Changes

''Dip isogons,'' or contours of equal dip in the plane of the section ([[:Image:Land_rig_example_drawing.png|Figure 3]]),<ref name=Ramsay_1967>Ramsay, J. G., 1967, Folding and fracturing of rocks: New York and London, McGraw-Hill, 568 p.</ref> can be used to fill in the geological section. The isogons can be located from projected dipmeter data and projected or correlated between the wellbores following the rules described by Ramsay and Huber.<ref name=Ramsay_etal_1987>Ramsay, J. G., and M. I. Huber, 1987, The techniques of modern structural geology__Volume 2, in Folds and Fractures: Orlando, FL, Academic Press, 700 p.</ref> A series of dip segments along the various isogons helps the geologist sketch the fold profiles. Interpolation between the isogons can also be carried out using the dip domain methods previously described or by cubic spline interpolation.<ref name=McCoss_1987>McCoss, A. M., 1987, Practical section drawing through folded layers using sequentially rotated cubic interpolators: Journal of Structural Geology, v. 9, p. 365-370.</ref>

''Dip isogons,'' or contours of equal dip in the plane of the section ([[:Image:Land_rig_example_drawing.png|Figure 3]]),<ref name=Ramsay_1967>Ramsay, J. G., 1967, Folding and fracturing of rocks: New York and London, McGraw-Hill, 568 p.</ref> can be used to fill in the geological section. The isogons can be located from projected dipmeter data and projected or correlated between the wellbores following the rules described by Ramsay and Huber.<ref name=Ramsay_etal_1987>Ramsay, J. G., and M. I. Huber, 1987, The techniques of modern structural geology__Volume 2, in Folds and Fractures: Orlando, FL, Academic Press, 700 p.</ref> A series of dip segments along the various isogons helps the geologist sketch the fold profiles. Interpolation between the isogons can also be carried out using the dip domain methods previously described or by cubic spline interpolation.<ref name=McCoss_1987>McCoss, A. M., 1987, Practical section drawing through folded layers using sequentially rotated cubic interpolators: Journal of Structural Geology, v. 9, p. 365-370.</ref>

[[File:Land rig example drawing.png|thumbnail|'''Figure 3.''' Cross section through an asymmetrical ramp anticline, Whitney Canyon field, Wyoming, with SCAT and isogen data superimposed. Uncomformities, axial planes, and inflection surfaces have been identified from the diameter data and projected away from the well bore. Isogens are contours of equal dip<ref name=Ramsay_1967></ref> and can constrain the shapes of folds in section. (From Lammerson, 1982.)]]

[[File:Land rig example drawing.png|left|thumbnail|'''Figure 3.''' Cross section through an asymmetrical ramp anticline, Whitney Canyon field, Wyoming, with SCAT and isogen data superimposed. Uncomformities, axial planes, and inflection surfaces have been identified from the diameter data and projected away from the well bore. Isogens are contours of equal dip<ref name=Ramsay_1967></ref> and can constrain the shapes of folds in section. (From Lammerson, 1982.)]]

===Relationship of folding and faulting===

===Relationship of folding and faulting===

Modern theories of structural geology generally relate the formation of folds to accommodation on irregular fault surfaces.<ref name=Hamblin_1965>Hamblin, W. K., 1965, Origin of "reverse drag" on the downthrown side of normal faults: Geological Society of America Bulletin, v. 76, p. 1145-1164.</ref> <ref name=Dahlstrom_1970 />) Generally, the folds are more obvious on seismic sections than faults, but fortunately there are geometric rules that allow us to predict one shape from the other<ref name=Suppe_1983>Suppe, J., 1983, Geometry and kinematics of fault-bend folding: American Journal of Science, v. 283, p. 684-721.</ref> <ref name=Verrall_1982>Verrall, P., 1982, Structural interpretation with applications to North Sea problems: Geological Society of London.Course Notes No 3, JAPEC (UK).</ref> <ref name=Gibbs_1983 />; Williams and Vann, 1987<ref name=Williams_etal_1987>Williams, G., and I. Vann, 1987, The geometry of listric normal faults and deformation in their hanging walls: Journal of Structural Geology, v. 9, p. 789-795.</ref> <ref name=Groshong_1989a /> in both extensional and compressional examples. An example of a cross section solution explaining the relationship between extensional rollover and listric faults is shown in [[:Image:Drive-mechanisms-and-recovery_fig1.png|Figure 4]].  

Modern theories of structural geology generally relate the formation of folds to accommodation on irregular fault surfaces.<ref name=Hamblin_1965>Hamblin, W. K., 1965, Origin of "reverse drag" on the downthrown side of normal faults: Geological Society of America Bulletin, v. 76, p. 1145-1164.</ref> <ref name=Dahlstrom_1970 />) Generally, the folds are more obvious on seismic sections than faults, but fortunately there are geometric rules that allow us to predict one shape from the other<ref name=Suppe_1983>Suppe, J., 1983, Geometry and kinematics of fault-bend folding: American Journal of Science, v. 283, p. 684-721.</ref> <ref name=Verrall_1982>Verrall, P., 1982, Structural interpretation with applications to North Sea problems: Geological Society of London.Course Notes No 3, JAPEC (UK).</ref> <ref name=Gibbs_1983 />; Williams and Vann, 1987<ref name=Williams_etal_1987>Williams, G., and I. Vann, 1987, The geometry of listric normal faults and deformation in their hanging walls: Journal of Structural Geology, v. 9, p. 789-795.</ref> <ref name=Groshong_1989a /> in both extensional and compressional examples. An example of a cross section solution explaining the relationship between extensional rollover and listric faults is shown in [[:Image:Drive-mechanisms-and-recovery_fig1.png|Figure 4]].  

[[File:Drive-mechanisms-and-recovery fig1.png|thumbnail|left|'''Figure 4.''' Modeling extensional fault shapes from the rollover geometry. (a) the groshong (1989b)<ref>Groshong, R. H., 1989b, Structural style and balanced cross sections in extensional terranes: Houston Geological Society Short Course Notes, Feb. 24-25, 128 p.</ref> method uses oblique simple shear with a reference grid constructed with a spacing equal to the fault heave. Distance 2 from the rollover up to regional elevation of the same reference bed is transferred to 2&prime;; likewise, 2&prime; + 4 is transferred to 4&prime; and so on to complete the fault trajectory. Interpolation between these points is carried out using a half grid spacing. (b) fault trajectory reconstruction by the groshong (1989b)<ref>Groshong, R. H., 1989b, Structural style and balanced cross sections in extensional terranes: Houston Geological Society Short Course Notes, Feb. 24-25, 128 p.</ref> method uses simultaneous modeling of three horizons. Dashed trajectories are individual solutions; solid lines are the preferred solution. (From Hossack, unpubl. Data, 1988.)]]

[[File:Drive-mechanisms-and-recovery fig1.png|thumbnail'''Figure 4.''' Modeling extensional fault shapes from the rollover geometry. (a) the groshong (1989b)<ref>Groshong, R. H., 1989b, Structural style and balanced cross sections in extensional terranes: Houston Geological Society Short Course Notes, Feb. 24-25, 128 p.</ref> method uses oblique simple shear with a reference grid constructed with a spacing equal to the fault heave. Distance 2 from the rollover up to regional elevation of the same reference bed is transferred to 2&prime;; likewise, 2&prime; + 4 is transferred to 4&prime; and so on to complete the fault trajectory. Interpolation between these points is carried out using a half grid spacing. (b) fault trajectory reconstruction by the groshong (1989b)<ref>Groshong, R. H., 1989b, Structural style and balanced cross sections in extensional terranes: Houston Geological Society Short Course Notes, Feb. 24-25, 128 p.</ref> method uses simultaneous modeling of three horizons. Dashed trajectories are individual solutions; solid lines are the preferred solution. (From Hossack, unpubl. Data, 1988.)]]

===Balanced cross sections===

===Balanced cross sections===