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Dip Subtraction and Rotation
Dip subtraction, also called dip rotation, is used to translate
actual dip to dip with regional dip removed. The result is used to
assess the actual angles of crossbedding or fault planes relative to
horizontal strata. If you do not have a dip removed arrow plot, you
may have to perform this calculation on a few dips to find
depositional dip patterns. The equations are:
1: NEWDIP = Arccos(Cos SD * Cos
DIP + Sin SD * Sin DIP * Cos(AZM - SDAZ))
2: ANGLS = Arccos((Cos DIP - Cos
SD * Cos NEWDIP) / (Sin SD * Sin NEWDIP))
3: IF Sin (AZM - SDAZ) >= 0
4: THEN NEWAZM = SDAZ + 180 -
ANGLS
5: Otherwise NEWAZM = SDAZ - 180
+ ANGLS
6: NEWAZM = 360 * Frac((NEWAZM +
360) / 360)
In a July 2017 email, Charles Berg at ResDip pointed out
that NEWDIP could exceed 90 degrees and that we usually do
not report dips this way. An additional step provided by
Charles corrects this problem:
7: IF NEWDIP > 90
8: THEN NEWDIP = 180 - NEWDIP
9: AND NEWAZM = 180 + NEWAZM
Where:
ANGLS = intermediate term
AZM = true dip azimuth before structural dip removal
DIP = true dip angle before structural dip removal
NEWDIP = dip after structural dip removal
NEWAZM = azimuth after structural dip removal
SD = structural (regional) dip to remove
SDAZ = azimuth of structural dip
It
is sometimes necessary or desirable to project the actual dip
onto a new azimuth. This "apparent dip" calculation is used to
prepare dips for presentation on a stick diagram at arbitrary
cross section orientations, such as the line of section between
two wells, or along the section of a seismic line. The equation
is:
10: PROJDIP = Arctan (Tan DIP *
Cos (PROJAZM - AZM))
Where:
AZM = true dip azimuth before rotation
DIP = true dip angle before rotation
PROJDIP = projected dip
PROJAZM = projected azimuth
METALOG "/DIP"
SPREADSHEET--Spreadsheet for Dip Subtraction
This
spreadsheet provides a tool for Dip Subtraction (Regional Dip
Removal) and Dip (Apparent Dip in any arbitrary azimuth). Dip
azimuth is trimmed to within a 360 degree compass.
SPR-25 META/LOG DIP SUBTRACTION CALCULATOR
Regional dip removal -- dip subtraction.
Example from META/LOG "DIP"
spreadsheet for dip subtraction or rotation.
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