No one wants to drill a dry water well, find little to no underground water, find the water isn’t even potable, or simply miss an aquifer. Water held in aquifers is known as groundwater. An aquifer is a sediment or rock in a geological formation, group of formations, or part of a formation that is saturated and sufficiently permeable to transmit economic quantities of water to wells and springs. For millennia, humans have tapped into aquifers; however, even with the latest drilling techniques, we can still drill unsuccessful water wells.
Aquifers can be a lifeline for land development, infrastructure projects, or even sustaining ranching and farming operations. There are different types of aquifers, including confined, perched, semiconfined, unconfined, and aquitard. Interestingly, we investigate the existence and type of aquifers with non-drilling technology. If you’re wondering how to tap into this concealed non-renewable natural resource, Geophysical prospecting is the key to locating suitable places to drill successful groundwater wells.
When finding water beneath the Earth’s surface, traditional methods (like blind drilling) might not always cut it. That’s where geophysical surveying, the science of using physical properties of the Earth’s materials to study its subsurface, comes into play. Geophysicists employ various techniques to detect and characterize shallow and deep aquifers. Here’s how they do it:
Geophysical methods to locate underground water at a glance
Surface Magnetic Resonance: Surface magnetic resonance (SMR) imaging isn’t just for medical scans. You can also use it to scan the underground and map pockets of groundwater. SMR, also known as surface NMR, proton magnetic resonance, and magnetic resonance sounding, is a cost-effective, non-invasive, and powerful ground-based geophysical technique used to detect and measure groundwater compared to traditional drilling. SMR measurements are directly sensitive to the presence of water and its interaction with grain surfaces, allowing you to differentiate water bound in tiny pores from (mobile) water stored in large pores. SMR measurements can also provide unambiguous estimates of hydraulic conductivity and specific yield. By studying the response of hydrogen atoms in water to a magnetic field, geophysicists can assess the location and extent of aquifers and their hydrogeologic properties.
Compared to other geophysical surface methods that might also be sensitive to water stored in porous sediments and rocks, the SMR method has a significant advantage because it is exclusively sensitive to the protons in water molecules. If the detection area of the SMR measurement is full of protons, the measured signal strength is at its maximum, while no protons mean a zero signal. Between these two end-points, the quantification of the water content is relatively easy, whereas other geophysical methods are ambiguous in this regard.
The SRM method is the most expensive and reliable geophysical method for locating groundwater because it tells you whether you have water, where, and how much! If your budget is generous, this is the method for you.
Electrical Resistivity Imaging (ERI): The ERI method measures subsurface electrical resistivity (inverse of electrical conductivity) by artificially generating electric currents that are injected into the ground with an array of electrodes pushed into the ground or in boreholes, and the resulting potential differences (voltages) are measured with the same electrodes. Different underground materials like water, rocks, or soil have distinct electrical properties. By analyzing the variations in the measured electrical resistivity, geophysicists can map the presence and characteristics of aquifers. Lithology, mineralogy, pore fluid chemistry, clay, organic matter, and water content affect the electrical resistivity of the medium. The depth of investigation of a 2D ERI survey can typically vary from tens to hundreds of meters, depending on the instrument’s capabilities. ERI is the most widely used exploration method for detecting groundwater because it is cost-effective, quick, and the second-most reliable technique after SRM. Electrical resistivity readings are taken and recorded using an electrical resistivity instrument, the resistivity meter, like the one shown below.
Electromagnetics (EM): EM methods like aquifers are excellent at mapping conductive targets. These methods measure the magnetic and electric fields associated with natural or artificially generated subsurface currents. The nature of the energy source defines whether an EM method is passive or active. Passive EM methods employ natural energy as an incoming plane of electromagnetic waves, which are the source recorded by receivers. Active EM methods require a controlled or artificial energy source (e.g., transmitter) and receivers, costing more than passive methods.
Passive EM methods used for aquifer detection and mapping include AMT (Audio-Magnetotellurics); they are cost-effective, practical to deploy, and can penetrate deep into the geological formations (50 m to ~1,000 m). Natural AMT signals come from various induced currents caused by thunderstorms and activity in the ionosphere. This method requires two pairs of shallowly buried electrodes and two or three shallowly buried magnetic sensors, typically within a 25 x 25 m or 50 x 50 m size footprint. The photograph below shows the setup of an AMT site.
Active EM methods for groundwater exploration include transient electromagnetic (TEM) soundings. TEM obtains fast vertical electrical resistivity soundings that respond strongly to conductive materials. TEM is an inductive (active) method that does not need direct galvanic (electrical) contact with the ground (like electrical resistivity). A transmitter wire loop placed on the ground induces a transient electrical current within the subsurface and the receiver antenna(s), which then measures the rate of change of the magnetic field associated with that current as it propagates through the earth. Depending on the size of the transmitter loop, it is possible to reach depths of 200 to 800 meters. The field photographs shown below illustrate an example of a TEM-sounding station.
Seismoelectric method: the seismoelectric method is relatively new in the geophysical area and claims to work best in geological settings with clastic sedimentary packages that can hold water. The technique measures seismoelectric or “electrokinetic” signals from fluid-bearing soils, sediments, and rock created by transforming electromagnetic waves from seismic waves generated by an active seismic source. Seismoelectric signals are produced whenever groundwater is forced to move by the pressure changes related to the migration of seismic waves. As the seismic waves move through the subsurface, they encounter geological interfaces that separate materials of different hydrological properties. The moving seismic waves squeeze the rock matrix in the aquifers. The less-compressible water moves minimal distances relative to the rock matrix. The water carries free ionic charges away from its partners bound to pore surfaces, creating a charge separation at a geological interface and forming an electrical dipole. The electrical dipole radiates electromagnetic waves that propagate to the surface and are sensed by an antennae array of four electrodes that detect an electric field.
Output from the electro-seismic inversion is typically a plot showing hydraulic conductivity versus depth. This interpretation assumes a one-dimensional (1D) layered Earth. If many 1D soundings are measured in close vicinity, a pseudo-2D cross-section can be generated, and an aquifer might be revealed. This method is claimed to reach great depths (< 800 m) in clastic sedimentary settings. Common applications of the seismoelectric method include groundwater exploration, aquifer characterization (depth and thickness), detection of hydrogeological boundaries and units, estimation of hydraulic conductivity, detection of contaminant plumes in the groundwater, and the detection of oil-water contact in the subsurface. The field photograph shown below illustrates a seismoelectric-sounding site.
Seismic Refraction: You may have heard of earthquakes, but did you know seismic waves can also be used for aquifer exploration? Seismic refraction is a type of seismic method. In general, seismic methods record the movement of vibrations through the ground with their wave speed and path, revealing Earth’s internal structure, strength, or stability of the subsurface materials.
Seismic refraction involves sending seismic waves into the Earth’s subsurface and analyzing each sensor’s first seismic arrivals (refracted waves). Geophysicists can determine the depth and thickness of different rock layers, including aquifers, by studying the time it takes for waves to return. It relies on the physical principle that the seismic velocity of the sampled layers increases with depth; thus, seismic energy that has traveled through deeper rock layers will overtake seismic energy traveling through slower, shallower rock layers and arrive first at more distant geophones.
The size and type of the seismic source dictate the depth of investigation. A hand-held sledgehammer commonly reaches ~30 m depth; a seismic gun can reach ~45-50 m; a weight drop can exceed ~45 m; a mechanical impact hammer can reach ~100 m; a surface wave vibrator can reach ~150 m depending on the local geology; and explosives can reach ~250 m. All seismic systems for seismic refraction surveys require three components: a seismic source (either artificial or natural) that generates the vibrations, sensors (geophones on land, hydrophones in water) connected to a cable to detect the seismic waves as they move through the subsurface, and a seismograph, the instrument that records the ground motion signal for later analysis. The field photograph shown below is of a seismic setup. After data collection, seismic refraction inversion software is used to find a combination of layer thicknesses and seismic velocities, yielding a seismic velocity model to explain the measured data.
Geophysicists measure body waves, which break down into compressional and shear waves (Vs) and surface waves (Vs), to investigate the subsurface conditions. Compressional P-wave seismic velocities (Vp) for water can vary from 1,400 to 1,500 m/s; 800 to 2,2200 m/s for unconsolidated saturated sand; 500 to 1,500 m/s for unconsolidated saturated sand and gravel; and ~1,700 m/s for the unconsolidated saturated glacial till. Vp for consolidated materials such as limestone (2,000 to 6,000 m/s), sandstone and shale (2,000 to 4,500 m/s), basalt (5,400 to 6,400 m/s), granite (5,000 to 6,000 m/s), and metamorphic rocks (3,500 to 7,000 m/s) are always faster than for unconsolidated materials. Due to their nature, measured Vs are always lesser than the Vp values for these materials.
Seismic refraction has been typically used to map bedrock location, topography, strength, and general geological stratigraphy. However, it has found application in hydrogeological studies. Aquifers that can be defined by one or more seismic velocity surfaces, such as soft clastic deposits (alluvial or glacial deposits) in consolidated soft sedimentary rocks (limestone or sandstone) underlain by a metamorphic or igneous rock or saturated unconsolidated deposits overlain by unsaturated unconsolidated deposits, are ideally suited for seismic refraction methods.
Ground-Penetrating Radar (GPR): a GPR survey uses electromagnetic or radar pulses to image the subsurface conditions. A transmitting antenna sends radar waves into the subsurface, which bounce back to the ground surface when they encounter changes in material properties. This principle helps geophysicists identify shallow aquifers and groundwater pollution pockets. This method works for relatively shallow groundwater mapping surveys. The GPR method suffers when a survey is done over saturated materials and clays because the radar pulses attenuate as they penetrate the subsurface materials. The depth of investigation can be controlled by changing the antennae. For instance, low-frequency antennas can reach more profoundly but do not offer deeper exploration depths than other methods; conversely, high-frequency antennas can be used for shallower depths. Depending on the type of GPR instrument, it can be pushed or pulled. The photograph below shows a GPR survey in push mode. Generally, the GPR method is not used for groundwater exploration beyond 5 to 6 m depth.
Unlocking the benefits of using geophysics to locate groundwater
Traditional drilling methods can be expensive and may only sometimes yield positive results. Geophysics can reduce the risk of becoming costly dry holes by providing valuable insights into the subsurface before drilling begins. For example, geophysicists can design geophysical exploration surveys to save time and money while reducing risk. The results of a geophysical survey can identify the most appropriate place to drill rather than randomly selecting a drill site that might or might not yield water. Electrical resistivity, or electrical conductivity (its inverse), is the best geophysical property to investigate the presence of groundwater because electrical current flows easily in moist, wet, and saturated earth materials. Depending on the measure of electrical resistivity, you can get an idea of the water quality and if it is suitable for human consumption. In addition, the results can also reveal subsurface site conditions (dangerous or safe) before drilling.
Geophysical methods are also non-invasive, minimizing disturbance to the environment. This is especially important in areas with fragile ecosystems. A geophysical survey can detect contaminated aquifers from their non-contaminated parts, depending on the methods. The investigation depth is not limited as long as you explore within the uppermost ~1,000 m.
You can also accurately map aquifer size, depth, water quality, hydraulic conductivity, and specific yield. This precision is invaluable for optimizing the design of water extraction systems.
Understanding aquifer characteristics can help you plan sustainable water extraction strategies and ensure a long-term water supply for your needs.
One final word on geophysics and aquifers:
Geophysics offers an innovative and cost-effective solution for harnessing shallow to deep aquifers for land development, industrial operations, ranching, or farming. It helps you make informed decisions about where and how to access groundwater.
Sometimes, using one geophysical method is barely enough to find groundwater. The more geophysical methods you apply in search of groundwater exploration, the higher the chances of success.
So, the next time you require groundwater, remember that geophysics is your ally in locating aquifers. It’s not just science; it’s a pathway to freshwater security and a more sustainable future.
Ready to explore the depths? Let’s talk.