Remote Sensing for Geologic Mapping and Analysis Using spatial and spectral information to characterize geologic features

Textbook for geologists (2002) Can study remote sensing from a geologic perspective in a UW Geology Dept. course

Brandberg Massif, Namibia Granitic intrusion in desert

Geologic map of the Sheep Mt. Anticline, Wyoming, based on air photos. From Banjeree and Mitra. 2004. AAPG Bulletin 88(9):1227-1237.

Learning Objectives What are the primary applications of remote sensing for geologists? Be able to define lithology, structure, and landform, and recognize common examples in air photos. How do drainage patterns help us interpret geology? Why are spectral data especially important to geologists? How can aerial photography contribute to soil mapping? What do we mean by 1st through 5th order soil maps? What are absorption features, and why are they important? What is gossan? How can we use spectral features to find hydrocarbons?

Application of images to… Lithology Structure Landforms Drainage Soils What do each of these mean?? Lithology—rock types. Structure—folding, faulting, deformation. Landforms—terrain features formed by natural processes that appear similar wherever found (erosional or depositional). Drainage patterns—aggregate of flow channels whether or not with water in them. Soils—soil types expressed on the surface.

Devil’s Tower – Lithology? Structure? Surrounding rock?? (Air Photo Courtesy Louis Maher, Jr.)

Types of Lithology Igneous Sedimentary Metamorphic What are each of these, and what are some examples?? Igneous: rock formed directly from molten mass: intrusive (granite) or extrusive (basalt) are common examples. Sedimentary: sediment deposited and lithified (turned to rock) after being transported by water, wind and/or gravity: sandstone, siltstone, shale, limestone are common examples Metamorphic: formed by action of heat or pressure on previously existing rock: gneiss, schist, slate, and quartzite are common examples

What kind of rock is this? What clues are you using? Igneous Rocks (basalt flows) at Craters of the Moon, ID (Air Photo Courtesy Louis Maher, Jr.)

Sedimentary Rocks (badlands) in South Dakota What kind of rock? Sedimentary Rocks (badlands) in South Dakota (Air Photo Courtesy Louis Maher, Jr.)

What’s the scale of this photo? What kind of rock? What’s the scale of this photo? Metamorphic Rock from Grand Canyon (not an air photo!) (Courtesy American Geological Institute)

Mapping lithology using imagery. Interpretation requires knowledge of relationships between the lithology and: Climate Topography Drainage pattern Jointing and faulting Texture Vegetation Spectral properties of minerals that comprise rock types Photointerpretation clues: tone, size, context, shape, etc. Interpreters should be trained to understand these relationships on images and in the field

Climate Affects the way rocks weather. Affects the associations of vegetation with particular rock types Affects soil formation from rock parent material Affects erosional patterns All of these influence the appearance of different rocks in images.

Drainage patterns Drainage patterns are easy to see on images Offer clues to many geologic characteristics of an area (e.g., topography, bedrock, surface texture and hardness, jointing, etc.) Obvious importance for hydrologic mapping, modeling, and management Often influence human land use

Drainage patterns A. Dendritic B. Parallel Dendritic: horizontal sediment or uniformly (homogeneous) resistant bedrock; gentle slope Parallel: moderate to steep slopes fine textured deposits or fractured bedrock or in areas of parallel elongate landforms Trellis: dipping or folded bedrock Rectangular: jointed or faulted bedrock Radial: volcanoes, domes, basins Annular: domes or basins Multibasinal - flat-lying glacial terrain; karst (limestone) terrain Contorted: metamorphic rocks disc.gsfc.nasa.gov/.../ geo_images_4/Fig4.1.gif Originally from Howard, 1967

What kind of drainage is this? What causes it? Rectangular drainage on Volga River (caused by faulting) (Satellite image)

Dendritic drainage pattern Type of drainage? Dendritic drainage pattern (Photo courtesy Michael Collier)

Igneous rocks on air photos: Basalt flow example Topography: flat to hilly Drainage: parallel or internal Photo tone: dark or sometimes spotted Gully type: none (not erosive) Igneous Rocks (basalt flows) at Craters of the Moon, ID (Air Photo Courtesy Louis Maher, Jr.)

Sedimentary rocks on air photos: Example – sandstone in arid climate Topography: flat or table like (mesas, etc.) but can be highly eroded Drainage: dendritic Photo tone: light and banded (can vary considerably) Gully type: none to deep depending on steepness Where is this? Can you name these features? Sedimentary rocks (sandstone) at Castle Valley, UT (Air Photo Courtesy Louis Maher, Jr.)

Geologic Structure Geologic structures are any features caused by deformation of rock (folding, faulting, etc.) Structure is important for trapping hydrocarbons, controlling water flow, understanding stratigraphy, etc. Includes: Strike and dip Folds (e.g., anticlines, synclines, domes, basins, etc.) Faults (e.g., normal, reverse, horst and graben, etc.) Joints Unconformities

Can you name this Wyoming feature? Sheep Mountain anticline in Bighorn Basin of Wyoming (Air Photo Courtesy Louis Maher, Jr.)

What are the linear features? Type of rock? What are the linear features? Sandstone jointing in Arches National Park (Air Photo Courtesy Louis Maher, Jr.)

Mapping and analyzing geologic structure with images Interpreter looks for changes in tone and texture that represent boundaries between geologic units Works best where vegetation cover is minimal But…can sometimes see changes in underlying strata related to changes in overlying vegetation Can sometimes enhance edges with digital filters Can use stereo techniques to measure elevation changes for calculating dip angles

Geologic map of Wyoming’s Casper Arch (Image Courtesy NASA.)

Landsat 8 image

Landforms and Geomorphology Definition of landforms varies with discipline Geologist may have different view than soil scientist or hydrologist Creating a landform key is important aspect of image interpretation Landforms are strongly influenced by underlying geology and climate

Broad categories of landforms Coastal and oceanic (e.g., fjord, ismuth, beach, etc.) Erosional landforms (e.g., canyon, cuesta, gully, etc.) Fluvial (river related) landforms (e.g., braided channel) Mountain and glacial landforms (e.g., cirque, peak, etc.) Slope landforms (e.g., terrace, cuesta, plain) Volcanic landforms (e.g., cinder cone, lava flow) Depositional landforms (e.g., alluvial fans) Etc. (there are many ways to think about landforms)

Glacial moraine near Pinedale, Wyoming What kind of landform? Where? (Air Photo Courtesy Louis Maher, Jr.) What kind of landform? Where?

Great Sand Dunes, Colorado (Air Photo Courtesy Louis Maher, Jr.)

Mapping and analyzing landforms with high spatial resolution images Identifying landforms on images requires using many clues Topography Drainage pattern Drainage texture Photo tone and texture Vegetation patterns Land use patterns Scale of landform determines scale of imagery necessary to map. Landforms occur across scales.

Soils Soils can be mapped at a wide range of scales and precision 1st order surveys are most detailed and 5th order are least Lower (1st, etc.) order surveys require detail found in air photos Almost all soil mapping requires a combination of field survey and remote sensing Typical project uses manually interpreted aerial photography followed by field work to label the interpreted units

Examples of soil map order Small plot level 1st order 1:8,000 scale Detailed soil map 2nd order 1:20,000 scale Soil association map 4th order 1:250,000 scale Statewide soil map 5th order 1:1,000,000 scale

Soil survey on air photo (From Wikipedia)

Spectral applications Minerals have distinct reflectance signatures with spectrally narrow features Lithologic mapping depends largely on discrimination of minerals Broad-scale structural mapping can also benefit from satellite RS Mineral and petroleum exploration is more efficient with imagery

Spectral Absorption Features

Spectral matching: Software looks for best match of unknown spectra (from image) to known spectra (from libraries) Is this a good match??

Or image classification… Choose appropriate bands for the minerals of interest Use image classification to group image pixels into classes based on those bands Combine or split classes as needed to make a map of mineralogy/lithology. Iron oxide on moon. Map NOT based on fieldwork! http://www.psrd.hawaii.edu/July04/newMineral.html

Mineral and petroleum exploration Spectral data allow geologists to narrow search areas and eliminate unproductive ground work Can map large structures, diagnostic mineralogy and lithology, and outcrop locations quickly Mineral clues can point to areas associated with gold, silver, copper, and other metal-bearing minerals Petroleum is usually more deeply buried and requires structural analysis, but often there are surface clues

Case Study: SW Utah ore prospecting Oxidized iron ore called “gossan” by prospectors can indicate mineralization associated with valuable minerals Gossan has a distinctive look (left). Often gossan occurs with other minerals like copper (below) or gold (Modified from the NASA RS Tutorial)

Landsat-based prospecting in Utah Part of a Landsat image covering SW Utah (near Zion NP) White Mountain Classic gossan staining Basin deposits Wah Wah Mts. (block fault) Prospectors look for telltale gossan and enhance imagery to make it stand out

Band ratios – enhancement of spectral features This image shows the ratio of two spectral bands to highlight gossan, which appears as yellow/brown area Ratio of TM bands 7/5 (Mid-IR bands) is often good for enhancing mineralogy

Advantage of spectral prospecting for gossan Landsat imagery free Spectral information in Landsat sufficient to create believable map of gossan Significantly narrows the search area to constrain ground-based prospecting Potentially increases profitability

Other examples: Copper prospecting in Nevada with ASTER imagery

Hyperspectral mapping of a mining district in Utah

Case study 2: Petroleum prospecting Petroleum requires source of hydrocarbons and a trapping formation (Modified from the NASA RS Tutorial)

Contribution of multispectral satellite imagery? Focus is on identifying trapping structures Satellite imagery allows rapid survey of large areas at low cost Lithology mapping allows identification of key formations Mapping of fracture patterns useful for understanding traps – fractures let hydrocarbons migrate through rock Satellite surveys must be followed up by surface exploration and usually drilling to understand buried structures

Landsat MSS image with “hazy” tones linked to know hydrocarbons Anadarko Basin in Oklahoma Source

Ratio image (composite of three band ratios) of area A from previous slide. Oil-bearing formation looks reddish. Turns out that hydro-carbons leaking through surface rocks were altering them spectrally Rocks associated with key formations are spectrally different Source

Similar leak of hydrocarbon gas in Wind River Basin, Wyoming caused the alteration of rocks in tan oval area in center of this image.

Vegetation effects Geologists and remote sensing scientists in Michigan have found stressed vegetation in vicinity of hydrocarbon gas leaks Shows up in the “red edge” of the vegetation spectral curve Even without leaks, vegetation can be associated with particular formations or it can follow structural features and fractures

Summary High spatial or spectral resolution imagery is widely used for various aspects of geology Choice of images depends on scale, spectral requirements, etc. Image interpretation for geology usually requires a coupled field component Interpreters must have comprehensive knowledge of a broad set of indicators that give clues to underlying geology