I Basic Techniques in Structural Geology Field measurements

I. Basic Techniques in Structural Geology • • Field measurements and mapping Terminology on folds and folds Stereographic projections From maps to cross-sections Growth Strata Fault related folds Seismic Imaging

• Twiss and Moores, ‘Structural geology’, Chapter 2. • C. M. R. Fowler, ‘The Solid Earth, An introduction to Global Geophysics’ • Shaw, Connors and Suppe, ‘Seismic Interpretation of Contractional Faultrelated Folds’ (AAPG Seismic Altlas, #53) • http: //principles. ou. edu/seismic_explo/refle ct/reflect. html

$‘Snell’s laws’ - There is no energy refracted if i>ic, where the critical angle$

‘Snell’s laws’ - There is no energy refracted if i>ic, where the critical angle is defined by ic= sin-1(V 1/V 2) Rock Granite Basalt Limestone Sandstone Shale Vp (km/s) 5. 0 5. 5 6. 0 4. 2 2. 5

$Seismic Imaging Techniques Source geophones Direct ic time ic Reflected Refracted • Seismic reflection$

Seismic Imaging Techniques Source geophones Direct ic time ic Reflected Refracted • Seismic reflection • Seismic refraction

Seismic Imaging Techniques Source ic geophones V 1 Travel time of P wave ic V 2 V 1 < V 2 • Seismic refraction • Seismic reflection Critical distance: xc Crossover distance: Xcross

Seismic Imaging Techniques Source geophones Travel time of P wave V 1 V 2 V 1 < V 2 • Seismic refraction • Seismic reflection Critical distance: xc Crossover distance: Xcross

Seismic Reflection Source geophones Reflection coefficient A typical value for R is 0. 001 Reflectors reflect contrasts of acoustic impedance: Polarity of reflected wave depends on sign of reflection coefficient

Simple ‘zero-offset’ Reflection survey • An ‘image’ of the subsurface is obtained by plotting seismograms side by side. • Reflections are generally faint • The ‘image’ obtained this way is in two-way time, not depth. (to convert to depth the velocity needs to be determined). • The cost scales with the number of sources For these reasons it is advantageous to deploy lines of geophones (with a range of ‘offsets’)

Seismic Reflection Source A z geophones Two-way travel time is: x C B Or - t 0 is the two-way normal incidence travel time: t 0 - An horizontal reflector generates an hyperbola in time - Velocity, V 1, and depth, z, can be determined by plotting t 2 as function of x 2.

Common Mid- Point (CMP) Stacking The seismograms corresponding to the various offsets can be corrected to account for the effect of the offset on the arrival time (Normal Move Out), and then stacked to simulate a ‘zero offset’ seismograms with enhanced signal to noise ratio. The Normal Move Out is :

Common Mid- Point Stacking • In case of multiple layers the t 2 -x 2 plot yields the ‘Root Mean Square velocity’, VRMS, (also called stacking velocity): • The equation is used to correct for NMO before stacking. • VRMS relates to interval velocity according to Dix’s equation • Interval velocities and thicknesses are determined from

Unmigrated Seismic Reflection Profile - Seismograms are plotted side by side. - Vertical axis is the two-way travel time - A Common Mid-Point (CMP) stacked profile show records as if shots and geophones were coincident

Migration • In a stacked profile all reflections are plotted as if they were coming from vertical ray paths. This is a ‘distorted’ view of the sub-surface. • Diffractions Migration aims at correcting these distortions and diffractions (assuming that all reflections are in the plane of the vertical section along the geophones line).

Distortions

Distortions Buried focus

Distortions An example with Synthetic seismograms

$Diffractions$

Diffractions

Unmigrated Seismic Reflection Profile Migrated Seismic Reflection Profile Still not directly an image of the subsurface.

Most Common ‘Artifacts’ • Multiples (Sediment/Basement interface or water/sea bottom interface in marine survey) • Sideswipes (reflections out of the plane of the section) can mess up the migration process. • Incorrectly migrated diffractions (they look like anticlines but are not) • Pull-up and Pull-down (not really artifacts)

Multiples geophones 0 t 1 2. t 1 Time Source Primary reflection First multiple

Reflection seismic Line DLC 9708 (Hopper et al. , 1997). Extent of corresponding sparker seismic lines marked by thick line at top. Three first multiples can be seen lower in the section

Shortcomings in seismic images of folds Folds can be distorted or only partially imaged in seismic sections. Two common shortcomings are: (1) Overlapping reflections in unmigrated or under-migrated sections; (2) lack of imaging of steeply dipping fold limbs. NB: Note also pullup.

(Camerlo&Benson, AAPG, 2006)

Deformation since Sueyi Time