Environmental and Exploration Geophysics Ipages.geo.wvu.edu/~wilson/geol454/lect6/Res3.pdf · of...

Tom Wilson, Department of Geology and Geography

Environmental and Exploration Geophysics I

tom.h.wilson

[email protected]

Department of Geology and Geography

West Virginia University

Morgantown, WV

Resistivity III

Stratigraphic correlations from borehole log

response


SP

Shallow

induction log Density and

neutron

porosityHigh ,

fresh waterLow sp,

fresh water

sandstone

Logging arrangements

SP log – passive log measurement


Measures natural current

flow produced between

drilling mud, formation

water and shale intervals

of differing resistivity

SP

Shallow


neutron

porosity

Electrode configurations - logging


Source

electrode

Sink

electrode

“the ground”

sinkPotential

electrode

Potential

electrode

Source

electrode

d1

d2d3

d4

Think how the electrodes associated with the logging

tool correspond to those of the surface array


V

A BNM

+ -

d1 d2

d3 d4

Downhole configuration


Source

electrode

Sink

electrode

d1

d2

1 2 3 4

1 1 1 1

4

iV

d d d d

d1 is the distance from A (the

source electrode) to M (a

potential electrode).

We need to use

the factor 4

How about this arrangement –

lateral log configuration


d2

d1

1 2 3 4

1 1 1 1

4

iV

d d d d

1 3

1 3

1 10 0

4

1 1

4

iV

d d

i

d d

Source

electrode

Sink

electrode

Normal log arrangement


d3

Source

electrode

Sink

electrode

d1

d4

1

1

1 4

10 0 0

4

1

4

1 1

4

iV

d

i

d

or

i

d d

The 4 remains since the source

electrode is buried

d2

If d4 is not

too large

The lateral log


V

A BNM

+ -

d1 d1 d2

Induction log


Reminiscent of the terrain

conductivity survey, but with

the transmitter and receiver

coils in the borehole.

The secondary current is

induced.

A type of log run in holes drilled

with non-conductive oil-based

muds or air drilled.

Stratigraphic correlations from borehole log

response


SP

Shallow


neutron

porosity

Example data from a Wenner sounding?


See interactive Excel file at

http://www.geo.wvu.edu/~wilson/geol454/ApparentRes.xls

Remember how to compute a?

We went through this briefly last time

Determine k? We have 1from the shortest offset and can

estimate 2 based on the long-offset response


Doesn’t look like a reasonable

2 for the observed response,

so k is probably wrong.

2 estimated from response,

then k is calculated

Also have a look at this xls file

2LayerReflection.xls


Linked on the class pages at

http://www.geo.wvu.edu/~wilson/geo252/2LayerReflection.xls

How does the resistivity vary as a function of

depth? Make a guess and see how you do.


Change this to spacing instead of effective penetration

Consider equivalent solutions

With about 10 % error equivalence suggests


Depth could vary

from about 6 to 13

meters within 10%

error.

While conductivity

could vary from

about 105 to 310

-m

Ap

pa

rent re

sis

tivity (

-m)

Resistivity (-m)

Inflection

point

4% errorthe computer may give you an “exact” answer but geology

doesn’t behave that way – consider equivalent solutions!


Depth could vary

from about 7.2 to

9.5 meters within

4% error.

While conductivity

could vary from

about 122 to 185

-m

Ap

pa

rent re

sis

tivity (

-m)

Resistivity (-m)

Inflection

point

½ distance to inflection point

provides an approximate

estimate of depth to the interface

Multi electrode instrumentation provides

sounding and profiling capabilities


AGI’s Sting and Swift or SuperSting

Finding the sink holesNational Corvette Museum in Bowling Green, Kentucky


Useful techniques in many areas prone to

sinkhole development and collapse


Along with terrain conductivity, resistivity is useful for

locating and mapping contaminant plume distribution


Finding fracture zones to locate water and oil

and gas wells


Tri-potential resistivity –

a method used to detect fracture zones


A simple 4-electrode system also

provides multiple observations at

a single spacing - The method is

referred to as the tri-potential

resistivity method

Normal Wenner array configurationCPPC Current-Potential-Potential-Current electrode configuration


A conductive fracture zone

would likely be one that is

water filled

High conductivity relative

to the country rock = low

resistivity

What is the geometrical factor?

CPCPcurrent potential current potential electrode configuration


The CPPC and CPCP

electrode configurations

both reveal the presence of

a low resistivity zone


CCPP


The CCPP electrode

arrangement reveals the

opposite response




V

a

Potential electrodes

a

+ -

i

i

Current electrodes

C PC

P

a = 15 meters

CCPP

A total of three measurements from one set up. Not

bad for a relatively inexpensive resistivity meter.


CPPC CPCP CCPP

Geometrical

factors2a 3a -6a


Tripotential resistivity measurements help establish the association

of a topographic lineament with a possible fracture zone

The work of Dr. Rauch and

some of his students


Good Devonian shale wells are located near fracture zones

Dr. Rauch and students


CCPP

CCPP

The fracture zone response

Is this a wet or dry fracture

zone?

Dr. Rauch and students

Looking ahead to the resistivity lab: skim through

Frohlich’s paper (see class page for paper link)


Some background information about the resistivity lab

Refer to Frohlich

and part 1 of the

resistivity

computer lab

Farmland – gently rolling topography


S5

S4

S3

S2

S1

Control

well 37

Control

well 16


Note Drill Hole Locations

along the profile line at left

and below

Glacial outwash overlying pre-glacial channels

that sit on dolomitic limestone bedrock


Shallow and

deeper aquifers

The bedrock is also a source of water, but has

high dissolved ion concentration


Bedrock aquifers, may

have increased

concentrations of

dissolved solids and

lower resistivity.

This lowered resistivity

makes it difficult to

“see” the bedrock

interface.

The resistivity contrast

can be minimal.


Modeling results suggest

this range may be a little

more variable and extend

from ~ 50 to 135 -m

Limestone bedrock generally

appears to have higher

resistivity – up to about 60 -m

This is your interpretation template


S1 S5S4S3S2

Look over discussion of this

section on pages 347 & 348

of Frohlich’s paper

10m

100m

L/2 where L is the current electrode spacing

Copy over the resistivity data


Copy over the folder IX1D-Res

Bring up IX1D

Bring up SS1 – Think about what the data

might be telling you about the subsurface


We will develop a qualitative interpretation of this sounding

using “inflection point” and “extrapolation” rules.

A. Where are the inflection points?

This is a Schlumberger sounding and the x axis

is usually labeled in terms of AB/2 or l OR L/2

Increasing depth

Data collected using

the Schlumberger array


B. How many layers are there and

C. What are their depths?

Inflection points

Refer to more recent, less noisy, data set S1

The rising and falling apparent resistivity trends provide insights

into relative differences of layer resistivity


1 23

4 5 6

?

1 = 23-m

2 = 32-m

3 = 23-m

4 = 81-m

5 = 49-m6 = 58-m

Remember AB/2 = l = L/2

depending on text/paper

The result is consistent with the basic

drill hole data


Limestone bedrock

(~35 m)

thickness depth Interp

25 3.5 3.2

103 8.8 12

40 23 35

75

ft

Constrained

using nearby

drill hole data

Our interpretation is a little more detailed.

We’ll get into details next time


Grey Clay

gravel

Grey Clay

Near-surface fresh

water gravel

Sandy clay

Bedrock

thickness depth Interp

25 3.2 3.2

103 8.8 12

40 23 35

75

SS1

Questions about Chapter 5 problems?

Have a look at this in-class problem


5 meters

2=180-m

1=26-m

Current

electrode

Potential

electrode

P

What is the potential measured at P given current of 0.5 amps.

Questions … ?


Get started on the in-class problem. I

will not pick up, but will check you off

before leaving.


• Finish up in-class problems (will discuss before leaving)

• Any other questions about Chapter 5 problems? (remember to

measure and calculate angles)

•Don’t forget - problems 5.1-5.3 due next class

• Start looking over Frohlich’s paper and get familiar with the basic

problems he addresses in his paper. We’ll be doing computer modeling

next week.

•Writing section – 1st draft chapter summary due September 22nd.

•Mid term exam September 29th

• No class October 4th and 6th

• Resistivity lab due Thursday October 13th

Looking ahead

Environmental and Exploration Geophysics Ipages.geo.wvu.edu/~wilson/geol454/lect6/Res3.pdf · of...

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