Glossary

AEM footprint
A measure of the area that an AEM system is able to remotely sense at any one time. This footprint shifts as the system moves along a flightline. The footprint or resolution of the system gets broader as the altitude at which the electrical current is transmitted and received increases.

airborne electromagnetics (AEM)
A type of airborne geophysics survey which involves a low-flying aircraft emitting an electrical current toward the ground. The current induces a magnetic field that penetrates the ground and allows a secondary electrical current to flow through the ground. A receiver on the aircraft detects the secondary current which can be used to create bulk conductivity maps of the landscape.

airborne gamma-ray data
Gamma-rays are a natural type of radioactivity and are constantly emitted from the land surface. The gamma-ray emissions from the elements potassium, thorium and uranium are used in aerial surveys to distinguish between erosional and depositional areas, soils and their source rocks.

airborne geophysics
Geophysics is the study of the below-surface physical properties of the earth, such as density, magnetisation and electrical conductivity. Airborne geophysics refers to this study from the air; for example, through airborne electromagnetics. Airborne geophysics is useful for mapping the physical properties of the earth and verifying data obtained from ground surveys.

airborne magnetic survey
A survey conducted from the air which records the natural magnetic characteristics of rocks on or below the surface of the earth. These surveys help identify the distribution and abundance of magnetic minerals in the landscape.

alluvial plain
A plain or flat area featuring weathered material that has been transported and deposited by the movement of water. The formation of an alluvial plain occurs over a long period of time. It is often formed through weathering and erosion, with the resulting material being transported by rivers or creeks to the lower plain.

alluvium
Alluvium is loose, unconsolidated soil or sediment which has been eroded and deposited by water. Alluvium includes materials such as silt, clay, sand and gravel.

aquifer
An underground geological formation or rock layer able to store and yield water. Aquifers may be replenished through rainfall or from connections with surface water bodies. Some formations feature the layering of aquifers. For example, the Great Artesian Basin comprises several layers of aquifers confined by beds of sedimentary rock.

basement
Basement refers to the solid rock below the sedimentary rock cover. Sedimentary rocks are formed from deposits of weathered rock remains. Over time, the increasing weight and pressure from the build up of sediment compacts and cements the sediments together, forming sedimentary rock.

borehole
A borehole is a narrow, typically vertical hole drilled in soil or rock to sample the area below the surface, or withdraw groundwater from an aquifer. Boreholes used to withdraw water are known as wells. Wells often feature casing to prevent caving, surface contamination and drawing of sand or sediment.

break-of-slope zone
The part of the landscape where the land surface begins to flatten out from steeper slopes. The break-of-slope zone is important in the context of groundwater, as rain falling on hilltops often recharges groundwater systems before it moves down slope. At the break-of-slope zone, there is often little gradient to maintain groundwater flow so it may accumulate and be forced to the surface.

bulk electrical conductivity (EC)
In the context of AEM, bulk electrical conductivity refers to the combined conductivity of soil, clay, rock and water. In order to distinguish between these causes of conductivity, raw AEM data require calibration and ground-truthing. This is done by evaluating the strength, form and elevation of the measured electrical current, in combination with information obtained from boreholes.

conductivity depth imaging (CDI)
Conductivity depth imaging refers to the presentation of conductivity data as images showing the degree and depth of conductivity. In the context of AEM, conductivity data are presented as maps and cross-sections. The maps show the location and average conductivity at a specific depth range, while the cross-sections show the degree and depth of conductivity across specific flightlines.

depositional area
Erosion is the process by which material such as soil, sediment, regolith or rock is moved from one place in the landscape to another. A depositional area refers to the area where the transported material settles, such as an alluvial plain.

depth-to-basement
Sedimentary rocks are formed from the deposits of the weathered remains of other rocks. Over time, the increasing weight and pressure from the build up of sediment compacts and cements the sediments together, forming sedimentary rock. Depth-to-basement refers to the depth of this sedimentary cover above the underlying bedrock.

dryland salinity
Dryland salinity is the accumulation and concentration of salt near the land surface in non-irrigated areas. It occurs where vegetation change increases the amount of water recharging the groundwater system. This increase raises the watertable and transports salts towards the land surface, where they can accumulate and become concentrated through evaporation.

erosion
The process by which weathered material such as soil, sediment, regolith and rock is transported from one place on the landscape to another. The forces which transport these materials include wind, water, ice and gravity.

erosional area
An erosional area is an area that features removal of soil, sediment, regolith or rock fragments from the landscape through erosion. Examples of erosional areas are river valleys, glacial valleys and coastal cliffs.

flightlines
Flightlines are the paths in which aircraft fly to obtain airborne geophysical data. Airborne geophysical surveys are flown along a series of equally spaced parallel flightlines, which are usually orientated north-south or east-west.

frequency domain AEM system
AEM involves a low flying aircraft emitting an electrical current toward the ground and a receiver on the aircraft detecting and measuring a secondary current. A frequency domain AEM system emits a current at a specific frequency, and measures how the secondary current received changes across frequency bands.

gaining section of a stream
Surface and groundwater resources are often connected. Flows between the two vary over time and are influenced by factors such as rainfall and soil material. This connectivity can be categorised by gaining or losing sections. A gaining section of a stream occurs where groundwater discharge contributes significantly to the stream flow volume.

geomorphology
The surface of the earth is being continuously modified by a combination of processes. These include volcanic processes and movement of sediment and rock by water, wind, gravity and ice. Geomorphology refers to the study of the earth’s landforms and the processes that shape them.

geophysical data
Data about the physics of the earth and its environment, such as the shape of the earth and its internal structure and composition. Examples include electromagnetic, heat flow and radioactivity data.

ground-truthing
Ground-truthing is the collection and testing of physical samples to help verify or calibrate data that have been collected remotely or automatically. Ground-truthing is used to calibrate data obtained from AEM surveys. For example, bores can be drilled to obtain information on the soil material and its capacity to store or transmit water and salt.

groundwater recharge
Groundwater recharge refers to the process where water below the land surface is replenished. Recharge occurs when water moves downward through pores in the soil and rock to the watertable. Recharge can occur by direct infiltration of rainfall or by leakage from surface water bodies.

inversion
In the context of AEM, inversion refers to the mathematical methodology used to separate the earth’s geoelectric properties from the observed electromagnetic interactions. This methodology converts the AEM data into cross-sections of the earth, with conductivity displayed vertically.
landform units  Landform units refer to a single, unique type of landform, such as a dune, floodplain, channel or wetland. Landform information can be used to verify AEM data.

losing section of a stream
Surface and groundwater resources are often connected. Flows between the two vary over time and are influenced by factors such as rainfall and soil material. This connectivity can be categorised by gaining or losing sections. A losing section of a stream occurs where a river or stream contributes water to the groundwater system. An example of a losing section is a river flush zone, in which surface water is lost to the underlying sediments.

microsiemens 
Siemens is the unit of measurement for electrical conductance. A microsiemens is one millionth (0.000001) of a siemens.

natural gamma log system
A natural gamma log system consists of a gamma-ray detecting probe, cable and counter instrument. When placed down a borehole, it records the level of naturally occurring gamma ray emissions from the rocks and soil around a borehole. It is used to estimate the quantity of clay in the rocks in the walls of the borehole, which may indicate the potential of that rock to hold water.

palaeochannel
Palaeochannels are alluvial aquifers created by ancient river systems that have eroded deeply into the landscape. They are inactive and have been filled with more recent alluvial sediments.

palaeovalley
A palaeovalley is an ancient, valley system which has been covered by more recent alluvium sediments.

perched groundwater
Perched groundwater is an aquifer located above the main watertable. It can occur where there is an underlying layer of impermeable rock or sediment above the main watertable. Perched groundwater may discharge to the land surface, for example, through seepage or as a spring.

regolith
Regolith refers to disintegrated and decomposed rock fragments, including soil, located above the solid rock or basement. The regolith zone is the area where many aquifers occur. The composition of the regolith material strongly influences groundwater quality through the presence of salts and acid-generating materials.

remote sensing
Remote sensing is the collection of information about an object without physical contact. In the context of AEM, remote sensing uses instruments mounted on satellites or in planes to collect data to produce images of the earth’s surface.

river flush zones
River flush zones occur where surface water is lost to the underlying sediments, recharging the groundwater system. These zones are categorised as losing sections. A losing section of a stream occurs where a river or stream contributes water to the groundwater system.

salt store
Salt store is the amount and distribution of salt present, typically in the regolith or a particular geological formation. The salt present in the salt store may be found within soil, clay, rock and water.

saprolite
Saprolite is a clay rich, thoroughly decomposed rock, formed by the process of weathering. The formation of saprolite occurs at the original site of the parent rock. As it does not move during formation, saprolite commonly retains the structure of the parent rock.

seepage sites
Seepage sites are generally small areas where groundwater percolates slowly to the land surface. Seepage often occurs when the flow of groundwater slows, resulting in groundwater accumulation and movement toward the land surface. Seepage sites are prone to surface salinity due to saline groundwater accumulating and concentrating near the land surface.

soil water
Soil water is water that occupies the pore spaces between soil particles. Some rainfall flows through the soil to recharge groundwater systems, but much will be retained in the soil for use by plants and other organisms.

stratigraphic unit
Stratigraphy is the study of the layering of rock and soil. It provides an understanding of the processes that created a landscape. A stratigraphic unit refers to a single and unique type of rock or soil layering.

sump
A sump is a low lying area that collects water. In the context of AEM, a sump is an area with a low lying watertable that collects nearby groundwater. A sump is prone to salinity due to this groundwater collection.

time-domain AEM system
AEM involves a low flying aircraft emitting an electrical current toward the ground and a receiver on the aircraft detecting and measuring a secondary current. A time-domain AEM system emits a current at a specific frequency for short periods of time, and measures how the secondary current received changes over these periods of time.

total dissolved solids
Total dissolved solids is a measure of the combined dissolved substances in a liquid. It is used as a measure of water salinity because some of the substances are salts. Water can be classified by milligrams of total dissolved solids per litre. In the context of AEM, total dissolved salt is used to understand groundwater salinity in survey areas. Water salinity is classified by milligrams of total dissolved solids per litre (mg/L). Fresh water is less than 1500 mg/L, brackish water is between 1500 and 5000 mg/L, and saline water is greater than 5000 mg/L.

total salt
Total salt indicates the amount of salt present in a measured area, and is expressed as tonnes per hectare per metre. In the context of AEM it is used to identify the location of salt stores in a study area.

watertable
The watertable refers to the upper surface of groundwater. The soil or rock is saturated below the watertable and unsaturated above it. As rising watertables mobilise and transport salts toward the surface, the depth to the watertable is an important indicator of salinity risk.