Lecture 8a - Laser and Optical techniques

Geophysical Techniques and Topographic Measurements

Importance of Positional Data

  • Positional and topographic measurements are crucial for geophysical techniques.
  • GPS is essential for providing positional data in geophysical surveys (e.g., EM surveys).
  • Elevation measurements are vital for applying corrections to gravity data.
  • Measurement position and topographic data are often part of the data inversion process.
  • Remote sensing and topographic data provide background imagery, survey context, and help in survey planning.
  • Aerial imagery can help identify fractures in surface rocks, supporting geophysical interpretations.
  • Topographic data is directly relevant in active areas (e.g., tectonic motion, volcanic processes).
  • Geophysics provides insight into subsurface processes, while topographic/remote sensing quantifies surface effects.

Processes Causing Topographic Change

  • Topographic changes are categorized into three broad categories:
    • Tectonics: Plate motion, mountain building (orogeny), earthquakes, volcanoes, sedimentary basin formation with compaction.
      • Large scale mantle processes.
      • Scales of thousands of kilometers, rates of millimeters to centimeters per year.
    • Crustal Processes: Landslides, lava flows.
      • Shallower Crustal processes.
      • Smaller spatial scales, more rapid.
    • Surface Processes: Individual events.
      • Purely surface processes.
      • Changes covering areas up to tens of kilometers, sometimes almost instantaneous (e.g., large landslide).
    • Anthropogenic Influences: Water abstraction (subsidence), quarrying, infrastructure projects.
      • Water abstraction can lead to substance over broad local scales and quarrying and infrastructure represents direct surface modification at site based scales.

Quantitative Estimates of Spatial Scales and Rates of Change

  • Mantle processes related to plate tectonics operate over 10001000s of km with plate motions of mmmm to cmcm per year.
  • Crystal processes operate over smaller spatial scales and can be more rapid.
  • Surface changes can cover areas up to tens of kilometers and be almost instantaneous (large landslide).

Measurement Techniques

  • A wide range of techniques is needed due to the wide ranges of scales involved.
  • This lecture aims to provide an overview of techniques used and to help identify good techniques for measuring different processes.
  • For example, to support geophysical investigations of a volcanic system, what methods would you choose to provide suitable topographic or remote sensing data?

Laser-Based Methods: Total Station

  • Total stations are used by surveyors to make accurate measurements.
  • Combine surveying theodolites and laser distance measurement.
  • Fire a laser pulse to a target, measure the time of flight, and calculate the distance (line length).
  • Most accurate measurements require a mirror or prism target.
  • Measurements are usually made over distances of a few hundred meters, but multiple kilometers are possible with reflectors.
  • Modern instruments don't always require reflectors.
  • Individual measurements provide a 3D point position with millimeter accuracy.
  • Useful when not many measurements are needed, but high accuracy is required.
Soufriere Hills Volcano Monitoring (Montserrat, 1995)
  • Total station used to monitor deformation.
  • Decreasing line lengths were measured over time.
  • Advantage: High accuracy and remote, safe distance (over a kilometer) from the growing dome.
  • Limitations: Measurements taken individually, time-consuming, reflectors required for long-range measurements.
  • Reflectors became dirty with ash and water, risky to clean, eventually lost.

Terrestrial Laser Scanner (TLS)

  • A very fast and fully automated total station.
  • Uses a laser and automated scanning platform to make hundreds of thousands of point distance measurements per second.
  • Each measurement represents the 3D coordinates of a point on the surrounding topography.
  • Output data are called point clouds (millions of measurements).
  • Underlying measurement principles are the same as for a total station, accuracy tends to be broadly similar.
  • Reflectors are not used directly in TLS surveys.
  • Used in a wide range of applications for topographic change monitoring where a combination of accuracy and spatial coverage is required.
Rockfall Monitoring in Norway
  • TLS used to monitor a rockfall area.
  • Surface displacement determined by comparing repeated TLS surveys.
  • Greens - zero change; yellow and orange - deformation (up to 18 cm over two years).
  • Deformation monitored for indications of acceleration that would precede potential collapse.
Lava Flow Monitoring on Mount Etna, Sicily
  • Long-range laser scanner used.
  • Repeated small lava flows built up a large lava delta at the headwall of a steep valley.
  • Laser scanner allowed assessment of lava accumulation.
  • Red colors outline the shape of the delta and show elevation change (up to 70 meters).
  • Combined topographic data with thermal camera images.
  • Detected active lava flows, providing insight into lava delta growth.
Advantages and Disadvantages of TLS
  • Advantages: Relatively accurate, portable, easy to deploy, broad spatial coverage.
  • Disadvantages: Not cheap, requires line of sight, ground-based in rugged terrain (may have areas that can't be observed, so the instrument has to be moved to multiple different viewpoints), data gaps.
  • Good for repeat measurements of active areas affected by rapid erosion or deep position, active deep deformation or volcanic activity.

Airborne Laser Scanning (ALS) or LIDAR

  • Laser scanners mounted on moving platforms for greater coverage.
  • Specialized survey aircraft used for greatest coverage.
  • LIDAR: Light Detection and Ranging.
  • Measurements every meter or so.
  • Positional accuracy is around a few tens of centimeters due to aircraft position uncertainties.
  • ALS advantages: huge area coverage and delivers relatively high density data over the area.
  • Resulting point clouds can be huge (hundreds of millions of points).
  • Limitations: Expensive, systems not always easily available.
ALS Survey Over Mount Etna
  • Many tens of square kilometers surveyed.
  • Very limited occlusions due to the instrument looking almost vertically down.
  • However, vertical cliff faces might be better measured from the ground.

Photogrammetry

  • Deriving topographic data from photographs.
  • Developed rapidly during World War II for gathering intelligence from aerial photographs.
  • Until about 2010, most work was carried out using specialized software and dedicated images taken from specialist survey aircraft just like LIDAR.
  • Freely available satellite data are used to provide global digital elevation models (DEMs) with accuracy and resolution of around tens to hundreds of meters.
  • Aerial images are still used, although the use of conventional survey aircraft has been completely overtaken by the use of small uncrewed airborne systems or UAS or drones.
  • Can provide millimeter to centimeter scale resolution and accuracy for site scales of a few hundred meters.
  • Just like laser scanning, we can also capture images from the ground, and this can enable dense or extended time series to be acquired.
  • This new breed of software incorporates advances from computer vision technologies, and you'll see it as identified as using structure from motion.
  • Structure refers to the shape of the 3D3D scene, and the motion refers to the photographs being taken from multiple different positions.
  • Increasingly used for all sorts of tasks, particularly in geomorphology.
Creeping Landslide Monitoring Using UAS Surveys
  • Repeated UAS surveys create repeated topographic models or DEMs.
  • Subtracting sequential DEMs calculates vertical differences (vertical change).
  • Can quantify vertical change in the topography.
  • Image analysis can be used to track horizontal motion.
Volcano Colima Hazard Analysis (Mexico)
  • Tourist overflights with photographs used to build 3D models.
  • Models are sufficiently accurate and contribute to hazard analysis.
  • Allows quantification of dome growth rates and identification of unstable areas.

Summary

  • Positional measurements make important contributions to geophysical surveys.
  • Wide range of spatial and temporal scales necessitates a wide range of measurement methods.
  • Laser-based techniques:
    • Tend to be accurate but expensive.
    • Accuracy and spatial coverage vary with the platform.
  • Photogrammetric processing of imagery:
    • More affordable, possibly slightly less accurate.
    • Widespread availability of drones makes it a popular technique for site-based topographic data acquisition.