Geology Reference
In-Depth Information
Parallel
20˚Angle
30˚Angle
120
120
120
(a)
(d)
(g)
100
100
100
80
80
80
60
60
60
40
40
40
20
20
20
0
0
0
40
40
40
(b)
(e)
(h)
35
35
35
30
30
30
25
25
25
20
20
20
15
15
15
10
10
10
5
5
5
0
0
0
0
500
1000
1500
2000
0
500
1000
1500
2000
2500
3000
3500
0
500
1000
1500
2000
2500
3000
3500
Position
Position
Position
300
100
100
(c)
(f)
(i)
250
80
80
200
60
60
150
40
40
100
20
20
50
0
0
0
0
20
40
60
80
100
0
20
40
60
80
100
0
20
40
60
80
100
Thickness (pixels)
Thickness (pixels)
Thickness (pixels)
Fig. 7.
Results of the outcrop step-back experiment. When the camera's focal plane is held parallel to the wall surface,
the grey-scale series defi ning the brick-mortar couplets shows no spatial trend (a and b) and results in a strong periodic
signal at a thickness of 31 pixels (c). As the angle between the camera and the wall is increased to 20° and 30°, foreshort-
ening of the lower portion of the photograph relative to the upper part results in compression of the thicknesses of the
brick-mortar couplets (d, e, g, h). Additionally, as the section becomes compressed, there is a marked degradation of the
periodic signal (f and i). Given the simplicity and constancy of the brick-mortar couplets in this model, and the degree of
signal degradation observed, foreshortening of an outcrop driven by step-back can have a major impact on any quantitative
analysis of grey-scale data series.
to the stratigraphic section. As illustrated in
the previous discussions, variation in the strike
geometry across the outcrop as well as the degree
of outcrop step-back can have an impact on
the reliability of the positional relationships
within the grey-scale data and, as such, can affect
the validity of any conclusions drawn from a
quantitative analysis of grey-scale data. In fact, the
spectrogram derived from the Picco di Carnizza
section (Cozzi
et al
., 2005, Fig. A) shows a fre-
quency shift across the section. This shift in fre-
quency is at least as likely to be driven by such
a photographic artefact as it is to refl ect any real
long-term changes in the rate of accommodation
space creation during deposition.
THE ROLE OF LATERAL VARIABILITY
IN GREY-SCALE DATA
One of the great strengths of traditional fi eld
observation is the way in which an experienced
geologist is able to draw data from along a bed's
lateral exposure. Given the complexity of the
stratal expression of peritidal carbonate succes-
sions, can the grey-scale technique be adapted to
integrate lateral stratigraphic content and if so,
what impact does such integration have on the
analysis of the resultant data series? Interestingly,
the image analysis shareware Scion Image will
average grey-scale values across a row of pixels.
As such, if the geometry of the image is carefully
