Sediment Analysis

Wentworth Scale

phi (f)

f = - log2 mm                                      2-f = mm

wt        =      20 grams

2 mm      = 2 grams

1 mm =     4 grams              % error=

0.5 mm = 4 grams                 wt after/wt beginning

0.25 mm = 6 grams                 * 100

0.125 mm = 2 grams

0.062 mm = 2 grams

< 0.062 mm ---------

20 grams

I. Histogram (Bar graph)

II. Cumulative Frequency Curve

III. Plot on Probability Paper

a. Statistical Measurements

1. Mean Grain Size

2. Inclusive Standard Deviation

3. Skewness

4. Kurtosis

(I) Construct Histogram

Weight percent = wt of fraction/total weight * 100

2 mm = 2/20 * 100 = 10%

1 mm = 4/20 * 100 = 20%

0.5 mm = 4/20 * 100 = 20%

0.250 mm = 6/20 * 100 = 30%

0.125 mm = 2/20 * 100 = 10%

0.062 mm = 2/20 * 100 = 10%

Ideal Frequency Curve

ideal distribution

(normal)

mode - inflection point ( point where curve changes direction)

median - grain size where half on one side and half on the other

mean - average size

Skewed

( + ) or ( - ) skewed curves

Skewness = degree of asymmetry

Kurtosis - measurement of the peakedness

leptokurtic

platykurtic

normal

(KG = 1.00)

Comparison of sorting in the central part of curve to the sorting in the tails

• when sorting in the tails is poorer than in the

central portion = leptokuric KG > 1.00

• when sorting in the tails is better than in the

central portion = platykuric KG < 1.00

Cumulative Frequency Curve

Cumulative Curve

2 mm = 10%                                 10%

1 mm = 20%                                 30%

0.5 mm = 20%                                  50%

0.25 mm = 30%                                  80%

0.125 mm = 10%                          90%

0.062 mm = 10%                          100%

Purpose to straighten out curve

Ideal

Plot cumulative curve on probability paper

Folk

Graphic Mean

Mz     = ( f 16 + f 50 + f 84 ) / 3

= ( -0.75 + 0.10 + 0.75) / 3

Mz = coarse sand

= 0.03

2. Inclusive Graphic Standard Deviation

dI = ( f 84 - f 16 ) + ( f 95 - f 5 )

4          6.6

dI under 0.35 f very well sorted

0.35 - 0.50 f well sorted

0.50 - 0.71 f moderately well sorted

0.71 - 1.0 f moderately sorted

1.0 - 2.0 f poorly sorted

2.0 - 4.0 f very poorly sorted

> 4.0 f extremely poorly sorted

3. Skewness (Graphic)

SKG = (f16 + f84 - 2 f50) + (f5 + f95 -2 f50)

(f84 - f16)           2 (f95 - f5)

4. Kurtosis

KG = (f95 - f5)

2.44(f75 - f25)

Sand Analysis (Class Results)

I                  II                      III                      IV

2 mm         22.4         0.0                     0.0                     0.38

1 mm         17.9         2.3                      1.1                      1.89

0.5 mm            30.1         62.2                     71.8                     61.59

0.25 mm         29.3          2.33               1.2                     35.14

0.125 mm 0.4                  29.0             25.9                     4.91

0.062 mm     0         0.2                         0                      1.13

< 0.062 mm                                                                                     0.38

1. Mz =

2. Inclusive Standard Deviation (dI)

dI = 0.35f or less - very well sorted

> 4.0f - extremely poorly sorted

3. Skewness

SKI = (f16 + f84 - 2 f50) + (f5 + f95 - 2 f50)

2 (f84 - f16)         2 (f95 - f5)

coarse         fine                  excess      fine     material = positive skewness

excess coarse material = negative skew

Kurtosis "Peakedness of Curve"

KG = (f95 -f5)

2.44 (f75 - f25)

Sorting in tails and sorting in central portion

If center portion is better sorted than tails = excessively peaked (leptokurtic)

If tails are better sorted then center = platykurtic curve

strongly platykurtic curve = bimodal