The answer here will depend on the approximate target depth. Having said that, it sounds like a difficult target and environment for geophysical measurements. One obvious candidate is ground penetrating rader in case of relatively shallow target depths (2-10 m). However, signal penetration may be difficult in silty-clay till. Alternatively, electrical resistivity tomography or electromagnetic induction methods are good candidates.

Thank you. All targets are very shallow. The target depth of the trench bottom is 6-7 m. Target depth of the top of the water surface in the trench is approximately 3-4 m. Target depth of the water table surrounding the trenches is 2-3 m.

In that case ERT and EMI may both be practical too. Of course, there success will depend on the presence of an electrical conductivity contrast to seperate the different layers of interest.

Thank you again. Would the saturated vs. unsaturated condition within the waste disposal trench and in the surrounding slity-clay till provide the electrical conductivity contrast needed to identify the water surface in the till and in the trench?

The bulk electrical conductivity is indeed influenced by saturation. If all else remains equal, the electrical conductivity is thus a good proxy for soil water content. However, the electrical conductivity of the water in the pores also needs to be considered. In applications with waste, this pore water conductivity can also vary strongly spatially and thus we have two superpositioned effects here that can complicate the interpretation of bulk electrical conductivity measurements.

The most effective are the electric and electromagnetic methods. At the same time, seismics, gravity, magnetic, temperature, NMR, etc. are quite applicable. However, we must begin such an investigation from the development of intial physical-geological model (PGM) of the medium which after obtaning geophysical data and their analysis will transform to a final PGM. The initial PGM must include contrast physical properties, depth of water table occurrence, kind of water (fresh of percent of salinity/contamination), geometrical size of water pool, etc.

Thank you Dr. Eppelbaum and Dr. Huisman.

There are two sets of 5 disposal trenches that are immediately adjacent to each other with only 1 to 3 m of native till separating the individual trenches (each trench is 100-200m long and 8 to 10 m wide). The waste forms are highly variable within a trench and between trenches - metal drums, soil, concrete casks, wood, etc. The trenches are capped with 3 m of the native glacial till. Each trench has some volume of accumulated water (i.e., a trench-specific water table) within the waste. As mentioned by Dr. Huisman, I would expect significant pore water in this waste and in the surrounding silty-clay, even above the water table.

A geophysical study was conducted here in the late 1980s (seismic refraction, magnetometry, electromagnetic terrain conductivity, and electrical resistivity). This earlier work easily differentiated the overall area of the disposals vs. the undisturbed till, but the methods either did not have the resolution, or the study was not designed, to identify the depth of the trench, the water level, etc. for each of the individual trenches. (Also, it appears that the electrical resistivity technique was used only to identify the thickness of the trench caps.) Can present-day electric and electromagnetic methods and the other methods you mention (using present-day data processing techniques) provide the resolution needed to differentiate the bottom surface elevation and water table elevation for each individual adjacent trench? Or are these features on too small of a scale and too close together to attempt to differentiate them using remote methods?

Dear Paul,

I agree with Johan.  It is a very difficult target in that environment, but the water in the trenches should be very conductive due to the contamination.  So GPR and  ERT will be good options, if the conductivities play along.  

Having said that, don't forget about a time domain IP system.  If the water is more conductive than the clays, it will have a lower IP effect and might just be he way.  You target will then be the lowest IP in the section.

Hope this helps.

Kind regards

Stoffel

Thank you Stoffel. That is helpful, and I will look into the use of time domain IP. 

Best wishes,

Paul

Usually electrical resistivity method is used for shallow depth and seismic methods are used for deep water table.

Thank you Dr. Al-Ansari. 

the electrical resistivity method is best  also seismic refraction surveys

You can use any electrical resistivity method.

I think the best method is Electromagnetic method by using Ground Penetrating Radar ( GPR ) where, the electromagnetic energy will be absorbed at the water table.

Also you can use electrical resistivity method

Thank you Dr. Abdalaziz.

I mean the best solution  is using electrical resistivity method (sounding), hovever you have to pay attention at the development of clay layer and an interpretation will be derived from this.

Ludo

Use electrical resistivity method, preferably vertical electric sounding inversion method. The Advanced Geosciences Inc. 1D software will enable you see saturated zone or the aquifer horizon, usually indicated with a blue persistent color. Top of this blue is the water table depth. This depth is always indicated.  Knowledge of the  hydrogeology of your study area is necessary. Seasonal fluctuation of water table is not usually much in the absence of drought.