Interpretation of Geophysical Data to Find Groundwater

Using SimPEG (Simulation and Parameter Estimation in Geophysics) software to conduct geophysical data analysis and inversion, many parameters of hydrogeologic interest can be determined. These include:

• Depth to bedrock
• Depth and thickness of aquifer
• Geographical location and extent of aquifer
• Presence of saltwater and contaminants in aquifer

In many geologic settings, fresh water is electrically resistive compared to most rocks.

Using this principle, the distribution of underground aquifers can be determined and ‘imaged’. The IRIS Syscal R1+ Switch 72 resistivity metre will allow for exploration of groundwater systems and aquifers up to 100–150 metres depths throughout Mon state. The maximum depth will vary considerably, depending upon the hydrogeologic structure in the particular survey area. In Mon state, we are looking for conductive regions where bedrock is more fractured and permeable to groundwater flow than the surrounding, intact bedrock.

To demonstrate how geophysics can be effective in finding groundwater and the best locations to drill, we can see the following example below from a water project in Bangladesh, performed by Canadian geophysicists Alastair McClymont and others. A ~1 km long DC-Resistivity profile was laid out. After using software to perform geophysical inversion (as described in the previous article), the following model was obtained. While In this example, fresh water is a resistive anomaly with respect to the surrounding rocks -the opposite of the situation we anticipate in Mon state- the concept of searching for anomalies in the data remains the same.

The SS unit (surficial sand layer, pink-coloured) only extends to ~10 m depths, and is the primary aquifer in this area, with many shallow wells drilled in it. However, because it is unconfined, the SS layer is unprotected from surface contamination: >86% of water samples tested were contaminated with E. coli bacteria. In contrast, the moderately deep aquifers (M units, pink/red coloured) and deep aquifers (D units, orange/red coloured) shown are better places to drill water wells, because they are confined beneath impermeable, clay-rich layers, protecting them from contamination from surface water.

In this case, geophysics provided the following valuable information to drillers:

• Which locations to drill at to likely find the uncontaminated M and D aquifers
• How deep to drill at these locations to likely find water
• Where NOT to drill, in order to avoid the shallow, contaminated SS layer

Another example below from the same study shows 7 DC-Resistivity profiles, superimposed upon a map of an area where drillers needed to know where to drill wells to find groundwater. When interpreted together, the profiles show 2 linear, spatially continuous resistive bodies (dark red coloured): units A and B. The resistive units were interpreted to be sandstone aquifers, and sites were identified along these areas at which to drill wells.

These examples, in addition to decades of geophysical research, show that unlike traditional methods such as drilling and surface geological exploration, geophysical methods such as DC-resistivity allow for high-resolution, fully 3-dimensional imaging of the distribution of groundwater underground. This reduces the high costs, time, and effort involved in finding optimal, high-yielding locations with good water quality at which to drill groundwater wells.