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Mapping The Colorado River Terraces, Texas, US
TEM CASE STUDY ● USA, 2026
01 – The challenge
Texas’ State Geological Survey needed a way to map electrical conductivity across walkable terrain quickly – something their existing airborne and ground-based TEM methods couldn’t do together.
The Near Surface Observatory (NSO) at the Bureau of Economic Geology has long used airborne and ground-based TEM methods to support geologic mapping, groundwater studies, and soil and water salinization research. But none of their existing instruments could rapidly acquire continuous electrical conductivity cross sections on foot, across the varied terrain their statewide programs actually demand.
02 – The context
A new geophysical instrument acquired by the Near Surface Observatory at the Bureau of Economic Geology (BEG), The University of Texas at Austin – tested along the Colorado River near Austin, Texas, and now supporting two active statewide research programs.
PARTNER
Bureau of Economic Geology, Near Surface Observatory (The University of Texas at Austin)
COUNTRY
United States, North America
TERRAIN
Alluvial terrace, limestone bedrock
APPLICATIONS
STATEMAP geologic mapping, Brazos River Alluvium Aquifer study
TECHNOLOGY
TEM2Go
YEAR
2026
The Bureau of Economic Geology is the State Geological Survey of Texas, established in 1909 as the oldest research unit at The University of Texas at Austin.
03 – The solution
Ground-Based Transient Electromagnetic Survey
TEM2Go is carried across the landscape by two people with backpacks – one carrying the transmitter coil, the other trailing about 15 metres behind with the receiver – controlled entirely via an app on a smartphone or tablet, with inverted depth sections displayed in real time as the team walks.
Methodology
01
Survey Planning
Profiles were planned along the lower and intermediate alluvial terraces of the Colorado River at Commons Ford Ranch Metropolitan Park, Austin.
02
TEM2Go Acquisition
The two-person team walked the terrain in March 2026, the transmitter and receiver separated by roughly 15 metres, collecting continuous conductivity data to depths of 50 metres or more.
03
Real Time Inversion
Depth sections were inverted and displayed in real time via the instrument’s app, allowing the team to assess data quality directly in the field.
04
Geological Interpretation
Resistivity variation across the 800-metre cross section was used to distinguish saturated alluvium, unsaturated alluvium, and underlying limestone bedrock.
04 – The results
The resulting 800-metre cross section resolved distinct subsurface layering across two river terraces. On the lower terrace, conductive, water-saturated alluvium overlies more resistive limestone bedrock at a depth of 10 to 15 metres. On the adjacent intermediate terrace, a resistive unsaturated zone sits above conductive saturated alluvium, itself underlain by resistive bedrock – two meaningfully different subsurface profiles, resolved in a single continuous walking survey.
200 Ωm
Resistivity contrast resolved in a single survey (range)
3
Distinct geological units identified from one model
100m
Vertical depth of subsurface resolved
The Region
01
The Balcones Fault Zone
Austin sits directly on the Balcones Fault Zone, where the limestone Hill Country meets the Blackland Prairie to the east. Displacement across the zone reaches roughly 1,000 feet in places – which is why bedrock crops up at shallow depths here, rather than staying buried far below.
02
Named Terraces
The Colorado River’s terraces through Austin aren’t anonymous – six are individually named (Sand Beach, Riverview, First Street, Sixth Street, Capitol, and Asylum), each sitting at a different height above the river as it cut down through the landscape over time.
03
A Fractured Aquifer Nearby
West of this site, the Edwards Aquifer – one of the most productive in the country and the primary water source for San Antonio – works the same way this survey’s target zones do: water moving through fractures and karst, not through the rock itself.
04
STATEMAP
This survey feeds into STATEMAP, the national program that funds detailed state-level geologic mapping across the US – meaning this one 800m cross section is part of a mapping effort spanning the whole country, not just this stretch of the Colorado River.
05 The impact
One of the first instruments of its kind – now supporting two statewide research programs.
TEM2Go is already supporting the BEG/USGS cooperative STATEMAP geologic mapping program and a Texas Water Development Board-sponsored study of the Brazos River Alluvium Aquifer – giving Texas’ State Geological Survey a genuinely new way to map the ground beneath the state’s varied terrain, on foot, in real time.
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