Bureau of Mines Report of Investigations/l976 

Use of Surface Joint and Photolinear 

Data for Predicting Subsurface 

Coal Cleat Orientation 

UNITED STATES DEPARTMENT OF THE INTERIOR 
Report of Investigations 8120 

Use of Surface Joint and Photolinear 

Data for Predicting Subsurface 

Coal Cleat Orientation 

By W. P.'Diamond, C. M. McCulloch, and B. M. Bench 

Pittsburgh Mining and Safety Research Center, Pittsburgh, Pa. 

UNITED STATES DEPARTMENT OF THE INTERIOR 

Thomas S. Kleppe, Secretary 

BUREAU OF MINES 

Thomas V. Falkie, Director 
This publication has been cataloged as follows: 

d 

Diamond, W P 

Use of surface joint and photolinear data for predicting 

subsurface coal cleat orientation, by W. P. Diamond, C. M. 

McCulloch, and B. M. Bench. [Washington] U.S. Bureau of 

Mines C1976I 

13 p. illus., tables. (U.S. Bureau of Mines. Report of investigations 8120) 

Includes bibliography. 

1. Coal mines and mining. I. McCulloch, Charles M., jt. auth. 

11. Bench, Bernard M., jt. auth. 111. U.S. Bureau of Mines. IV. 

Title. (Series) 

TN23.U7 no. 8120 622.06173 

U.S. Dept. of the Int. Library 
CONTENTS 

Page 

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

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

Determination and analysis of cleat orientation .......................... 2 

Techniques for prediction of cleat orientation .. ......................... 5 

Analysis of regional directional data .................................... 5 

Analysis of local directional data ....................................... 9 

Criteria for cleat prediction ............................................ 11 

Conclusion ............................................................... 11 

References ............................................................... 13 

ILLUSTRATIONS 

1 . Plan view of directional permeability due to cleat orientation ....... 2 

2 . Structure. cleat orientation. and mine locations in study area ....... 3 

3 . Cleat orientation. Osage No . 3 mine. Monongalia County. W . Va ........ 4 

4 . Composite rose diagram of cleat orientation .......................... 4 

5 . Cleat orientation. Blacksville No . 1 mine. Greene County. Pa . . and Monongalia County. W . Va ....................................... 5 

6 . Composite rose diagram of principal surface joint trends ............. 6 

7 . Composite rose diagram of principal infrared photograph 

photolinears ....................................................... 6 . 

8 . Composite rose diagram of principal photoindex photolinears .......... 7 

TABUS 

1 . Principal directional sets in order of dominance ..................... 7 

2 . Fundamental regional systems from directional data ................... 8 

3 . Coal cleat. duminant surface joint. and photolinear orientations of 

the study area ..................................................... 10 
USE OF SURFACE JOINT AND PHOTOLINEAR DATA FOR PREDICTING 

SUBSURFACE COAL CLEAT ORIENTATION 

by 

Wo Po Diamond, 1 C, Mo McCul loch, 1 and Bo Me Bench 1 

ABSTRACT 

Coalbeds exhibit a directional permeability, with the maximum permeability oriented parallel to the face cleat. Thus, the most efficient pattern of 

vertical degasification holes for a coalbed is dependent upon cleat orientation. Surface joints, infrared photolinears, and Ronchi grating photolinears 

were investigated as potential estimators of subsurface cleat orientation. 

Cleat orientations were masured in 18 mines, in the Pittsburgh coalbed in 

southwestern Pennsylvania and northern West Virginia to be used as a standard 

against which prediction techniques would be evaluated. Two cleat systems 

with a strong geographic segregation were measured in the study area: 

N 67" W - N 28" E (95" separation) for the north, and N 76" W - N 17" E 

(93" separation) for the south. 

Directional data obtained from the three techniques investigated yielded 

satisfactory regional predictions of cleat orientation. Analysis of surface 

joints was the only technique considered sufficiently reliable on a local 

basis. Prediction of cleat orientation is based on the pairing of the principal directional sets into all reasonable combinations of fundamental systems. 

The system or systems composed of the most dominant sets nearest to 90" separation are likely to be a reliable prediction of cleat orientation. The most 

dominant set of the system selected is likely to be the face cleat. 

INTRODUCTION 

Cleat is the naturally occurring vertical fracture in coal. Two cleat 

sets are cmonly oriented perpendicular to each other to form a fundamental 

system (&).2 The more dominant fracture is the face cleat, which cuts across 

bedding surfaces in the coal and may extend for many feet horizontally. The 

butt cleat is a short, poorly developed fracture that commonly is truncated 

by the face cleat. 

l~eologist. 

a~nderlined numbers in parentheses refer to the list of references at the end 

of this report. 
1- 

(Butt cleat , , /Face 

II 

cleat 

I It has been demonstrated by the Bureau of 

1 Mines (L) that coalbeds 

exhibit a directional permeA- Degasificat~on ability directly related to 

hole ---- I -J-1 

-L---- the cleat system. Degasi- I I -r Drainoge ellipse f ication experiments con- 

/ ' - A,' , _i_. ducted underground in coal 

- I -- I 1 I I-- I I mines have shown that 

horizont a1 holes drilled 

perpendicular to, and thereFIGURE 1. - Plan view of directional permeobi lity due fore intersecting, the 

to cleat orientation. largest number of face 

cleats will yield 2.5 to 10 

times asmuch gas as holes drilled perpendicular to the butt cleat. This 

observation can in turn be related to the placement of vertical degasification 

holes drilled from the surface into a coalbed. A vertical hole into a coalbed 

with a we 11-developed face and butt cleat (for example, the Pittsburgh coalbed) 

will collect gas at a higher rate from the face cleat direction than from the 

butt cleat direction. Therefore, an elliptical drainage pattern (fig. 1) will 

be developed with the long axis parallel to the face cleat. 

To determine the most efficient pattern for vertical degasification holes 

in an area of virgin coal, the subsurface cleat orientation must be determined, 

In conjunction with an extensive investigation of the Pittsburgh coalbed in a 

39-quadrangle area in southwestern Pennsylvania and northern West Virginia, a 

study was undertaken to evaluate several techniques for estimating cleat orientation. The area of study was particularly well suited for the proposed 

investigation owing to the well-developed cleat system of the Pittsburgh coal 

and the numerous underground mines, which provided excellent control data on 

cleat orientation. 

DETERMINATION AND ANALYSIS OF CLEAT ORIENTATION 

Cleat orientations were measured in 18 mines operating in the Ptttsburgh 

coalbed (fig. 2). Data were plotted on a semicircular rose diagram for evaluation of cle'at orientation for each mine. A single, well-defined cleat system, such as for the Osage No. 3 mine (fig. 3), was typically observed. The 

mean face and butt cleat orientation of each mine was plotted on a composite 

rose diagram (fig. 4) to demonstrate the regional trends. 

The composite rose diagram (fig. 4) indicates the presence of two nearly 

orthogonal cleat systems. The two fundamental cleat systems calculated for 

the study area are N 67" W - N 28" E (95" separation), and N 76" W - N 17" E 

(93" separation). The first system is composed of the readings from mines 

located in the northern part of the study area (Nos. 1-7, fig. 2) plus one 

mine (No. 9) more closely associated geographically with the southern part. 

The second system is composed of readings from the mines (Nos. 8 and 10-18) 

located in the southern part of the area. 
FIGURE 2. - Structure, cleat orientation, and mine locations in study area. 
Face cleat 

N 

I Butt cleat T 

250 readings total 

0 

u 

20 

Scale of reodings 

FIGURE 3. - Cleat orientation, Osage No. 3 mine, 

Monongalia County, W. Va. 

N 

4 Butt cleat 

Face 

cleat 

Nickelsen and Hough (4) , in their study of jointing 

in the Appalachian Plateau 

of Pennsylvania, reported 

similar segregations, concluding that there existed 

a "number of locally dominant trends. " They also 

noted an "overlap of trends 

dominant in adjacent local 

areas and that isolated 

occurrences of any given 

joint set may occur far from 

the area of its greatest 

dominance. " The latter conc lusion was observed for the 

Humphrey No. 7 mine (No. 9), 

which was geographically 

within the southern group of 

mines, but the cleat orientation was similar to those 

of the northern mines. The 

Blacksville No. 1 mine 

(No. 10) to the west of the 

Humphrey No. 7 mine has a 

bimodal face cleat peak 

(fig. 5). The most dominant 

face cleat, N 75" W, is 

similar to that of the 

southern group of mines 

where the mine is geographically located. The secondary face cleat, N 63" W, is 

similar to that observed in 

36 readings total 0 

U 

10 the northern group of mines. 

The Pursglove No. 15 mine 

Scale of readings (No. 12), located adjacent 

to mines 9 and 10, has a 

FIGURE 4. - Composite rose diagram of cleat orientation. suggestion of a bimodal face 

and butt cleat similar to 

that of the Blacksville No. 1 mine. However, the separation of the peaks is 

not considered sufficient to calculate two separate fundamental systems. 

The separation of the cleat readings into two distinct systems indicates 

approximately a 10" rotation of the cleat system, clockwise northward through 

the area. The rotation of the cleat system is similar to the change in trend 

observed for the fold axis of the area (fig. 2). The butt cleats are essentially parallel to the axial trends of folds, and the face cleats are perpendicular to them. It has been proposed by Nickelsen and Hough (5) and reiterated by McCulloch and others (2) that the face cleats were formed as 
N Butt cleat 

Face 

cleat 

I 

250 readings total 0 - 20 

Scale of read ings 

FIGURE 5. - Cleat orientation, Blacksvil le No. 1 mine, 

Greene County, Pa., and Monongal ia 

County, We Va. 

extension fractures oriented 

parallel to the greatest 

principal compression stress 

axis that produced the 

area's folds. The origin of 

butt cleats is obscure, but 

they are possibly release 

fractures formed perpendic - 

ular to the greatest principal compressionalstress axis 

during erosion and uplift. 

If this is true, then rocks 

other than coal potentially 

have similar joint orientation and dominance, hence 

the study of surface joints 

and photolinears to determine if they can be used to 

predict cleat orientation in 

virgin coal areas. 

TECHNIQUES FOR PREDICTION OF CLEAT ORIENTATION 

Three techniques were imrestigated for possible use in cleat prediction. 

Surface joints were measured for each of the 39 quadrangles of the study area. 

A minimum of 10 sample locations per quadrangle; with at least 10 readings per 

location, were obtained. Joints of varying prominence were measured in all 

lithologies in varying bed thicknesses to obtain a broad estimate of joint 

sets present in each quadrangle. Infrared aerial photographs were examined 

stereoscopically for linears. Owing to the masking effect of the urban and 

mining culture on the area, photolinear interpretation was based predominantly 

on straight water courses, stream valleys, and abrupt right-angle bends of 

streams. Thirteen photoindex sheets covering the study area were inspected 

for photolinears using a Ronchi defraction grating. This technique (5) combines many indistinguishable small parallel linear features into a more readily observable directional trend. Because the scale of the photoindex sheets 

is 1 inch equals 1 mile, only very general trends can be differentiated; 

individual linears, which could be seen with the infrared photographs at a 

scale of 1 inch equals 2,000 feet, could not be identified. 

ANALYSIS OF REGIONAL DIRECTIONAL DATA 

For the three techniques investigated, the directional data were originally compiled and plotted on rose diagrams for individual 7-1/2-minute quadrangle or photoindex sheet areas. Peaks were defined by comparing the number 

of readings comprising each high point on a rose diagram with the number of 

readings in the adjacent 5" intervals. A decline of one-third the number of 

readings comprising a high point was used to define the limits of an individual peak, with the first 5' interval containing less than two-thirds of the 

high point not being included within the peak. However, if the first 5O 

interval to decline one-third the number of readings, and the next adjacent 
difference of 10 percent in 

the number of readings is 

0 10 considered sufficient to 

233 readings total I assign different orders of 

Scale of readings dominance. 

FIGURE 7, - Composite rose diagram of ~rincipal infrared Trends established by photograph photolinears. the Ronchi inspection of 

photoindex sheets could not 

be assigned meaningful orders of dominance based on the composite rose diagram. This is the result of a small number of readings and the high degree 

of similarity between trends obtained from all photoindex sheets, which precludes the enhancement of one trend over another. However, it was possible to 

assign orders of dominance to the'data based on a visual impression of image 

"strength" as each index sheet was viewed through the Ronchi grating. 
N 

The principal regional 

sets of table 1 are paired 

into combinations of fundamental systems in table 2. 

Two surface joint systems (Nos. 2 and 4) are composed only of joint sets of 

the first order of dominance. 

Both systems are composed of 

the N 15" E' set. System 4, 

with a 91" separation, more 

closely approaches the ideal 

0 10 fundamental system's 90" 104 readings total - separation than does sysScale of readings tern 2, which has a 72" 

separation. System 4 

FIGURE 8. - Composite rose diagram of principal photo- (*N 76" W' - N 15" E' ) is 

quite similar in orientation index ineors. to the *N 76" W' - N 17" E' 

cleat system established 

underground. The N 76" W' set of system 4 is the most dominant set of system 4, and it parallels the face cleat. The N 76" W' set could also be paired 

with the N 27" E" set (system 5), but the separation of 103" is greater and 

the N 27" E" set is less dominant than the N 15" E' set. 

TABLE 1. - Principal directional sets in order of dominance 

NOTE.--Sets of equal dominance are listed in the same column of a data source. 

Data source 

Surface joints ................... 

Photolinears (TR photographs) .... 

Photolinears (photoindex sheets). 

Surface joints ................... Photolinears (IR photographs). ... Photolinears (photoindex sheets). 

West 

I 

N 57" W 

N 76" W 

N 65" W 

- 

N 70" W 

I1 

- 

- 

N 25" W 

- 

N 44" W 

East 

I 

N 15" E 

N 20" E 

N 27" E 

v 

- 

- 

N 13" W 

- 

- - 

I11 

N 34" W, 

- 

N 79" W 

N 40" W 

N 23" W 

IV 

- 

- 

- 

- 

N 09" W 

I1 

N 27" E 

N 01" E 

N 77" E 

I11 

N 37" E 

N 75" E 

N 12" E 

IV 

N 47" E 

- 

N 55" E 
TABLE 2. - Fundamental regional systems from directional data 

NOTE. --Superscripts indicate the order of dominance within the directional 

group (east or west) for each data source, and an asterisk indicates 

the dominant trend of each system. 

Data source 

Cleat : 

1. .................................... 2. .................................... 

Surface joints : 

1. .................................... 2..................................... 

3. .................................... 4... .................................. 5. .................................... 6 ..................................... 

Photolinears (IR photographs): 

1 ................................... .. 2 ..................................... 

3 ..................................... 

4 ..................................... 

5 .................... ... .............. 6 ............................. .. ..... 

Photolinears (photoindex sheets): 

1.. ............................... .... 2 ..................................... 

3 ..................................... 

4 ...................................... 

5 ...................................... 

The N 57" W' set, if not paired with N 15O E' , can be paired with 

N 27" E" (system 1) for an 84O separation or with N 37" El ' ' (system 3) for 

a 94" separation. System 1 (*N 57" W' - N 27" El') is reasonably similar to 

the *N 67" W' - N 28" E' cleat system established underground. System 3 

(*N 57" W' - N 37" E' ' ' ) , though not substantially dissimilar from the 

*N 67" W' - N 28" E' ' cleat system, is, however, composed of one set of the 

third order of dominance. The *N 57" W' set of systems 1 and 3 is the most 

dominant set of the systems and is reasonably similar to the face cleat 

orientation. 

The first infrared photolinear system (*N 65" W' - N 20" E' ) is composed 

of the most dominant westerly set and the most dominant easterly set. This 

system best estimates the established *N 67" W' - N 28" E' coal cleat system. 

The *N 65" W1 set is the most dominant of the system, and it nearly parallels 

the face cleat. The N 20" E' set, if not paired with N 65" W' , could be 

paired with N 79" W' " (system 2), which is similar to the +N 76" W' -N 17" E' 

Degrees of 

separation 

95 

93 

84 

72 

94 

91 

103 

81 

85 

99 

80 

60 

100 

88 

97 

82 

100 

99 

86 

West 

*N 67" w1 

*N 76" WI 

*N 57" w1 

*N 57" W~ 

*N 57" W1 

*N 76" wt 

*N 76" wr 

*N 34" wlir 

*N 65" W' 

N79"~"~ 

N79"~'~~ 

N40°~'ft 

*N25"wn 

N13"WV 

*N 70" Wf 

*N 70" W' 

N23"W1" 

*N44"w1' 

N09"~'" 

System 

East 

N 28' E' 

N 17' E' 

N 27" E" 

N 15' E' 

N 37" EN' 

N 15" E' 

N 27" E" 

N 47" ErV 

N 20" E' 

*N20°Et 

*NOlOErl 

*N20°Ef 

N75"~*~' 

*N75"E1" 

N 27" E' 

N 12" EII1 

*N77"E" 

N55"EIv 

*N77"Er1 
cleat system. However, photolinear system 2 contains one set of the third 

order and has a larger divergence from 90' than does system 1. The N 79" W" ' 

set can also be paired with N 01" E' ' (system 3) for an 80' separation. This 

system, while having one set (*N 01' E" ) that diverges 16" from the butt 

cleat of the *N 76' W' - N 17" El cleat system, has one set (N 79" W' ' ' ) that 

only diverges 3" from the face cleat. However, system 3 is composed of one 

set 02 the third order, and more significantly the most dominant set of 

the system *N 01" E' ' is in the butt cleat direction. The fourth, fifth, and 

sixth photolinear systems are each composed of one component of third or lower 

order of dominance and have no similarity to the established cleat systems. 

The first photoindex system (*N 70" W' - N 27" E' ) is composed of the two 

most dominant sets. This system is similar to the *N 67" W' - N 28" E' cleat 

system. The *N 70" W' set is the most dominant of the system, and parallels 

the face cleat. The *N 70" W' set could additionally be paired with N 12"~"' 

set to approximate the *N 76' W' - N 17" E' cleat system. This second system, 

however, contains one component of the third order. The remaining three photoindex systems are composed of sets of second and lower orders of dominance and 

are not similar to the established cleat systems. 

ANALYSIS OF LOCAL DIRECTIONAL DATA 

The relationship between coal cleats measured in individual mines and 

the directional trends established by measuring joints and photolinears 

directly overlying the mines is given in table 3. Surface joints provided the 

best estimator of cleat, averaging within 27.4" of the face cleat and 56 -6" 

of the butt cleat. It was possible to correctly predict the face cleat 

direction for 14 of the 18 mines using the surface joint data. The photolinear data were considerably less reliable, with averages of '23.7" for the 

face cleat and '19.4" for the butt cleat. The face cleat direction was correctly predicted in only 7 of the 18 mines. 

The high degree of divergence observed for the photolinear analysis is 

primarily the result of a dominant westerly photolinear set in the vicinity 

of N 15" - 30" W. This is similar to the *N 25" W" set observed on the 

photolinear composite rose diagram (fig. 7), and the N 34" W' ' ' set on the 

surface joint composite rose diagram (fig. 6). Lattman and Nickelsen (2) , in an investigation of the correlation between surface joints and photolinears 

in a small (6-square-mile) area of central Pennsylvania, determined that even 

though the face cleat generally correlated with the dominant surface joint and 

photolinear sets, coal cleat orientations were not necessarily reflected in 

surface joints in shale and sandstones. It was also found that the joints in 

shales and sandstones produced the strongest photolinears. It seems possible 

that in light of these observations, a surface joint trend that is not parallel to a cleat orientation may be intensified as a photolinear in local 

areas. 
TABLE 3. - Coal cleat, dominant surface joint, and photolin

orientations of the study area 

No. I Mine 

Face 

cleat 

1 

2 

3 

4 

5 

6 

7 

8 

9 

10 

11 

12 

13 

14 

15 

16 

17 

18 

- 

l 

Butt 1 S;w:;ce 

cleat 

Montour 4.. ........ Mathies. ........... Westland..... ...... Somerset No. 60.. . . Vesta No. 5. ....... Marianna No. 58.... 

Gateway.. . . . . . . . . . . Shannopin .......... Humphrey No. 7..... 

Blacksville No. 1.. 

Blacksville No. 2.. 

Pursglove No. 15. . . Osage No. 3.. . . . . . . Arkwright... . ..... . Federal No. 2. . . . . . Consol No. 93... . . . Loveridge ......... . Consol No. 20 .... .. Average divergence. 

Insufficient data. 

joints 

East 

*N 37" E 

N 27" E 

N 23" E 

N 27" E 

N 14" E 

N 12" E 

N 13" E 

N 23" E 

N 20" E 

N 27" E 

*N 13" E 

N 17" E 

N 17" E 

N 19" E 

N 28" E 

*N 17" E 

N 17" E 

a Indeterminate. 

Degrees of 

divergence 

West 

10 

5 

3 

3 

10 

4 

10 

7 

10 

9 

12 

6 

7 

3 

9 

7 

5 

13 

7.4 

We

NOTE.--An asterisk indicates the dominant set of each system. If no asterisk 

of equal dominance, 

East 

12 

0 

2 

3 

16 

16 

15 

5 

7 

7 

4 

2 

0 

7 

11 

1 

2 

9 

6.6 
Analysis of directional data for a limited area is hampered by problems 

not nonnally encountered in a regional approach. Surface joint measurenrents 

are limited by the number and location of outcrops in the areas of interest. 

As outcrops becoxre less numerous and farther away from the area, the validity 

of the analysis decreases. Under these conditions, photointerpretation would 

perhaps be beneficial because it does not rely on outcrops far data. However, 

photoanalysis, which depends on the availability of quality photographs and 

an experienced photointerpre ter , was signif ic.ant ly less reliable than field 

measurements of joints in the area investigated. Both methods are limited by 

the small number of directional measurements usually obtainable for a small 

area. With small samples, individual trends stand out less clearly, which 

makes analysis difficult. 

CRITERIA FOR CLEAT EXEDICTION 

The pairing of the established sets into all reasonable cambinations of 

fundamental systems is of primary importance in analysis of each type of directional data. Which system or systems are to be used to predict the cleat 

orientations is based on the relative dminance of the sets of each system as 

determined by the number of readings comprising the corresponding peaks on 

rose diagrams. A sys teln composed of the most dominant set and a perpendicular 

set of the first or second order of dominance will most likely give the best 

estimation of cleat orientation. The most dominant set of the selected system 

is most likely the face cleat di-rection. 

Cleat systems of similar, but different, orientation may be present. 

Here, two regional fundamental joint systems were in fact composed of sets of 

the required orders of dorninance and correlated with the observed cleat 

systems. In the analysis of linears from infrared photographs and linears on 

photoindex sheets, one system satisfied the dminancy requirements and was 

correlative with one cleat system, and other sys tems were reasonably correlative with the remaining cleat system. But these latter systems would be 

rejected because one component lacks the necessary relative dominance. The 

rejection of an estimator for the second cleat system is not detrimental since 

the two cleat systems are separated by only 10". The directional 

systems not correlative with established cleat systems were rejected i n a l l 

cases because they did not meet the dominance requirements. 

CONCLUSION 

The regional orientation of subsurface cleat systems in southwestern 

Pennsylvania and northern West Virginia is reflected in the fracture trends of 

other rock types. An analysis of the directional trends of surface joints, 

linears determined from examination of infrared photography, and Pinears 

observed on photoindex sheets using a Ronchi grating should provide a reliable 

estimate of cleat orientation in areas of virgin coal where similar geologic 

conditions exist. The procedure depends upon assigning a r~lative dominance 

to the princTpal directional sets obtained by the three methods and pairing 

them into fundamental systems. The analysis of surface joints is the only 

technique considered sufficiently reliable to be used on a local basis, but 

even this may fail if there is a strong divergence between cleats in coal and 
joints in the overburden. Reliable cleat prediction for a small area depends 

on the measurement of enough linear features to permit differentiation of 

individual trends. 

Even though inspection of photoindex sheets with a Ronchi grating did 

allow prediction of one of the regional cleat systems, less confidence is 

placed on this technique than in the analysis of surface joint data and 

infrared photolinears. This is because the dominance of Ronchi grating trends 

is based on a visual impression of image "strength" as opposed to a more 

analytical approach for the other techniques. 
REFERENCES 

1. Kissell, F. N. The Methane Migration and Storage Characteristics of the 

Pittsburgh, Pocahontas No. 3, and Oklahoma Hartshorne Coalbed. 

BuMines RI 7667, 1972, 22 pp. 

2. Lattman, L. H. , and R. P. Nickelsen. Photogeologic Fracture-Trace Mapping 

in Appalachian Plateau. AAPG Bull. , v. 42, 1958, pp. 2238-2245. 

3. McCulloch, C. M. , M. Deul , and P. W. Jeran. Cleat in Bituminous Coalbeds. 

BuMines RI 7910, 1974, 25 pp. 

4. Nickelsen, R. , and V. D. Hough. Jointing in the Appalachian Plateau of 

Pennsylvania. Geol. Soc. Am. Bull., v. 78, 1967, pp. 609-630. 

5. Pohn, H. A. Analysis of Images and Photographs by a Ronchi Grating. 

Remote Sensor Application Studies Progress Report, July 1, 1968, to 

June 30, 1969, 9 pp. f available from National Technical Information 

Service, Springfield , Va . , PB 197 108.