Seismic Reflection Data what it is how it

Seismic Reflection Data: what it is, how it can be used, & an application at Elk Hills, CA - Hudec and Martin, 2004

Seismic Reflection Data: Definition Seismic Reflection subject to or caused by an earth vibration return of a wave from a surface that it strikes into the medium through which it has traveled Records seismic waves at the surface that are reflected off of subsurface stratigraphic interfaces - Clay, 1990 Images the subsurface using acoustic methods

Seismic Reflection Data: Acquisition • Source • Receivers reflection patterns are described by Snell's Law - Kansas Geological Survey grid of receivers for 3 D survey: inlines 12. 5 m for Wytch Farm Oil Field 16. 7 m for Elk Hills Oil Field cross lines Spacing

Seismic Reflection Data: Processing Common Midpoint Method - http: //walter. kessinger. com/work/seisx_processing. html

Seismic Reflection Data: Processing two-way travel time (ms) Seismic x-section - Kansas Geological Survey • interface reflects energy proportional to impedance difference impedance = velocity x porosity • subsurface geometries are reconstructed • stratigraphic sections show up layered

Seismic Reflection Data: Resolution Vertical Horizontal Minimum separation between two features such that we can tell that there are two features rather than only one

Seismic Reflection Data: Interpretation Time slice Cross -section

Seismic Reflection Data: Interpretation 2 D image of normal faults from offshore Lebanon: http: //www. mines. edu/academic/geology/faculty/btrudgil/research. html due to faulting Stratigraphic variations due to depositional changes Faulting on the order of 30+ meters

Structural Applications Individual layers and faults (Scale for this x-section) Various scales of interpretation Stratigraphic packages and fault zones - Kattenhorn and Pollard, 2001 - Fort et al. , 2004

Seismic Reflection Data: Application Elk Hills Oil Field 20 miles east of the San Andreas Fault - http: //rst. gsfc. nasa. gov

Elk Hills: Motivation Part 1 of 2 step project Ultimate goal: fracture (stress) prediction across a specified stratigraphic layer Elastic models forward model for slip-induced stress perturbations • fracture pattern today is a composite of fracturing during each distinct slip event • slip-induced stresses vary as faults interact • evolutionary history important!

Elk Hills: Motivation Gain insight into fault geometry and timing at Elk Hills Industry: implications for hydrocarbon entrapment and charge General: implications for evolution of thrust systems

Elk Hills: Stratigraphy A’ Mc. Donald structure B’ NW S R - Thinning onto highs - Thickening across faults N 31 S Use sedimentary features to constrain fault movements with time of deposition of specific layers B A 29 Syn-depositional growth faulting: 2 mi A West A’ B East B’

Elk Hills: Stratigraphic Constraints on Fault Timing: 2 D analysis (cross sections) 3 D analysis (isochores) Chronological fault evolution model

Elk Hills: Stratigraphic Constraints Calitroleum Pseudowell analysis A’ N West ms A MYA 4 -A 2 mi Wilhelm A Calitroleum Pseudowell thickness plots A’ A BRR SW NE A’

Elk Hills: Stratigraphic Constraints Cross section analysis Fault Movement Indicators Example 1: Onlap Active faulting and uplift during deposition Syn-faulting strata

Elk Hills: Stratigraphic Constraints Cross section analysis Fault Movement Indicators Example 2: Offset beds of equal thickness Faulting post deposition Pre-faulting strata

Elk Hills: Stratigraphic Constraints Cross section analysis Fault Movement Indicators SW NE

Elk Hills: Stratigraphic Constraints Cross section analysis Fault Movement Indicators 0. 5 mile 100 ms Offset onlap: 1. 3 R>1 R 2. 3 R>BRR

Elk Hills: Stratigraphic Constraints x 1 x 2 x 3 x 4 x 5 MYA 4 -A Wilhelm Calitroleum 500 ms BRR 1 mile Mc. Donald Isochore: line drawn through points of equal vertical (apparent) thickness of a unit

Elk Hills: Stratigraphic Constraints Two Signatures: 1. Close contours A A’ A’ a) a) fault cut interval at time A (syn tectonic) 2. Thin beds A A’ Structural high b) c) b) fault below interval c) filled in (syn tectonic) paleo high (post tectonic)

Elk Hills: Stratigraphic Constraints 5 R A 2 R 3 R 1 R 5 R BRR Mc. Donald A’ 1 R 3 R 2 R Isochore: Mc. Donald to Base Reef Ridge N 2 R A’ 1 R Structural high - active fault 2 R 3 R 3 R 1 R 7 ft 7 A 5 R 6 R 5 R 0 1 0 2 2 miles 4 6 km Fault cut Structural high - active fault 6 R No expression - inactive fault

Elk Hills: Stratigraphic Constraints Conclusions from all stratigraphic analyses A A’ 5 R 1 R fault initiations bracketed between horizons: 3 R 2 R west Active faults 6 R 2 R, 3 R, 1 R, 5 R 2 R, 3 R, 1 R Mc. Donald 7, 2 R, 3 R B’ B 6 R - Reid, 1990 7 east

Elk Hills: Stratigraphic Constraints A’ N B’ 2 mi 31 Conclusions from all Analyses S A 29 R B EAST WEST 3 R Stage 1 B’ B pre-Mc. Donald (mid Miocene) 7 Stage 1 A’ A pre-Mc. Donald (mid Miocene) Stage 2 3 R B’ B 6 R Stage 2 A’ A 2 R pre-Mc. Donald (mid Miocene) 7 A’ A Post MYA-4 A (late Pliocene) 1 R Stage 3 R 3 R 2 pre-Base Reef Ridge (early Pliocene) A Stage 4 5 R 1 R A’ 3 R 2 R pre-Wilhlem (mid Pliocene)

Elk Hills: Stratigraphic Constraints Fault Geometry & Timing input for elastic models determine faulting related stress perturbations