Just like oil, structures such as faults and folds play an important part in determining the migration and trapping of groundwater. Open gravels will have a much greater permeability than clays, and sandstones will be somewhere in between. Faults may act as barriers to flow, if they are silicified for instance or have a thick gouge, or they may act as conduits if they are zones of intense fracturing. Structural geology provides a means to both assess the character of faults and understand the thickness and distribution of aquifers and aquicludes. I have provided consultancy services for both these aspects in North Canterbury, New Zealand.