Seismic refraction and reflection projects and the traditional field camp

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Seismic refraction and reflection projects and
the traditional field camp

• Bob Bauer and Eric Sandvol University of Missouri
• Branson Field Lab

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Presentation Overview

• Traditional aspects of our course
• Evolution of our incorporation of geophysics
  projects
• Instructional context for our geophysics projects
• Seismic refraction project for all students
• Advanced seismic refraction and reflection option
  for students

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Four weeks of a traditional field course

• Introductory field methods
• Stratigraphic sections
• Sedimentary facies and stratigraphy
• Mapping of folded and faulted sedimentary units
• Regional geology instruction and 4-day trip
• Yellowstone, Tetons, SRP, Beartooth, Heart Mtn
  detachment
• Hard-rock structural analysis

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Traditional 5th and 6th week

• 5th week projects in mapping of more complex
  folded and faulted sedimentary rocks
• 6th week projects in structural analysis and
  mapping in Precambrian metamorphic and plutonics
  rocks

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The Evolution of our approach

• Several years of short seismic refraction and
  hydrology projects
• Nature Conservancy land in Red Canyon

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In 2005 we instituted a new approach to the 5th
and 6th weeks of our course based on an NSF CCLI
equipment grant

• Seismic equipment
• Geometrics - Geode 24 channel seismic recorders
  and packaged software
• Seismic cable
• Geophones
• Tough-book laptop
• ( existing laptops)

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The 5th week instruction introduces all students
to several new projects that can be completed in
the same general location

• Shallow seismic refraction (Eric Sandvol)
• Groundwater hydrology (Don Siegel)
• Surfacewater hydrology (Laura Lautz)
• Stream terrace mapping (Dennis Dahms)

• No pre-requisite courses in geophysics,
  hydrogeology or geomorphology- All projects
  completed by three-person teams- Include lecture
  and field instruction daily to small groups of
  teams -supergroups

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For the 6th week of instruction, students choose
one of three advanced project options

• Hydrogeology (Siegel and Lautz)
• Multiple (5) 1-day projects (2/group)
• Daily reports due each evening
• Hard-rock structural analysis and mapping
• (Bauer)
• 4-day mapping and data collection ( 3/group)
• Evening data plotting analysis
• 1 day to prepare map, data plots report
• Geophysics (Sandvol)
• Two projects (2/group analysis reports))
• 1 day each of data collection (all students)
• 1 day each of computer processing in camp
• 1 day to write second project report

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5th Week Seismic Refraction

• One-day project completed by all students
• Morning lecture to supergroup (9-12 people)
• Data collection by supergroups,
• Analysis reports by 3-person groups
• General objective give all students a basic
  background in how seismic waves can be used to
  image the subsurface
• Local objective determine whether seismic
  refraction techniques can be used to image the
  shallow floodplain strata or the groundwater
  table

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Introductory Lecture Instruction

• Snells law
• Travel times in a layered Earth
• Time-distance relationships
• Critical angle
• Travel times of seismic waves dipping layers
• Cross-over distance
• Experiment design
• Picking 1st arrivals
• Interpretation techniques

Reference reading in Burger (1992) Exploration
geophysics of the shallow subsurface
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Field Equipment Design

• 32-channel Geode Seismic Data Acquisitions system
  with a sledge hammer source
• Students design their own seismic profile to
  image shallow seismic boundaries (1.5-2 meters
  deep) beneath the floodplain
• Deploy thirty-two geophones and collect the data
  themselves

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Data analysis in the lab

• Using an interactive computer program on laptop
  computers students determine

• The number of layers that the data support
• The travel time of the first arriving P-waves
• The velocity and layer thicknesses for each layer
  using ray theory calculations

Formulate a 1-D seismic velocity model that best
fits the data
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Interpretation in geologic context

• Seismic experiment at the same field site as the
  ongoing hydrology projects
• Students use their measurements of groundwater
  depth to interpret their seismic velocity models
• Water table generally causes the largest velocity
  change at this site
• The students see how the shallow geophysical
  measurements can be integrated with the hydrology
  projects

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6th week geophysics option

• Two separate projects
• Refraction processing using time-term analysis
  and refraction tomography
• Reflection processing using muting, filtering,
  and normal moveout corrections
• Students learn to
• Design data acquisition for a target depth
• Determine if refraction or reflection data
  analysis is most appropriate for the problem
• Interpret the results in a familiar geologic
  context
• Most student have previous geophysics course

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Each project involves

• Data acquired using
• 32-channel Geometrics Geodes
• 10 Hz geophones
• Betsy (shotgun shell) source
• One-day data collection in an area where student
  have previously mapped
• One-day data analysis in the lab

• General instruction on
• the data acquisition process
• seismic survey design
• data analysis techniques

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Refraction data collection and processing

• Used thee time-term method to estimate refractor
  depth
• Software package Plotrefa? to calculate the
  velocities for an n-layer model
• Used a tomographic analysis to model travel time
  data

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Reflection data collection and processing using
seismic UNIX (SUNT)

• Survey design split spread, CDP gathers
• Normal move-out corrections
• Calculating layer thicknesses
• Stacking concepts (CMP and CDP stacking)
• Static corrections
• Fold calculations
• Processing steps
• Interpretations

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Conclusions

• Our two-part approach to teaching seismic field
  techniques and analysis allows us to
• Provide all students with basic instruction in
  elementary seismic techniques
• Provide students with specific interests in
  seismology with both field data collection
  experience and experience in applying a wide
  variety of seismic processing techniques
• Both project levels are provided in a field
  context already familiar to the students from
  previous projects

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Questions Discussion