Seismic refraction and reflection projects and the traditional field camp Bob Bauer and Eric Sandvol University of Missouri Branson Field Lab

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 Camp Branson

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 Camp Branson

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

The 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 5 th and 6 th 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)

The 5 th 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

For the 6 th 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

5 th 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 & 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 1 st arrivals Interpretation techniques Reference reading in Burger (1992) Exploration geophysics of the shallow subsurface

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

Data 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 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

6 th 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

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 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

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

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

Questions & Discussion