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Computer characterization of aspects of grainedge roughness using the scanning electron microscope
Marine Geology, 74 (1987) 291-294
291
Elsevier Science Publishers B.V., Amsterdam-- Printed in The Netherlands
Short Note
COMPUTER CHARACTERIZATION OF ASPECTS OF GRAINEDGE ROUGHNESS USING THE SCANNING ELECTRON MICROSCOPE A.T. WILLIAMS, R.G. MILES and G. TOUGH Coastal Research Unit, Science Department, Polytechnic of Wales, Pontypridd, Mid-Glarnorgan, Wales CF37 1DL (U.K.)
(Received May 13, 1986; revised and accepted July 23, 1986)
The abundance/absence of various quartz grain surface features is fundamental to any SEM analysis involving reconstruction of geological environments. For the many derived features t h a t can be seen on a quartz grain surface, "checklists" galore exist (Krinsley and Donahue, 1968; Cater, 1984) but the validity of specific examination techniques is still a matter of debate. One of the most extremely important mechanically derived parameters for this type of analysis is edge abrasion as roundness values are a dominant morphological feature found on quartz sand grains. For example, Goudie and Bull (1984) found that the main difference between two types of African sedimentary deposits was a total absence of edge-abraded grains in deeply weathered profiles, and on this basis compiled a slope evolution model for the area concerned. A commonly applied technique for grainslope analysis is t h a t suggested by Ehrlich and Weinberg (1970). The method approximates the closed curve by a Fourier series of degree twenty. The grain shape is then said to be represented by the twenty Fourier coefficients, with the leading coefficient set to unity. If the curve is reconstructed from these coefficients it is seen t h a t it gives a smooth representation of the boundary, reducing the severity of angular change and perhaps removing small nicks and abrasions which may be significant. Conse0025-3227/87/$03.50
quently the technique gives a global impression of the shape. The technique discussed in this paper specifically looks for small local characteristics, which accumulate to classify the grain. In particular it highlights a useful means of determining accurate numbers for grain-shape angle change regions - - a function of edge abrasion - - which eliminates the subjective approach to this problem. SEM grain-shape analyses (Figs.1 and 2) information were inputed into a computer using a digitiser so t h a t the grain edge was represented by a set of coordinate pairs which were processed in a two-stage manner. Initially the data was preprocessed to determine regions of interest and then each of these regions was examined in more detail. Since the data expressing a grain edge is composed of a large number of coordinate pairs, a simple preprocessor is used which can easily eliminate points of no interest (those on "smooth" sections of the curve where "smooth" is a definable angular quantity - - see below) and so dramatically reduce the number of points that must be considered in more detail. The same spacing of the digitised points is used for each grain. To preprocess the information, an angular check is applied to triples of coordinate pairs. If the resulting angle does not exceed four fifths of an arbitrary determined required
© 1987 Elsevier Science Publishers B.V.
292
Fig.1. SEM picture of a quartz sand grain from a modern beach.
a n g l e , e.g. 25 ° , t h e n t h e p o i n t is r e t a i n e d for f u r t h e r c h e c k i n g . O n c e all of the pairs h a v e b e e n p r e p r o c e s s e d t h e r e d u c e d s e t c a n be
further examined using parametric Lagrangian interpolation. Polynomials are determined independently
TABLE 1
TABLE 2
Numbers of "areas of change" for Fig.1
Number of "areas of change" for Fig.2
Specified change of edge angle (degrees)
Numbers of change areas
Specified change of edge angle (degrees)
Numbers of change areas
25 51 77 102 128 154
27 11 6 3 2 1
25 51 77 102 128 154
43 12 4 1 1 1
293
Fig.2. SEM picture of a quartz sand grain from a raised beach.
for the x and y coordinates, the independent variable being the distance between the points. This closely follows the procedure as outlined in McConalogne (1970). At each of the points these polynomials can be applied clockwise or anticlockwise so as to determine gradients of the incoming and outgoing tangents at this location. If the gradient information agrees to within the permissable tolerance values the point is assumed to be on a '~smooth" section of the curve and is discounted otherwise it is recorded as a point of sharp change. Since information is required concerning areas of interest, r a t h e r t ha n points o f interest, points which are relatively close are coalesced to determine regions of angular change exceeding the prescribed value. The areas of change
for Figs.1 and 2 are summed up in Tables I and 2, respectively. Grain-edge analysis determines the number of areas of change per grain-edge shape exceeding the specified, e.g. 25 ° angles. In practice 25-50 grains per sample suite are ample and all would be analysed in this manner. Any convenient statistical test, e.g. Chi-Squares, Mann-Whitney, could now be utilized for differences/non-differences with respect to grain angular changes, so t hat this param et er can be quantitatively assessed.
References Cater, J.M.L., 1984. An application of scanning electron microscopyof quartz sand surface textures to the environmental diagnosisof Neogenecarbonatesediments,Finestra Basin, south-east Spain. Sedimentology,31:717-731
294 Goudie, A. and Bull, P.A., 1984. Slope process change and colluvinm deposition in Swaziland: an SEM analysis. Earth Surf. Proc. Landforms, 9: 289-299. Ehrlich, R. and Weinberg, B., 1970. An exact method for the characterization of grain shapes. J. Sediment. Petrol., 40: 205-212.
Krinsley, D.H. and Donahue, J., 1968. Environmental interpretation of sand grain surface textures by electron microscopy. Geol. Soc. Am. Bull., 79: 743-748. McConalogne, D.J., 1970. A quasi-intrinsic scheme for passing a smooth curve through a discrete set of points. Comp. J., 13(4): 392-396.
291
Elsevier Science Publishers B.V., Amsterdam-- Printed in The Netherlands
Short Note
COMPUTER CHARACTERIZATION OF ASPECTS OF GRAINEDGE ROUGHNESS USING THE SCANNING ELECTRON MICROSCOPE A.T. WILLIAMS, R.G. MILES and G. TOUGH Coastal Research Unit, Science Department, Polytechnic of Wales, Pontypridd, Mid-Glarnorgan, Wales CF37 1DL (U.K.)
(Received May 13, 1986; revised and accepted July 23, 1986)
The abundance/absence of various quartz grain surface features is fundamental to any SEM analysis involving reconstruction of geological environments. For the many derived features t h a t can be seen on a quartz grain surface, "checklists" galore exist (Krinsley and Donahue, 1968; Cater, 1984) but the validity of specific examination techniques is still a matter of debate. One of the most extremely important mechanically derived parameters for this type of analysis is edge abrasion as roundness values are a dominant morphological feature found on quartz sand grains. For example, Goudie and Bull (1984) found that the main difference between two types of African sedimentary deposits was a total absence of edge-abraded grains in deeply weathered profiles, and on this basis compiled a slope evolution model for the area concerned. A commonly applied technique for grainslope analysis is t h a t suggested by Ehrlich and Weinberg (1970). The method approximates the closed curve by a Fourier series of degree twenty. The grain shape is then said to be represented by the twenty Fourier coefficients, with the leading coefficient set to unity. If the curve is reconstructed from these coefficients it is seen t h a t it gives a smooth representation of the boundary, reducing the severity of angular change and perhaps removing small nicks and abrasions which may be significant. Conse0025-3227/87/$03.50
quently the technique gives a global impression of the shape. The technique discussed in this paper specifically looks for small local characteristics, which accumulate to classify the grain. In particular it highlights a useful means of determining accurate numbers for grain-shape angle change regions - - a function of edge abrasion - - which eliminates the subjective approach to this problem. SEM grain-shape analyses (Figs.1 and 2) information were inputed into a computer using a digitiser so t h a t the grain edge was represented by a set of coordinate pairs which were processed in a two-stage manner. Initially the data was preprocessed to determine regions of interest and then each of these regions was examined in more detail. Since the data expressing a grain edge is composed of a large number of coordinate pairs, a simple preprocessor is used which can easily eliminate points of no interest (those on "smooth" sections of the curve where "smooth" is a definable angular quantity - - see below) and so dramatically reduce the number of points that must be considered in more detail. The same spacing of the digitised points is used for each grain. To preprocess the information, an angular check is applied to triples of coordinate pairs. If the resulting angle does not exceed four fifths of an arbitrary determined required
© 1987 Elsevier Science Publishers B.V.
292
Fig.1. SEM picture of a quartz sand grain from a modern beach.
a n g l e , e.g. 25 ° , t h e n t h e p o i n t is r e t a i n e d for f u r t h e r c h e c k i n g . O n c e all of the pairs h a v e b e e n p r e p r o c e s s e d t h e r e d u c e d s e t c a n be
further examined using parametric Lagrangian interpolation. Polynomials are determined independently
TABLE 1
TABLE 2
Numbers of "areas of change" for Fig.1
Number of "areas of change" for Fig.2
Specified change of edge angle (degrees)
Numbers of change areas
Specified change of edge angle (degrees)
Numbers of change areas
25 51 77 102 128 154
27 11 6 3 2 1
25 51 77 102 128 154
43 12 4 1 1 1
293
Fig.2. SEM picture of a quartz sand grain from a raised beach.
for the x and y coordinates, the independent variable being the distance between the points. This closely follows the procedure as outlined in McConalogne (1970). At each of the points these polynomials can be applied clockwise or anticlockwise so as to determine gradients of the incoming and outgoing tangents at this location. If the gradient information agrees to within the permissable tolerance values the point is assumed to be on a '~smooth" section of the curve and is discounted otherwise it is recorded as a point of sharp change. Since information is required concerning areas of interest, r a t h e r t ha n points o f interest, points which are relatively close are coalesced to determine regions of angular change exceeding the prescribed value. The areas of change
for Figs.1 and 2 are summed up in Tables I and 2, respectively. Grain-edge analysis determines the number of areas of change per grain-edge shape exceeding the specified, e.g. 25 ° angles. In practice 25-50 grains per sample suite are ample and all would be analysed in this manner. Any convenient statistical test, e.g. Chi-Squares, Mann-Whitney, could now be utilized for differences/non-differences with respect to grain angular changes, so t hat this param et er can be quantitatively assessed.
References Cater, J.M.L., 1984. An application of scanning electron microscopyof quartz sand surface textures to the environmental diagnosisof Neogenecarbonatesediments,Finestra Basin, south-east Spain. Sedimentology,31:717-731
294 Goudie, A. and Bull, P.A., 1984. Slope process change and colluvinm deposition in Swaziland: an SEM analysis. Earth Surf. Proc. Landforms, 9: 289-299. Ehrlich, R. and Weinberg, B., 1970. An exact method for the characterization of grain shapes. J. Sediment. Petrol., 40: 205-212.
Krinsley, D.H. and Donahue, J., 1968. Environmental interpretation of sand grain surface textures by electron microscopy. Geol. Soc. Am. Bull., 79: 743-748. McConalogne, D.J., 1970. A quasi-intrinsic scheme for passing a smooth curve through a discrete set of points. Comp. J., 13(4): 392-396.