section measurement.ppt

7/27/2019 Section measurement.ppt

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News and information

Partner list posted Gas up vans before tomorrow morning

Leave some space in your notebook

NIU students with yellow GPS come see me withyour unit


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Practical field procedures

Estimate total thickness of section Graphical or trigonometric

Determine level of detail for your description Depends on purpose

Determine section starting and ending points Walk entire section if time permits

Determine measurement method


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Organizing your notebook

Purpose for day What is the level of detail you are aiming for?

Geographic location

Method used

Starting and ending point descriptions

Scale for graphical column


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Notebook layout


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Denver, CO strat column

Geologic ages along left Split columns for extra thick

sections

Weathering profile

Capture lithologic variabilitywithin units


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Phosphoria Fm.

Graphical representation ofthickness

Text description ofimportant lithological

characteristics


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Measuring stratigraphic sections

Why measure sections? Acquire fundamental geologic information

Needed for basic characterization of an area

Assist in mapping

Facilitate correlations Establish relative ages

Facies and sedimentologic analyses


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Where do we measure sections?

Is there a need? Basic geological characterization, mapping,

correlations, facies analysis

Good exposure

Easy access Others can duplicate or examine your work

Structurally simple Low potential for tectonic thickness variations


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Variables in section measurement

True thickness (T) Perpendicular distance from top to bottom

Slope distance (s orw) Distance from top to bottom measured along the surface

Surface slope (s) Bedding dip (d)

Traverse orientation (a)


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Sources of error

Surface slope 2-5 degrees?

Bedding dip 2-5 degrees?

Surface distance 1-2%?

Traverse orientation 2-5 degrees?

Strive for < 10% total thickness error


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Simplest case of section measurement

Horizontal landsurface

Traverse normal tostrike

Constant dip

t= wsin d


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General case of section measurement

Seven possibilities


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Measurement techniques

Compass and Tape Must know: s, d, s, a


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Measurement techniques

Hewett (1920) method

T

E

No need to measure

surface slope angle or

distance!

T= Ecos d


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Hewett method procedure

Determine eyeheight Measure strike and dip of section to be

measured

Design traverse If normal to strike

Set inclinometer to measured dip

If oblique to strike

Set inclinometer to apparent dip in the direction a


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Hewett method procedure

For the measured dip or apparent dip,calculate how much section thickness is equalto one eyeheight

T= Ecos d

Stand at base of section Site to new position along traverse

Move to newly cited position

Record T of stratigraphic thickness

Repeat the above steps until reaching the topof the section

Estimate thickness of last fraction of section


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Mertie method

Allows for traverses that are not normal tobedding strike

T= s (sin dcos s sina) +/- s(sin scos d)

Use (+) when slope and bedding dip in opposite directions

Use (-) when slope and bedding dip in the same direction When a= 90, this equation reduces to one of the

general cases previously discussed

section measurement.ppt

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