Groundwater - How do we find it?
Historically, people settled close to available fresh water, as water is essential for all life. Listen to Professor Anders Vest Christiansen discuss the role groundwater has played for human settlement, and the tools and technology used today to find and manage groundwater resources.
Historically, people settled in the vicinity of freshwater resources - as water is essential for all life.
We know this from Denmark, but as well from the early civilizations - that formed near the major rivers in North Africa and the Middle East.
In areas without immediate access to water - people early on invented solutions to get to the water.
A prominent example is the Roman Empire - where they built an elaborate system of aqueducts - transporting water from the distant mountain regions to Rome.
People also learned that you can get access to shallow ground water - by simply digging a hole in the landscape in the right position.
If you build up the hole with bricks or rocks you have a constant water supply.
Even today many people around the world - rely on drinking water from shallow open wells.
Today, when searching for groundwater for extraction - we're trying to answer several questions at the same time.
Firstly, we need to identify the water-bearing layers. We call these layers aquifers.
A water-filled sand layer is an excellent aquifer.
More clay-rich or otherwise compact material may hold water - but we cannot retrieve it. We call these layers aquitards.
Secondly, we would also like to identify the surface area - that recharges the aquifer with water and the rate at which it happens.
This recharge area may be quite far from the aquifer itself - but it's an important piece of information - to establish sustainable extraction rates for the well in the years to come.
Lastly, we'd like to know the quality of the water.
In many areas, especially if we're looking for groundwater close to the coast - the water may be saline.
It may be completely useless for drinking water or irrigation.
The search for groundwater can be done by drilling a hole in the ground - where you believe an aquifer is present.
This has been common practice for many years, but it's also led - to a number of unsuccessful boreholes finding no water.
Geophysical methods have found widespread use - as a tool to scan the subsurface for the hidden layers.
Providing images of the subsurface that can be used to pinpoint ideal targets - to put down your next drilling when searching for water.
The geophysical tools come in many forms, but for groundwater investigation - the most common ones are the electromagnetic methods.
The electromagnetic methods make use of two things: One, the fact that a varying magnetic field generates a varying electric field - and vice versa. Two, the fact that the earth can conduct current.
This means that if we generate a varying magnetic field at the surface - the subsurface will respond with a varying electric field.
This varying electric field will again have its own varying magnetic field - that we can pick up at the surface in an induction coil.
An example of such an electromagnetic measurement tool is this tTEM system.
It consists of three parts. In the front, an ATV pulling the device.
In the middle, we have the transmitter coil. It's basically a piece of wire that sets up the varying magnetic field. In the back, we have a receiver coil that picks up the answer from the ground below.
The shape and size of the magnetic response picked up in our receiver coil - can be translated into a model of the electrical properties of the subsurface.
The last step is to translate the electrical properties - of the layers in the subsurface into something more meaningful. We can do that because the electrical properties - depend on the rock type or sediment type in the subsurface.
Let's imagine that we have a very simple subsurface model consisting of dry sand - wet sand and some clays. The dry sand will have the highest resistance - to conducting electrical current. The wet sand will be more conductive. The clay will be best at conducting current. This allows us to interpret the electromagnetic signals - into a more meaningful model of the subsurface - where we can illuminate the otherwise hidden layers.
The mapping can be carried out directly on the ground - or even from a helicopter carrying a large conduction coil as a sling load. This way, large areas can be mapped quickly - providing images of the hidden layers in the subsurface.
Locating fresh groundwater has been a crucial task throughout civilization. Today, technical developments and advanced geophysical tools - allow us to scan the subsurface helping us reveal the hidden layers below ground. This can help us pinpoint the best targets - for putting down the next groundwater extraction well.