Geophysics 437: Application of
Methods in Environmental and Exploration Geophysics
Fall Term
2008
Weight
20%, due Wednesday, December 3, due 5:00 pm
Instructor: V. Kravchinsky
TA: D. Rippe
Assignment #5: Resistivity structure
of the
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Background |
(a)
Write a brief introduction to the resistivity survey (about one page). Include
a description of the conditions under which the measurements were taken.
Include a brief description of how the resistivity instrument operates, along
with operation procedures.
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(b)
Analyze the resistivity data (VES) obtained in 2005-2006 and 2007 (one line
includes 2005-2006 dataset, second line includes 2007 dataset). When proceeding the
data plot log10(ra) vs. log10(AB/2) to for each resistivity sounding location for
yourself (you do not have toprogram it yourselves, the plotting is done by
the subroutine rafind2),
where AB/2 is the half-spacing of the current electrodes in a Schlumberger
array and ra is the apparent resistivity. Include one-two most illustrative
examples in your report (see example here example1.jpg). To make the image you may interpolate the data
onto a regular grid using "griddata" in Matlab. Hint: for the grid
spacing of the vertical axis, you can use dy=0.1
in units of log10(distance).
When you take a look at the resistivity data you may notice some outstanding
points. You may remove some data points that seem to be too anomalous. You
can regularize the data by replacing these data point values with the average
of the nearest neighboring data points. Or you could apply smoothing the data
(any filter you want or running average) but it is no required because a
quality of our dataset is pretty high (thanks
to all hard-working student crews)J. Hand in the image with the regularized data if you
applied any smoothing. Plot a pseudosection of measured apparent resistivity
along every line. You may want to choose logarithmic vertical scale to
illustrate changes. |
Here is a map
showing the position of the profiles:
Note: gravity base
station positions do not correspond to our gravity dataset (they are given
for the survey of 2006). |
(c) Use
Zohdy's inversion method to obtain a model of the true subsurface resistive
structure for every line. To do this you will write a Matlab program and follow
the procedure discussed in class and in Zohdy (1989). Now make line plots and
an image similar to those in (b), but
replacing AB/2 by the true depth and
replacing ra by the true resistivity
r. Note: creating the
image will require re-gridding the data in Matlab. Try to explain the
resistivity structure/changes you obtained with horizontal distance and depth.
Plot model observed apparent resistivity, apparent resistivity and model true
resistivity at the same graph for every VES station (point of measure). After
computing theoretical (calculated) apparent resitivity and true resistivity
plot four cross-section graphs:
- observed apparent resistivity vs. logarithmic
pseudodepth
- apparent resistivity vs. logarithmic pseudodepth
- true resistivity vs. logarithmic true depth
- true resistivity vs. true depth
along both lines.
Include a brief description of the numerical method and programming procedure
in your report based on Zohdy’s method (about two pages).
(d)
Explain the geological meaning of your data (recall that we tried to cross the
paleoriver valley from south to north). Compare your result with the gravity
model from your previous assignment. Recommended volume for (d) part is about
one page.
Materials
to hand in:
1)
Hard copy of report, including figures (7±2pages).
2) Floppy or zip or CD disk with Matlab programs or send your program and input
files by email to me or TA.
Downloads:
Attached data files including rafind2.m and rhotran.m in the ZIP archive prepared by D. Rippe: Assignment_5_2008_data.zip
Example:
Resistivity file prepared by Volkan: example1.jpg