inclusions at that depth? glauconitic sands or pyritic...

Bhagwan, assuming that the matrix is what is more conductive and not the water at that depth, then there could be a layer or body of conductive mineralogy as Riadh has indicated. Based on your description of the lower resistivity occurring at 100 - 200 meters and that the alluvium is 600 - 800 feet thick wouldn't this change in conductivity indicate a transition from alluvium to paleo-bedrock? It seems like it might be a metallic deposit similar to roll-front uranium deposits.

Sorry can't really provide anything concrete.

Thank you very much, Riadh, Kevin and Marie, I hope to get some litholog data of the area or surroundings and will try to correlate. I would like to update you in course of time.........

1. Do you have the idea of the sub-surface aquifer disposition, then it becomes easier to address the problem.

2. When you say that the area have deeper fresh water - is it that the quality of the top aquifer system is saline? or you do not have any data for that part.

3. A lot of interpretation can be made for such low resisitivities -- have you tried changing the orientation of the electrodes?

4. Even after that they produce the same results, have you checked for any traces of conductive materials like electrolites / evaporites on the suface.

5. Sometimes even an underlain conductive clay can cause these - are their any instance of conductive clay or other over head surficial distrubance in the near vicinity

The Partially sorted Pebbles and gravels with clay rich cementing medium may be the aquifer material . If possible try to collect near by drilling sample and You can very well match with your resistivity values to understand the Geophysical Signatures..

Thanks Sakthi, I am in the process of collecting further information and drilling samples.

A lot of factors can contribute to this effect. To answer this research question, one must have a full knowledge of the local geology of the study area.I humbly wish to list the possible factors which I think you can cross check:

1. Are there surface inhomogenieties within a close range to the electrodes on the surface?

2. What material geological is the vadose zone and the sediments on the surface made of?

3. Are there any possible contamination from the surface- waste disposal etc.

4. Is the study area close to the coast; this is to rule out possible salt water intrusion.

5.Is there any formation in the area that has an associated ancient basinal brine that may be migrating upwards through localized fracture systems

6.lf you suspect 5 above, getting a fracture map of the study area and check if there is one close to your study area which could have served as conduit for saline water migration.

A lot of factors can contribute to this effect. To answer this research question, one must have a full knowledge of the local geology of the study area.I humbly wish to list the possible factors which I think you can cross check:

1. Are there surface inhomogenieties within a close range to the electrodes on the surface?

2. What material geological is the vadose zone and the sediments on the surface made of?

3. Are there any possible contamination from the surface- waste disposal etc.

4. Is the study area close to the coast; this is to rule out possible salt water intrusion.

5.Is there any formation in the area that has an associated ancient basinal brine that may be migrating upwards through localized fracture systems

6.lf you suspect 5 above, getting a fracture map of the study area and check if there is one close to your study area which could have served as conduit for saline water migration.