Seismic refraction and reflection Seismic refraction and reflection projects and the traditional field campprojects and the traditional field camp
Bob Bauer and Eric SandvolBob Bauer and Eric Sandvol University of MissouriUniversity of Missouri
Branson Field LabBranson Field Lab
Presentation OverviewPresentation 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
Camp Branson
Four weeks of a Four weeks of a traditional field coursetraditional 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
Camp Branson
Traditional 5Traditional 5thth and 6 and 6thth week 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
The Evolution of our approachThe Evolution of our approach• Several years of short
seismic refraction and hydrology projects
• Nature Conservancy land in Red Canyon
Objective:
To provide a wider array of project disciplines without expanding our pre-requisite courses -
Historical, Sed/Strat, Structure
In 2005 we instituted a new approach to the In 2005 we instituted a new approach to the 55thth and 6 and 6thth weeks of our course based on an weeks of our course based on an NSF CCLI equipment grantNSF CCLI equipment grant
• Seismic equipment:– Geometrics® - Geode 24
channel seismic recorders and packaged software
– Seismic cable– Geophones– Tough-book laptop
(+ existing laptops)
The 5The 5thth week instruction introduces all students to week instruction introduces all students to several new projects that can be completed in the several new projects that can be completed in the same general location: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
For the 6For the 6thth week of instruction, students week of instruction, students choose one of three advanced project optionschoose 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
55thth Week – Seismic Refraction 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
Introductory Lecture & InstructionIntroductory Lecture & Instruction
• Snell’s 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
Field Equipment & DesignField 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
Data analysis in the labData analysis in the lab
• 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
Using an interactive computer program on laptop computers students determine:
Formulate a 1-D seismic
velocity model that best
fits the data
Interpretation in geologic contextInterpretation 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
66thth week week geophysics optiongeophysics 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
Each project involves: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
Refraction data collection Refraction data collection and processingand 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
Reflection data collection Reflection data collection and processing using and processing using seismic UNIX (SUNT)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
ConclusionsConclusions
• 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
Questions & DiscussionQuestions & Discussion