Hi Carmen,

Do you want to track changes in water quality, or only to find the depth of the water table ? Could you precise also the type of soil expected and the level of detail that you want ?

Best regards

Julien Gance

Dear Carmen,

Few of my comments for your question:

1) Theoretically, water saturation and electric resistivity are quite well correlated because the electric flow mechanism in soil media is predominantly electrolytic. Therefore, a mini electric imaging array in this case might help. But it requires a professional resistivimeter (geophysical) equipment.

2) Practically, it is not easy to have a successful application of near-surface geophysics (or engineering and environmental geophysics as many people say) because of the small soil mass to be investigated and not-so-clear contrast in physical properties between target and the surrounding ones. Therefore, in your case, you need to know at least: i) roughly the depth to water table (say, In Bangkok soft clay, this varies from 0.5 to 1.5 m below the ground surface seasonally); ii) you should be able to take soil samples and measure resistivity in the laboratory (you can also measure conductivity and convert it to the resistivity). Once these two types of parameters are known you can design the measurements. To be frank, your depth of target is too shallow, thus the electrode spacing are are small and have to be very carefully designed.. In our case in Bangkok plain, we could detect well the water table at the depths mentioned above using electric imaging method. the upper part (unsaturated zone) above the groundwater table has much higher resistivity.

3) Suggestion: you may consider taking continuous sample up to the depth you desire to study. Measure the electric conductivity and try to correlate with other parameters you are investigating. Although this looks more a work of a chemist or a staff in an environmental engineering laboratory it can be a perfect geophysical work. researches are inter-disciplinary anyway nowadays.

Sincerely yours,

P. H. Giao

Asian Institute of Technology (AIT)

If you are interested in water content, the dielectric permittivity is the physical property of choice. The electrical resistivity is affected by many other factors besides water content. Methods that measure the dielectric permittivity of the soil are time domain reflectometry, capacitance sensors, and ground penetrating radar. To decide what is best for your application, more information is required as already indicated by the first reply.

Hi Carmen,

I misread the depth you mentioned (I thought it was 10 cm :-). In the case it is is 100 M the story would be different as it is deep enough. Implementation of an EI survey to reach that depth can be quite tedious in the field. FYI, we have done some EI surveys for Pusan and Mekong delta clay and could correlate well with soil layers up to 40m deep or so (but we did not reach 100-m depth). Again, if you can have soil samples, measurements of resistivity (or conductivity) will certainly help. This is the very approach of well log analysis in oil and gas upstream sector for determination of water saturation (note: you need to know additionally porosity and resistivity of pore water).

I also totally agree with Johan that measurements of dielectric permittivity can be very helpful.

Cheers,

P. H. Giao