Accessing shallow groundwater in a sustainable manner is crucial for increasing agricultural productivity and improving food security across Sub-Saharan Africa, and many parts of South Asia
Geophysical sensors: Improving water security by detecting shallow groundwater and depleted aquifersWater Security
The lack of access to irrigation is one of the most critical hurdles to agricultural productivity and food security in Sub-Saharan Africa, where only 5% of agricultural land is currently irrigated. The same is true is some parts of South Asia, such as the Eastern Indian states of Bihar, Orissa and Jharkhand. In part because agriculture has not developed in these regions, there appear to be large untapped reserves of groundwater, many of which are shallow and recharged by rain. Indeed, a recent study by the British Geological Survey found that 60% of African citizens live in areas where groundwater is less than 25 meters below the surface.
In this context, there are two challenges we seek to overcome.First, to help low-income smallholder farmers gain access to this valuable water, but without devastating non-recharged aquifers. Second, to identify aquifers that have been depleted because of population pressures and the proliferation of intensified agricultural practices, so that they can then be refilled through rainwater harvesting.
We are in the early stages of developing a novel sensor that can solve both these problems.
This sensor combines magnetic resonance sounding (MRS) with Transient Electro-Magnetic (TEM) ground resistivity mapping. It is currently designed and optimized to detect shallow—typically rainfed—water (up to 10m deep), which ensures that the water usage enabled by this technology is sustainable. It also detects the salinity of groundwater, which helps determine the interest in extracting it.
The MRS method detects the total water content of soils and rocks by exciting protons in the water molecules with an externally applied magnetic field of the Larmor frequency set by the local value of the Earth’s static magnetic field. After excitation the proton magnetic moments precess at the Larmor frequency around the Earth’s field producing a magnetic field that is detected by a receiver on the surface. Information about the pore size of the mineral grains in the aquifer can be extracted from the decay constant of the exponentially decaying fields, whose amplitude is proportional to the amount of water in the volume excited by the external field.
The TEM method applies a single pulse of current in a horizontal loop that creates a time varying magnetic field that induces Faraday currents in the ground. The field decays after the energizing pulse is shut off and the receiver observes the decay of the magnetic field induced by the transmitter. The characteristics of the decaying fields depend on the resistivity distribution in the ground, which in turn indicates the properties of the soil and the salinity of the water.
The combination of MRS and TEM mapping removes the ambiguities deriving from the traditional use of electrical or electromagnetic methods to determine the distribution of electrical resistivity in the subsurface where the resistivity of soil or rock depends on many factors like the porosity, salinity of the pore fluid, and water saturation. To date only a few surveys have addressed this problem by conducting separate MRS and TEM mapping measurements at the same site but with different equipment.
A variation of this technology can also be used to identify depleted aquifers and underground water pathways. Although we have not yet begun work on that problem, we have all tools and technologies to address the issue.
Related Program Areas
Food Security and Agriculture, Climate Change and Environmental Damage
TEM detector during field tests