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