Geophysics and Remote Sensing
Geophysical or remote sensing imagery can be integrated with other datasets to assist in the interpretation of groundwater surface water interactions. Geophysical surveys can provide mapping of the spatial (and temporal) variation in important properties such as groundwater chemistry (particularly salinity), soil moisture content and soil/sediment texture. Such surveys can map the geological and geomorphological features that control stream-aquifer connectivity. They can also be used to map indicators of groundwater discharge (such as vegetation types, waterlogged or saline areas) in the landscape. Geophysical and remote sensing has been used to identify hotspots in terms of saline groundwater discharge into streams, pollution plumes or recharge of fresh stream water into alluvial aquifers. Geophysical data collection can be undertaken at different scales using different platforms, including satellite, aircraft, vehicles or boats. Downhole geophysical surveys can also be undertaken on existing bores (Table 2).
Remote Sensing (airborne / satellite) Imagery
Remote sensing techniques rely on the fact that subsurface aquifers commonly have some mappable surface expression or indicator, such as vegetation or distinctive geomorphology. For example much of the large scale 'salt-mapping' that has been done by natural resource agencies has typically been done in this way - often by mapping different vegetation types known to be associated with salinity and/or waterlogging. More recent developments in airborne geophysical survey techniques have resulted in the capability to map subsurface features related to groundwater movement, rather than near-surface indicators.
Multi-spectral imagery, such as Landsat TM imagery can be processed and classified to map landscape indicators such as moisture content, vegetation type and stress, land use and terrain. These can be integrated with other data to identify seepage areas. The Near Infrared imagery is particularly useful as water is a strong absorber in this part of the spectrum. Pre-dawn thermal infrared imagery has been trialled as a mapping tool for identifying springs and seepage areas (Woolley, 1997). Hyperspectral imagery involves greater partitioning of the electromagnetic spectrum and has the potential to map more specific soil or land cover properties.
Airborne Electromagnetics maps the bulk electrical conductivity of geological material down to depths of greater than 100 metres. These surveys are useful for providing the overall geological framework, for identifying losing stream reaches that recharge aquifers and the distribution of salt stores relative to the surface drainage network. Figure 1 shows a section of the airborne electromagnetics survey flown in the Lower Balonne catchment of eastern Australia and calibrated using field data from boreholes. Saline water is more conductive when compared to fresh water. The low-conductivity (blue) signal in the northern section of the survey area is interpreted as a fresh groundwater plume, caused by leakage from the Maranoa River.
Figure 1: Airborne Electromagnetics image showing groundwater recharge zone in Lower Balonne catchment (Image: Bureau of Rural Sciences)
Airborne Magnetics measures the magnetic field to map geological boundaries or structures that can define or constrict groundwater flow. Airborne magnetics have been successfully employed in the Honeysuckle Creek catchment of southern Australia to map buried paleochannels filled with iron-rich gravels (Figure 2). Subsequent drilling confirmed these channels were pathways of preferred groundwater flow and were carrying saline groundwater. Where these intersect and are hydraulically connected with the modern stream network is important for salinity management.
Figure 2: Airborne Magnetics survey showing iron-rich gravels in buried river channels in Honeysuckle Creek catchment (Image from Bureau of Rural Sciences)
Airborne Radiometrics, where gamma radiation emissions are used to derive the concentration of thorium, uranium and potassium within the shallow soil profile. A spectrometer is used to count the number of gamma rays across multiple bands within the energy spectrum, with peaks in particular bands attributed to each of the three radioactive isotopes considered. The technique is used to provide detailed information about the characteristics of the soil and its parent geological material, including surface texture, weathering, leaching, soil depth and clay mineralogy (Bierwirth, 1997). This is useful in mapping landforms such as near-surface palaeochannels or estimate soil hydraulic properties. The method is typically incorporated in airborne geophysical surveys, but portable spectrometers can also be used in ground-based surveys.
Radar measures the reflectance of transmitted microwaves to interpret moisture content and chemical composition of the shallow soil profile. Ground penetrating radar involves measuring the reflectances from high frequency pulses to map near-surface geological features.
2. Ground Surveys
Electromagnetics, as well as airborne electromagnetics. There are also a number of either personally carried or vehicle mounted techniques that measure the secondary magnetic field from induced electrical currents. Various instruments are routinely used in ground-based surveys to map soil conductivity to various depths (Table 1). Such surveys have been used to map seepage from irrigation channels into surrounding sediments (ANCID, 2000), or to identify shallow saltwater intrusion in coastal settings.
| Technique | Signal Penetration Depth (m) |
|---|---|
| EM38 | 1-1.5 |
| EM31 | 6 |
| EM34 | 7.5-30 (depending on coil separation) |
Resistivity/Electrical Conductivity, uses an array of transmitters to introduce an electrical current and receiver electrodes to measure subsequent voltage differences. Distortion of the electrical field by conductivity variations due to salinity, texture or moisture content can be imaged vertically at various depths. Ground-based surveys can be undertaken as transects parallel or orthogonal to the surface water feature. Also the geo-electric array, either submersible or floating, can be towed behind boats along rivers and irrigation channels to map the electrical conductivity of the water column and underlying sediments (Allen & Merrick, 2004). The technique has been particularly effective in mapping seepage of highly saline groundwater and evaluating the effectiveness of salt interception schemes.
Seismic, where the reflections or refractions of seismic waves from a simple energy source at the surface are detected to map stratigraphy and geological structures. Ground-based seismic traverses can be useful in mapping features that constrain or control groundwater flow, such as the geometry and stratigraphy of alluvial aquifers, the bedrock structures and preferred pathways such as palaeochannels. While there are two main types of seismic survey, refraction and reflection - the most commonly used survey technique is refraction. A shockwave - most commonly from hitting either a metal plate with a sledgehammer or by firing a seismic gun (Figure 3) into the ground - bounces back off the different layers making up the underlying geology with differing strength. The harder the material, the more strongly the signal reflects back.
Figure 3:Seismic gun : fires a shotgun charge into the ground to generate a shockwave.
Borehole Logging
Borehole logging involves running either one or a combination of different geophysical tools down boreholes drilled into the ground. It is the most invasive of the three main methods of survey discussed however it is capable of giving a far more detailed picture of the hydrogeology. The main downhole techniques are summarised in Table 2.
| LOG | PARAMETERS MEASURED | APPLICATIONS |
|---|---|---|
| Caliper | Borehole or casing diameter. | Fracture identification, lithologic changes, and well construction. |
| Natural Gamma | Natural gamma radioactivity. | Lithology and estimation of clay content in overburden. |
| Fluid Temperature | Temperature of borehole fluid. | Indicates geothermal gradient, and water flow in borehole or between borehole and fractures. |
| Fluid Resistivity | Resistivity of borehole fluid. | Indicates water flow within borehole, or between borehole and fractures; and water quality. |
| Single Point Resistance | Resistance of materials between probe and ground surface electrode. | Lithology, fracture identification, and location of well screens. |
| Normal Resistivity | Apparent resistivity of material. | Lithology, and water quality. |
| Spontaneous Potential (SP) | Electrical potentials between probe and surface electrodes. | Lithology, water quality, and in some cases, fractures in resistive crystalline rock. |
| EM Conductivity (Induction) | Electrical conductivity in medium surrounding borehole. | Location of contaminant plumes, conductive clay units, or bedrock fractures. Monitor water quality changes over time. |
| Flowmeter | Continuous or point measurements of water flow in borehole. | Identification of permeable zones and apparent vertical hydraulic conductivity and flow direction. |
| Borehole Video | Provides visual record of lithology, fractures, well construction. | Lithologic logging; identification of fractures; examination of casing or well construction. |
| Acoustic Televiewer | Provides acoustically-generated image of boring walls. | Structural logging; identification and orientation of fractures and foliation; examination of casing or well construction. |
| Optical Televiewer | Provides optically-generated image of boring walls. | Lithologic & structural logging; identification and orientation of structure & lithologic changes; examination of casing or well construction. |
Advantages
- Provides opportunities for the rapid, non-invasive mapping of landscape parameters that either indicate or control groundwater-surface water interactions (such as groundwater salinity or aquifer texture).
- Can provide good spatial resolution in the vicinity of surface water features, while multiple surveys can provide information in terms of changes through time.
- Various platforms from satellite and aircraft to vehicles, boats and downhole loggers can be used to collect data of various resolution and geometry.
Disadvantages
- Undertaking and interpreting these surveys can be complex, requiring specific equipment, technical expertise and logistical support. The equipment used can be expensive to purchase or hire, and can require ongoing maintenance and calibration.
- The data processing following field collection to remove measurement artefacts or to derive mapped outputs can be complex and may require extensive calibration with other datasets, such as borehole logs and chemical analyses.
- The cost of commercially available data (such as satellite imagery) can vary significantly.
- Ground or water-based surveys can have significant logistical problems encountering obstacles such as fallen trees, snags, fences, rough terrain, shallow water levels, thick vegetation or boggy conditions.
Data Availability
There is the opportunity to use existing geophysical datasets generated for geological mapping and mineral exploration for catchment mapping. The Geoscience Portal provides information on historical geophysical surveys as well as other geoscience datasets. The State geological surveys also undertake regional surveys as part of their mapping initiatives, refer Catchment Mapping.
Case Studies
Border Rivers stream EC imaging
Relevant Links
ANCID A Review of Geophysical Equipment applied to Groundwater and Soil Investigation [PDF 3.3MB]
NDSP Review of Salinity Mapping Methods in the Australian Context
US Geological Survey Application of geophysical logging to hydrogeological investigations
References
Allen DA and Merrick NP, 2005. Surface water/groundwater interaction investigation using a towed geo-electric array. Conference Proceedings. Irrigation Association of Australia.
ANCID, 2000. Open channel seepage and control. Vol 1.1 Literature review of channel seepage identification and measurement [PDF 3.2MB]. Australian National Committee on Irrigation and Drainage. Prepared by Sinclair Knight Merz.
Bierwirth PN, 1997. The use of airborne gamma-emission data for detecting soil properties. Proceedings of the Third International Airborne Remote Sensing Conference and Exhibition. Copenhagen, Denmark.
Woolley DR, 1997. Thermal infrared survey - Darling River and Baldy-Yeoval areas, NSW. Report CNR 97.066 NSW Department of Land and Water Conservation.