GEOLOGICAL SURVEY CIRCULAR 879

Computer Analysis of 

Digital Well Logs
Computer Analysis of 

Digital Well Logs

By James H. Scott

GEOLOGICAL SURVEY CIRCULAR 879

1984
United States Department of the Interior 

WILLIAM P. CLARK, Secretary

Geological Survey 

Dallas L. Peck, Director

Free on application to Branch of Distribution, U.S. Geological Survey, 604 South Pickett Street, Alexandria, VA 22304
CONTENTS

Abstract ............................................ 1

Introduction .......................................... 1

Overview of computer programs ............................... 1

Detailed descriptions of computer programs ........................ 3

Summary and conclusions ................................... 16

References cited ........................................ 16

ILLUSTRATIONS

Page

FIGURE 1. Generalized flow chart of computer programs for analyzing digital well logs .............. 3

2. Graph showing calibration information used to transform neutron API units to porosity index, for holes

of various diameters ............................................ 4

3. Examples of computer plots of natural gamma-ray and neutron-neutron logs .............. 6

4. Examples of computer plots of dual-detector density and caliper logs .................. 8

5. Graph showing empirical hole-diameter correction data for magnetic susceptibility logs ........ 9

6-10. Examples of computer plots of logs:

6. Magnetic susceptibility and conductivity ................................ 10

7. Resistivity and single-point resistance ................................. 11

8. Sonic velocity and amplitude ....................................... 13

9. Absolute temperature and differential temperature computed at a depth lag interval of 1 ft . . 14

10. Induced polarization, resistivity, and self potential .......................... 15

TABLE

TABLE 1. Log-measurement values and transformed values produced by computer programs
COMPUTER ANALYSIS OF DIGITAL WELL LOGS

By JAMES H. SCOTT

ABSTRACT

A comprehensive system of computer programs has been developed by the U.S. Geological Survey for analyzing digital 

well logs. The programs are operational on a minicomputer 

in a research well-logging truck, making it possible to analyze 

and replot the logs while at the field site. The minicomputer 

also serves as a controller of digitizers, counters, and recorders 

during acquisition of well logs. The analytical programs are 

coordinated with the data acquisition programs in a flexible 

system that allows the operator to make changes quickly and 

easily in program variables such as calibration coefficients, 

measurement units, and plotting scales. The programs are designed to analyze the following well-logging measurements: 

natural gamma-ray, neutron-neutron, dual-detector density 

with caliper, magnetic susceptibility, single-point resistance, 

self potential, resistivity (normal and Wenner configurations), 

induced polarization, temperature, sonic delta-/, and sonic 

amplitude.

The computer programs are designed to make basic corrections for depth displacements, tool response characteristics, 

hole diameter, and borehole fluid effects (when applicable). 

Corrected well-log measurements are output to magnetic tape 

or plotter with measurement units transformed to petrophysical and chemical units of interest, such as grade of uranium 

mineralization in percent eU308, neutron porosity index in percent, and sonic velocity in kilometers per second.

INTRODUCTION

Well-logging service companies have used truckmounted computers for field-data acquisition and 

analysis for several years (Moseley, 1976; Kinkade 

and others, 1978; Best and others, 1978). These 

commercial computer-based systems are generally 

designed for speed, simplicity of operation, efficiency and reliability of data acquisition, and they 

have turned out to be highly successful for routine 

well logging. However, they are not flexible 

enough for research studies where ease of program modification is more important than 

standardization of results. The U.S. Geological 

Survey's research well-logging system uses a

Hewlett Packard 9845B minicomputer1 with 

BASIC programming language, 318K bytes of 

core memory, and cathode ray tube (CRT) 

graphics with a capability for hard-copy output of 

graphics and printed data. Programs and data are 

stored on cartridge magnetic tapes or flexible 

disks, and are read into the computer under 

keyboard or program control.

The general design of the analytical programs 

is described in the first section of this report. The 

second section contains detailed descriptions of individual programs with examples of results given 

for each one.

OVERVIEW OF COMPUTER PROGRAMS

All programs begin with a section that reads 

the digital field tapes or disks into the computer 

for rough editing. Invalid numbers and overlapping depth intervals are removed if they exist. 

A block of calibration information, normally stored 

at the end of the recorded data, is read, checked 

for validity, and stored for later use. Acquisition 

of field data is occasionally terminated prematurely because of a power failure or recording 

problem, and if this happens, calibration information is not available. This condition is detected by 

the program, and the user is prompted to enter 

the necessary calibration values from the 

keyboard.

Next, the program calls for keyboard entry of 

depth corrections, if needed, and for keyboard 

entry of the upper depth of valid data, which gives 

the user an opportunity to delete invalid data that 

occur in the vicinity of the borehole fluid level for 

some logs, particularly electric logs and sonic logs.

'Use of trade and company names in this report is for descriptive purposes only 

and does not constitute endorsement by the U.S. Geological Survey.
The new arrays of depths and log readings are 

computed at evenly spaced depth intervals by interpolation of recorded values at a spacing that 

is determined by the user. Evenly spaced depth 

intervals are needed to facilitate subsequent data 

manipulation of well-log data (such as filtering, applying hole-diameter corrections, drift corrections).

displayed on the CRT and that can be positioned 

at any point on the log trace. Special-function keys 

on the computer keyboard are used to move the 

cursor up and down along the log trace, to delete 

erroneous data points, or to move them to the left 

or right of their original position. After editing, 

the log can be filtered if necessary to remove 

noise, and then stored on magnetic tape, or output

TABLE 1. Log-measurement values and transformed values produced by computer programs

[API, American Petroleum Institute; SI, International System of Units]

Log type Log-measurement value Transformed value

Natural gamma 

Neu t ron-neut ron 

Dual-detector density 

Counts per second 

Counts per second 

Counts per second 

API units or ell^Og (percent). 

API units or porosity (percent).

 Compensated density grams per 

cubic centimeter.

Magnetic susceptibility Centimeter-granr-second Centimeter-granr-second (cgs) or

(cgs) units (apparent) SI units, corrected for drift

and hole-diameter variations.

Resistivity Ohm-meters (apparent) Ohm-meters corrected for

hole diameter and mud 

resistivity variations.

Sonic velocity Microseconds per foot Feet per second or

kilometers per second.

Ohms or Hertz 

Millivolts per voltMillivolt s -

Temperature 

Induced polarization 

Self potential 

Single-point resistance Ohms Ohms (not transformed).

Degrees Fahrenheit or degrees Centigrade. 

Millivolts per volt (not transformed). 

Millivolts (not transformed).

Basic corrections and calibration algorithms are 

then applied to the interpolated raw data. Depth 

corrections are applied for the vertical offsets of 

the various detectors located at different positions 

on the probes, dead-time corrections are applied 

to nuclear counting measurements, and linear or 

nonlinear calibration functions are applied to convert digitizer output units (millivolts) to log measurement units (ohm meters, microseconds per 

foot). After the basic corrections and calibration 

algorithms have been applied, the logs are plotted 

on the CRT and edited, if necessary, while displayed on the CRT. The editing routine provides 

for keyboard manipulation of a cursor that is also

as a paper copy of the CRT plot, or plotted on 

an X-Y plotter. In the next stage of operation, 

unit conversions are applied to convert the log 

measurement values to the desired final output 

values (for example, neutron counts per second to 

neutron porosity in percent). The measurement 

units and converted units for all logs discussed in 

this paper are given in table 1. After the unit conversions are applied, the logs may be replotted 

on the CRT, edited and (or) filtered again if necessary, and stored on magnetic tape or output as 

a paper copy of the CRT plot, or replotted on a 

high-resolution X-Y plotter. A generalized flow 

chart of the computer programs is given in figure 1.
\

Field data input from 

magnetic tape or disk

\

( Read deta

^ 

informetion aveilable?

Processed data input 

from output magnetic tape

.

Read data

Enter calibration information from keyboard >

Check validity of calibration data 

and compute calibration factors

Edit and remove invalid data

Enter and apply depth corrections

Remove data above top of valid log

Generate new data arrays with 

evenly spaced depth intervals

Apply basic corrections and 

calibration algorithms

Plot, adit, filter, output options:

1. Plot on CRT or X-Y plotter

2. Edit on CRT

3. Apply filters to remove noise

4. Record corrected, transformed data 

on output magnetic tape

NO .YES \UUIIB ( .s- -*i cxu j

Apply unit conversion options (for example, 

counts per second-»API units ^neutron porosity)

FIGURE 1. Generalized flow chart of computer programs for analyzing digital well logs. 

CRT, cathode ray tube; API, American Petroleum Institute.

DETAILED DESCRIPTIONS OF COMPUTER 

PROGRAMS

1. Natural gamma-ray and neutron-neutron log 

program

Gamma-ray (total count) and neutron-neutron 

(porosity) log analyses are combined in one program because digital field logs are obtained with 

a probe that simultaneously measures both parameters. The gamma-ray measurement is made with 

an Nal(Tl) detector mounted near the top of the

probe and the neutron measurement is made with 

an He-3 tube that detects neutrons scattered back 

to the detector from an Am(Be) source attached 

to the bottom of the probe, 40 cm (centimeters) 

below the detector. After field data are read into 

the computer from magnetic tape, the program 

compares the calibration check count rates with 

count rates known to be valid from previous calibration runs to assure that the probe sensitivity 

was within statistically acceptable limits when the 

field measurements were made. Then the logs are
scanned and invalid or redundant data points are 

deleted. Depth corrections are made for the different vertical positions of the two detectors on the 

probe, and for any changes that might be needed 

in the zero-depth reference point (such as casing 

collar versus ground level). New arrays at evenly 

spaced depth intervals are generated and deadtime corrections are applied using values of dead 

time determined by the two-source method (Scott, 

1980). Dead-time corrections are made rapidly by

use of matrix arithmetic operations available in 

the BASIC programming language. Only three 

program steps are needed to correct an array of 

observed count-rate values X, for dead time, T:

1.

2.

3.

When step 3 is executed, the corrected count rates 

are stored in array X. These matrix statements

MAT X =

MATX = X-(T), and

MAT X =

100

10

1.0

0.1

1000 2000 3000 

NEUTRON RESPONSE, IN API UNITS

4000 5000

FIGURE 2. Graph showing calibration information used to transform neutron API (American Petroleum 

Institute) units to porosity index, for holes of various diameters in inches (in.) and centimeters (cm).
are equivalent to the much slower FOR-NEXT 

loop evaluation of the standard form of the deadtime correction formula:

X(n=X(n/(l-X(D*T)

Corrections for down-hole dividers that reduce the 

observed count rate by dividing by 2, 4, or 8, are 

also made at this time. After these corrections are 

made the user can plot, edit, apply a smoothing 

filter to reduce statistical noise, and then re-record the log data on an output tape in units of 

counts per second.

In the next section of the program, the user 

can convert the gamma-ray log values from units 

of counts per second to API (American Petroleum 

Institute) units or to eU308 grade (percent), and 

the neutron-neutron log values from counts per 

second to API units or neutron porosity index 

(percent). Transformation of the gamma-ray and 

neutron-neutron logs to API units is accomplished 

with conversion factors established by empirical 

calibration of the logging probes in the API test 

pits at the University of Houston (American Petroleum Institute, 1974). API units are converted 

to porosity index (percent) by use of empirical 

calibration data obtained from the Birdwell-SSC 

test pits in Tulsa (Seismograph Service Corporation, 1973). Figure 2 gives an example of calibration data used to convert neutron API units to 

porosity. Because the conversion to porosity index 

depends on hole diameter, the program requires 

that the user either (1) specify a constant value 

of hole diameter, or (2) read a caliper log into the 

computer from an output tape. The program 

makes the conversion to porosity index by interpolating values between curves relating API units 

to porosity index for various values of hole diameter (see fig. 2). Transformation of the gamma-ray 

log units from counts per second to percent eU308 

is accomplished by an iterative numerical modeling 

procedure known as GAMLOG (Scott, 1962, 1963) 

for which coefficients have been derived for digitizing intervals of 0.2, 0.5, and 1.0 ft, for mudfilled and air-filled boreholes. The calibration factors relating count rate to percent eU308 (for an 

infinitely thick slab of uranium ore) and for making 

corrections for mud-filled holes of varying diameter were obtained from the U.S. Department of 

Energy-Bendix calibration pits in Grand Junction, 

Colo. (Mathews and others, 1978).

The GAMLOG procedure sometimes produces 

negative U308 grade values as the result of statistical fluctuation in count rate, particularly when

small (0.2 ft) digitizing intervals are used. An 

iterative routine is appended to the GAMLOG section of the program to test for the presence of 

negative grade values, and if any are found, to 

redistribute them along adjacent points of the log 

to avoid this physically impossible condition.

After the gamma-ray and neutron-neutron logs 

are transformed to API, eU308, or porosity units, 

the user can plot, edit, and filter the logs, and 

(or) record them on output tape.

The filtering routine that is used in this program 

is a Hamming smoothing filter described by 

Blackman and Tukey (1958) that conserves area 

under the curve. The number of points comprising 

the filter is selected by the user; values of 5-11 

have been found to be most effective in practical 

application.

Examples of unit conversion of analyzed 

gamma-ray and neutron-neutron logs are given in 

figure 3. 

2. Dual-detector density and caliper log program

This program analyzes gamma-gamma log density measurements and caliper log-hole diameter 

measurements made with a combination probe. The 

probe utilizes two collimated gamma-ray detectors 

spaced at 17 and 37 cm from a gamma-ray source 

(125 mCi Cs-137), and a single caliper arm that 

presses the detectors against the hole wall and 

measures hole diameter. The program starts by 

reading the digitized caliper and gamma-ray 

well-logging data from magnetic tape, and reading 

in calibration information that is recorded at the 

end of the logging data. Calibration information 

consists of count-rate values obtained when the 

probe is placed in density calibration test blocks, 

one aluminum and the other Plexiglas, having effective densities of 2.62 and 1.28 g/cm3 (grams per 

cubic centimeter), respectively. The program compares the calibration count rates with values obtained with the same probe previously, shortly 

after calibration runs were made in test pits at 

the Denver Federal Center, Colo. (Snodgrass, 

1976). If the observed count rates agree with the 

previously obtained values within reasonable 

limits of statistical error, the calibration coefficients obtained during the previous calibration 

runs in the test pits are used to convert log count 

rates to apparent density values. If not, the user 

is offered the opportunity of using new calibration 

coefficients that are computed from the count 

rates obtained in the aluminum and Plexiglas 

blocks.
600 6000

GAMMA RAY 

API units 

300 600

NEUTRON

API units

2000 4000

eU3 0 B GRADE 

percent

NEUTRON POROSITY 

percent

0.00

FIGURE 3. Examples of computer plots of natural gamma-ray and neutron-neutron logs: A, 

count rate, in counts per second (cps); B, American Petroleum Institute (API) units; 

C, uranium grade in percent eU308 and neutron porosity index in percent.

6
After the calibration coefficients are determined, the program searches for and removes any 

invalid data or redundant depth values. Depth corrections are then made for the offset positions of 

the various detectors on the probe and for error 

in the zero-depth reference point, if any. The 

caliper log values, digitized by a 5V2 digit DVM 

and recorded in millivolts, are converted to inches 

by fitting a polynomial curve to calibration readings stored at the end of the field-data tape. The 

order of the polynomial can be selected by the 

user. The caliper arm on the logging probe is pivoted at the top and swings out under spring pressure to make contact with the hole wall. When 

the arm is fully extended, the depth of the caliper 

measurement is shallower than when the arm is 

compressed against the probe, because of the 

geometry of the pivoting motion of the caliper 

arm. A depth correction is made by the program 

for this error by use of the following formula:

Zc = Za -(L-\/L2 -(C-D)2)

where Zc is corrected depth,

Za is apparent depth,

L is length of caliper arm,

C is caliper measurement (hole diameter), 

and

D is probe diameter.

New arrays of caliper and gamma-ray detector 

count-rate values are generated at evenly spaced 

depth intervals, and then corrections are applied 

for dead-time errors and downhole scaling of count 

rate (division by 8 in the probe). At this point 

the user can plot, edit, and filter the corrected 

near- and far-detector count rate and caliper logs, 

and (or) record them on output tape.

The next section of the program converts the 

near- and far-detector gamma-gamma logs from 

units of count rate to apparent-density (g/cm3), 

and applies hole-diameter corrections to the converted values. The user can again plot, edit, filter, 

and re-record the converted apparent-density 

data. The final section of the program applies a 

density compensation algorithm using the nearand far-detector apparent density values as input. 

The compensation algorithm and its development 

are described by Scott (1977, 1978b). After the 

compensated density values are computed, the 

user can plot, edit, filter, and record the final logs 

on output tape. This final set of corrected and 

compensated logs includes compensated density 

(g/cm3), amount of compensation (g/cm3), and

caliper (inches or centimeters). Figure 4 presents

examples of dual-detector density logs in the three

stages of analysis.

3. Magnetic susceptibility and conductivity log

programs

This program analyzes the magnetic susceptibility and conductivity logs obtained with a probe 

that makes both measurements by use of a single 

induction coil connected in a Maxwell bridge (Scott 

and others, 1981). The program reads the twochannel logging data stored on magnetic tape in 

the form of digitized voltage outputs of phase discriminators that separate the magnetic susceptibility response from the conductivity response of the 

probe. Invalid raw data values and overlapping 

depth values are removed, and depth corrections 

are made for zero reference-depth offset, if any. 

Next, a calibration factor is computed as a function of the logging sensitivity scale and a calibration check value obtained by switching a capacitor 

of known value into a loop surrounding the coil 

in the probe. The magnetic susceptibility log, initially recorded in millivolts, is converted to relative micro cgs (centimeter-gram-second) units by 

multiplication by the computed calibration factor. 

Minimum and maximum values of the converted 

magnetic susceptibility log and the conductivity 

log are determined, and appropriate plotting 

multiplication by the computed calibration factor. 

Minimum and maximum values of the converted 

magnetic susceptibility log and the conductivity 

log are determined, and appropriate plotting 

scales and ranges are computed. Next, the program prompts the user to enter measurementdrift correction tie points at depths of his choice. 

Measurement drift is caused by temperature variation of the sensing coil. If laboratory measurements of magnetic susceptibility are available from 

drill core or cuttings, these values may be used 

to establish drift-correction tie points. If not, the 

user must estimate values based on his knowledge 

of the lithology, or by arbitrarily assigning zero 

values of magnetic susceptibility to the lowest 

points along the log trace. If the drift of the log 

is linear with depth, which is usually true, only 

two tie points are needed one near the top and 

the other near the bottom of the log. If the drift 

is not linear, more tie points are needed. The drift 

correction is made by forcing the log trace to pass 

through the specified values of magnetic susceptibility at the tie points. Values in between the tie 

points are interpolated linearly if only two tie
NEAR DETECTOR 

cps

0 3000 6000 

H I | I h

200--

. 250--

300

FAR DETECTOR CALIPER

cps inches

) 3000 60

 1 1 | 1 1 

s~

> 'r~ 

1 1 T

00 cI 3 4 

 \ | 1 

- J- -

I

V

NEAR DETECTOR

g/cm 3

1 2 3 

60 Jr I I I =L

80--

FAR DETECTOR

g/cm 3 

1 2 3 4= " I " 4-

CALIPER

cm

5 7 

-L I I I I

COMPENSATED DENSITY

9/cm 3

1 2 3 

60 4= I I I =L

80--

COMPENSATION

g/cm 3

-1 0 1 

J= I I hFIGURE 4. Examples of computer plots of dual-detector density and caliper logs: A, gamma-gamma 

count rate, in counts per second (cps), and caliper in inches; B, apparent density in grams 

per cubic centimeter (g/cm3 ), and caliper, in centimeters (cm); C, compensated density.in 

grams per cubic centimeter (g/cm3), and compensation in grams per cubic centimeter (g/cm3).

8
points are used, and by use of a spline curve fitted 

to the tie points if more than two points are available. Values above and below the top and bottom

tie points are drift-corrected by linear extrapolation of data at the two tie points closest to the 

top and bottom of the log. After the drift correction is made, the user is prompted to plot, then 

to edit and (or) filter the magnetic susceptibility 

log, and to record it on output tape if desired.

The next section of the program corrects the 

magnetic susceptibility log for hole diameter, a 

correction that is established empirically by use 

of test pits such as those available at the Denver 

Federal Center, Colo. (Snodgrass, 1976). An example of empirical hole-diameter correction data 

is given in figure 5. Values of hole diameter are 

input to the computer by reading a caliper log output tape, or if a caliper log is not available, by 

entering a constant value of hole diameter from 

the keyboard.

conductivity log is plotted in relative form, and 

is not analyzed quantitively in the present program because of lack of reliable calibration information.

Figure 6 gives examples of magnetic susceptibility and conductivity logs before and after corrections for drift and hole-diameter variations have 

been applied.

4. Resistivity and single-point resistance log program

This program analyzes resistivity logs made 

with either the normal or the Wenner configuration (Dobrin, 1952) using electrode spacings of 8, 

16, 32, and (or) 64 in. (20.3, 40.6, 81.3, and (or) 

162.6 cm), and a single-point resis tance log made 

with the same probe. The program reads the fourchannel resistivity log data and the single-point 

resistance log data from magnetic tape where it 

is stored in the form of digitized voltage output 

of a four-channel solid-state resistivity module

2.5

HOLE DIAMETER, IN INCHES 

6789 10 11 12

oc 

P 2.0

3

o

UJ 

CC

cc 1.5

1.0 10 15 20 25 

HOLE DIAMETER, IN CENTIMETERS

30

FIGURE 5. Graph showing empirical hole-diameter correction data for magnetic susceptibility logs.

After the hole-diameter corrections are completed, the user can reapply the drift correction, 

and then plot, edit, filter and (or) record the 

analyzed data on output tape. This completes the 

analysis of the magnetic susceptibility log. The

(Scott and Farstad, 1977) that has been modified 

to provide single-point resistance log output on a 

fifth channel. The single-point resistance log is 

computed by dividing digitized samples of the voltage and current transmitted to the current elec-
MAGNETIC SUSCEPTIBILITY

micro cgs units 

0 20 40

CONDUCTIVITY 

relative

ai ai 

u.

OL LU 

a

200 --

250--

MAGNETIC SUSCEPTIBILITY 

micro cgs units

0 20 40

H I I 1 I I IMAGNETIC SUSCEPTIBILITY

micro SI units 

0 500

40--

co

DCat

S 60

OL 

Ul

a

80--

B

FIGURE 6. Examples of computer plots of magnetic susceptibility and conductivity logs: A, observed 

susceptibility in centimeter-gram-second (cgs) units and relative conductivity; B, susceptibility in 

centimeter-gram-second (cgs) units, corrected for drift only; C, susceptibility in International System 

of Units (SI), corrected for drift and hole diameter.

10
8" S 32" NORMAL Ra 

ohm-m

0 200 400 

I I I I I I I

16" S 64" NORMAL Ra 

ohm-m

0 200 400 

I I I I I I I

300

SINGLE POINT R 

ohms

0 300 600 

H I | I h

8* S 32" NORMAL Re 

ohm-m

0 200 400 

I I I | I I I

16" S 64" NORMAL Re 

ohm-m

0 200 400 

I I I I I I

FIGURE 7. Examples of computer plots of resistivity and single-point resistance logs: A, Apparent resistivity 

(Ra) 8-inch and 32-inch (8" & 32") normal spacings, in ohm-meters (ohm-m), for 16-inch and 64-inch (16" 

& 64") normal spacings, in ohm-meters (ohm-m), and single-point resistance (R), in ohms; B, resistivity 

corrected for borehole-fluid and hole-diameter effects (Re) for 8-inch and 32-inch (8" & 32") normal spacings, 

in ohm-meters (ohm-m), and for 16-inch and 64-inch (16" & 64") normal spacings, in ohm-meters (ohm-m).

11
trode on the logging probe. After resistivity and 

resistance logs are read into the computer, invalid 

data and overlapping depth values are removed 

and depth corrections are made. Scale factors are 

computed from the transmitter and receiver 

range- switch settings that are recorded on the 

field tape, and the four digitized voltage outputs 

from the resistivity module are converted to apparent resistivity values for the four electrode 

spacings. Appropriate scales and ranges are computed for plotting, and the user is prompted to 

plot, edit, and filter the logs and then record the 

processed logs on output tape if desired.

The next section of the program applies corrections for the presence of fluid in the borehole. The 

user is required to enter values of borehole fluid 

resistivity, probe diameter, and hole diameter 

from the keyboard. These parameters and the 

electrode spacing are used to compute the correction. If the hole diameter varies significantly, as 

is usually true, a caliper log may be read into the 

computer from an output tape to provide a value 

of hole diameter for every resistivity reading in 

the hole. The correction algorithm is based on resistivity departure curves published as a correction chart entitled, "Normal device, no invasion" 

by the Schlumberger Well Surveying Corporation 

(1949). The curves can only be expected to remove 

first-order effects, because they are based on the 

simplifying assumptions that the layer thickness 

is infinite and that there is no invasion of drilling 

fluid into the formation. The numerical algorithm 

was developed and described by Scott (1978a). 

After the correction algorithm has been applied, 

the user can again plot, edit and (or) filter the 

logs, then record them on output tape.

Figure 7 gives examples of analyzed resistivity 

and single-point resistance logs before and after 

applying corrections to the resistivity logs for 

borehole fluid resistivity and hole diameter.

5. Sonic velocity and amplitude log program

This program analyzes continuous velocity logs 

made with a two-receiver probe and a module that 

computes the reciprocal velocity (delta-0 of the 

sonic pulse traveling between the two receivers 

and measures the relative amplitude of the first 

arrival compressional-wave energy picked up by 

one or the other of the two receivers. The program reads the digitized delta-i and relative 

amplitude values from magnetic tape, edits invalid 

and redundant values, and then offers the user 

an opportunity to plot, edit, and filter the raw

data. A special filtering routine is available in this 

program for removing invalid delta-t values that 

are caused by errors in the electronic detection 

of first-arrival energy at the two receivers. This 

type of error, known as cycle skipping, is common 

in boreholes where fractured rock or severe rugosity causes poor signal-to-noise response of one or 

both of the receivers. The filtering technique 

works as follows. First, starting at the top of the 

log and working downward, a moving average is 

obtained over a specified number of points in a 

manner similar to the application of a boxcar filter, 

but with one important difference the center 

point is not included in the average. Then a test 

is made to see if the value of the center point 

deviates more than a specified amount from the 

value of the average. If the value of the center 

point is beyond the specified limit, it is considered 

to be in error because of cycle skipping, and is 

replaced by the average value of the nearest two 

valid adjacent points. This simple scheme is about 

90 percent effective in identifying and removing 

invalid data points caused by cycle skipping. In 

most applications, effective cycle-skip removal has 

been accomplished by using filter half-widths of 

5-10 points and delta-t deviation test limits of 10- 

20 microseconds/ft. After cycle-skipping errors 

have been removed, the log may be replotted, and 

any remaining cycle-skip points that are identified 

visually by the user can be deleted manually by 

the editing routine that is programmed for use 

with the CRT plot of the log.

The next part of the program converts the 

delta-£ values to velocity in units of either feet 

per second or kilometers per second, selectable by 

the user. After the conversion is complete, the 

user can replot, re-edit or refilter the logs with 

a Hamming smoothing filter, then record the final 

data on output tape. Figure 8 gives an example 

of a noisy delta-t log before and after cycle-skip 

filtering, and then after conversion to velocity 

units. 

6. Temperature log program

This program analyzes digitized absolute temperature logs and computes differential temperature logs from them. The program reads the absolute temperature log from magnetic tape, edits 

and removes invalid and redundant data, and then 

computes a differential temperature log using a 

depth-lag interval specified by the user, after 

which the logs may be plotted, then optionally 

edited and (or) smoothed by a Hamming filter as

12
200--

tu

UJ

&

Q

250--

300

SONIC DELTA-T 

micro sec/ft

100 

H I H

200

AMPLITUDE 

relative

50 100 

H I I I I I I h

SONIC DELTA-T 

micro sec/ft

100 

H \ h

200

200--

n:

Q

250

300

AMPLITUDE 

relative

0 50 100 

H I I I I I I I h

5000

UJ

60--

70

80 +

90--

SONIC VELOCITY 

feet/sec 

15000 

H I 

25000

SONIC VELOCITY 

km/sec

5 

I I I IFlGURE 8. Examples of computer plots of sonic velocity and amplitude logs: A, Unfiltered sonic 

transit time (delta-i), in microseconds per foot (micro sec/ft), and relative amplitude; B, Filtered sonic transit time (delta-i) in microseconds per foot (micro sec/ft), and smoothed relative 

amplitude; C, Filtered transit time data converted to sonic velocity in feet per second (ft/sec) 

and kilometers per second (km/sec).

13
desired. The user may record both the absolute 

and differential temperature logs on output tape 

after any stage of editing and smoothing. Figure 

9 gives an example of an absolute temperature log 

and the corresponding computed differential temperature log.

7. Induced polarization, resistivity, and self-potential log program

This program applies corrections and plots logs 

made with a digital IP (induced polarization) measurement system. In addition to the IP log, resistivity and self-potential values are recorded on

TEMPERATURE

degrees C

35 50

CO 

CCtu

Q_tu

O

700

800 

900 

1000 

1100 

1200 

1300 

1400

1500 

1600

-1

DIFFERENTIAL TEMP 

degrees C

0

 I 

FIGURE 9. Example of a computer plot of an absolute temperature log in degrees centigrade (C), and differential temperature (temp) log in degrees centigrade (C), computed at a depth lag interval of 1 ft.

14
magnetic tape as subsidiary logs because these 

measurements are needed by the logging module 

to compute IP values electronically. The program 

reads the field logs from magnetic tape, culls out 

invalid and redundant data, and then applies scale 

factors computed from module range-switch settings and electrode spacings. The program 

prompts the user to select options for plotting, 

editing, and filtering the data, then recording the 

reduced data on output tape if desired. An example of a plot of IP, resistivity, and self-potential 

logs is given in figure 10.

IP (16" Normal) 

»V/V

0 30 60 

H I I

30--

cn

CE 

UJ

X 

Q.

RESISTIVITY (16" N) 

ohm-m

0 200 400 

I I I | I I I

70--

80--

FIGURE 10. Example of a computer plot of an induced polarization (IP) log in millivolts per volt (mV/V), resistivity, for 

a 16-inch normal (16"N) electrode configuration, in ohm-meters (ohm-m), and self-potential (SP) in millivolts (mV).

15
SUMMARY AND CONCLUSIONS REFERENCES CITED

Interactive computer programs for analyzing digital well logs have been developed by the U.S. 

Geological Survey for use with a research well-logging system. The programs are implemented on 

a small computer that is installed in the field logging truck, making it possible and practical to 

analyze field logs at the well site if desired. The 

programs provide the user with the capability of 

applying corrections for changes in calibration, for 

applying digital smoothing filters, for correcting 

measurement drift, and for editing data displayed 

on the computer CRT. The programs are easily 

modified to accomodate unique conditions that are 

apt to occur in research logging. Computation is 

staged in such a manner that the effects of transformation from observed measurement units to 

computer units, and the effects of filtering and 

editing may be observed by making successive 

plots on the computer CRT. The user has the option of making paper copies of the CRT plots 

whenever he desires to do so. Plotting units and 

scales for depths and log parameters are flexible 

and can be changed by the user to fit his or her 

needs, whether they be to obtain a quick plot of 

data analyzed in the field or to make a finished 

copy for publication. Analyzed logs may be recorded on output magnetic tapes and these tapes 

may be read back by the computer program for 

reanalysis and replotting at any future date.

Experience in using these analytical programs 

on a small desktop computer, in contrast to using 

similar programs on a large timeshare computer, 

indicates that the desktop computer installed in 

the research logging truck offers the following advantages:

1. The desktop computer has turned out to be 

more reliable and has had far less "down" 

time than the large timeshare computer.

2. With greater flexibility and facility for 

operator interaction, the desktop computer 

programs yield better results with less user 

time and effort.

3. Analysis is possible and practical at the well 

site.

4. A cost reduction of about 50 percent per 

analyzed log is realized with the desktop 

computer as compared with the timeshare 

computer if the purchase and maintenance 

costs of the desktop computer are spread 

out over a 5-year period.

American Petroleum Institute, 1974, Recommended practice 

for standard calibration and format for nuclear well logs: 

Dallas, Tex., American Petroleum Institute, API RP 33, 

13 p.

Best, D. L., Gardner, J. S., and Dumanoir, J. L., 1978, A 

computer-processed wellsite log computation: SPWLA (Society of Professional Well Log Analysts) 19th Annual Logging Symposium, June 13-16, 1978, El Paso, Tex., Transactions, p. Z1-Z32.

Blackman, R. B., and Tukey, J. W., 1958, The measurement 

of power spectra: New York, Dover Publications, p. 95- 

100.

Dobrin, M. B., 1952, Introduction to geophysical prospecting: 

New York, McGraw-Hill, p. 372-376.

Kinkade, R., West, J., and Hallenburg, J., 1978, The CompuLog concept and practice: SPWLA 19th Annual Logging 

Symposium, June 13-16, 1978, El Paso, Tex., Transactions, p. S1-S9.

Mathews , M. A., Kiozumi, C. J., and Evans, H. B., 1978, 

DOE-Grand Junction logging model data synopsis: U.S. 

Department of Energy, Bendix Field Engineering Corporation report GJBX-76(78), 52 p.

Moseley, L. M., 1976, Field evaluation of direct digital well 

logging: SPWLA 17th Annual Logging Symposium, June 

9-12, Denver, Colo., Transactions, p. NN1-NN11.

Schlumberger Well Surveying Corporation, 1949, Resistivity 

departure curves: Schlumberger Well Surveying Corporation, p. 9.

Scott, J. H., 1962, The GAMLOG computer program: U.S. 

Atomic Energy Commission Report RME-143, 43 p.

Scott, J. H., 1963, Computer analysis of gamma-ray logs: 

Geophysics, v. 28, no. 3, p. 457-465.

 1977, Borehole compensation algorithms for a small-diameter, dual-detector density well-logging probe: SPWLA 

18th Annual Logging Symposium, June 5-8, 1977, Houston, Tex., Transactions, p. S1-S17.

 1978a, A FORTRAN algorithm for correcting normal 

resistivity logs for borehole diameter and mud resistivity: 

U.S. Geological Survey Open-File Report 78-779, 12 p.

 1978b, A computer program for borehole compensation 

of dual-detector density well logs: U.S. Geological Survey 

Open-File Report 78-515, 7 p. 

-1980, Pitfalls in determining the dead time of nuclear

well-logging probes: SPWLA 21st Annual Logging Symposium, July 8-11, 1980, Lafayette, La., Transactions p. 

H1-H11.

Scott, J. H., and Farstad, A. J., 1977, Electrical resistivity 

well-logging system with solid-state electronic circuitry: 

U.S. Geological Survey Open-File Report 77-144, 24 p.

Scott, J. H., Seeley, R. L., and Earth, J. J., 1981, A magnetic 

susceptibility well-logging system for mineral exploration: 

SPWLA 22d Annual Logging Symposium, June 23-26, 

1981, Mexico City, Hex., Transactions, p. CC1-CC21.

Seismograph Service Corporation, 1973, Birdwell builds pits, 

clients benefit: SEIS News, January-February, 1973, 4 p.

Snodgrass, J. J., 1976, Calibration models for geophysical 

borehole logging: U.S. Bureau of Mines Report of Investigations 8148, 21 p.

16

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