Seismic refraction surveys

Transcription

1 Seismic refraction surveys Seismic refraction surveys generate seismic waves that are refracted back to Earth s surface from velocity and density discontinuities at depth. Uses : Small-scale: geotechnical (building foundations), environmental (weathering layer, remediation), archaeological, sedimentary structures. Medium-scale: sedimentary basin architecture, depth to basement. Large-scale: structure of the crust and mantle, Moho. + depth conversion of seismic reflection data! Kearey & Brooks Chapter 5 Snell s law, critical angle i c and the conditions for reflection Snell's law Sin(θ 1 ) = Sin(θ ) 2 = const = p p is called the ray parameter, as it characterizes the entire ray path. Special case of the critical angle Sin(i c ) = Sin(90o ) Sin(i c ) = e.g. if = 2 km s 1, = 3 km s 1, i c = 42 o

2 Reflection versus refraction seismics: Reflection results. Line B - CDP Stack (Amazon margin, NE Brazil) Fan channel-levee system OBS 315 Late Miocene - Pleistocene Late Albian - Mid-Miocene Oceanic crust Seafloo SW r multi ple NE Moho?? Reflection versus refraction seismics: Refraction results. Line B - CDP Stack (Amazon margin, NE Brazil) Moho

3 Refraction: two horizontal layers z 1 T SA = T BD = Cosθ x = AB + 2 tanθz T SABD = T SA + T AB + T BD T SABD = 2z 1 + x 2 tanθz = x # 1 + 2z tanθ & Cosθ $ % Cosθ ' ( = x 2z # + 1 Sinθ & Cosθ $ % ' ( = x 2z + ( 1 Sin 2 θ) Cosθ T SABD = x + 2 zcosθ = x + 2z Sinθ =, Cosθ = 1 Sin 2 θ 2 2 ( ) 1/2 ( ) = $ 1 $ The refracted waves propagate in the deeper, faster medium, just below the material interface. Normally the first phases to arrive at a receiver, these are called head waves. " $ # " # 2 % % ' & ' & Two horizontal layers: Time-Distance Plot Critical distance x crit is where refracted phase is first observed. Cross-over distance x cros is where the travel times of the direct and refracted arrival are equal. traveltime T T distance x from source x cros Intercept time = 2z ( 2 1 ) 1/2 = 2 zcosθ = x cros + 2z ( 2 1 ) 1/2 " x cros = 2z + % $ ' # & x cros always > 2z The thickness z of the upper of the two layers can be determined from the cross-over distance and the velocities or from the intercept time and the velocities. z = 1 " % $ ' 2 # + & 1/2 x cros 1/2

4 Geophone number Wave propagation of direct, reflected and refracted waves Elapsed time after shot (s) Depth (m) Offset (m) Refracted arrivals from multiple layers e dw Hea ave wav Head ABCDEF is the refracted ray path through the bottom layer of a three-layer model. The traveltime curve for the direct and two head waves are shown above.

5 intercept time, two-layer case T SD = x + 2 zcosθ, = Sinθ By analogy: T ABCDEF = x V z 1Cosθ z 2Cosθ 2 3-layer case: two intercept times The velocities, and V 3 can be estimated from the slopes of the direct wave and the two head waves. z 1 and z 2 can be calculated from the two intercept times. where T n = x V n + n 1 i=1 $ θ i = sin 1 % & 2z i Cosθ i V i V i V n ' ( ) n-layer case: n-1 intercept times This gives the travel time, T n of a ray critically refracted along the top surface of the n th horizontal layer Typical field data from hammer-blow seismics practical =1.89 km/s V 3 =5.84 km/s

6 Common shot point gathers from 3 streamers (length 6, 15, & 6 km) Deviations from simple layering: Dipping layers Shoot down-dip Same type of T-x curve as for the horizontally layered case, but observation of two different apparent velocities v2d, v2u for the refracted wave that propagates at true velocity v2. Shoot up-dip

7 Down-dip traveltime: t 2 x ( ) = Up-dip: t 2 ' ( x) = xsin( θ +γ) + 2zcosθ v 2 v 1 xsin( θ γ) + 2z' cosθ v 2 v 1 θ = 1 2 (sin 1 ( / d ) + sin 1 ( / u )) γ = 1 2 (sin 1 ( / d ) sin 1 ( / u )) z = t i / 2cosθ, t i = 2z cosθ / h = z / cosγ, z ' = t i ' / 2cosθ h' = z'/ cosγ θ and γ can be estimated from the velocities, u and d and hence z and z and h and h can be calculated. See Kearey & Brooks (Chapter 5) Interpretation of irregular traveltime curve in terms of non-planar refractor geometries Reference (dashed lines) show the planar case M (e.g.) is nearer the surface than the reference interface, the actual travel time to M plots below the reference line. Conversely, that for N is above it. These observations can be quantified using the concept of delay time.

8 The concept of delay time Delay time: how much longer does it take to run the actual path (obliquely down! horizontal! obliquely up) compared to running the entire horizontal distance SR in the fast medium 2? We can think of the travel time of a refracted wave being made up of 3 parts: 1. the time it takes to travel between the source and receiver, S v R v, at velocity, 2. the time δ S to go from source point S to refraction point C at velocity, 3. an equivalent time δ R, to run from point D up to the receiver. t SR = δ S + SR + δ R δ S and δ R are called delay times Determining lateral variations in layer thickness from forward & reverse shooting The time t f to go from one end to a receiver (S f CDR), and then on to the other end, t r, (REFS r ), is longer than the total time, t total, to go from end to end (S f CDEFS r ), because of the extra times to travel from the interface to the receiver, along DR and ER. t f h R t r t f + t r = t total + 2δ R δ R = 1 2 (t f + t r t total ) h R = δ R ( 2 2 ) 1/2 t f, t r and t total can be read off from a traveltime versus distance plot, and the delay time δ R can be calculated. The depth h R to the interface beneath R can then be calculated from the delay time and the velocities,. Many receivers! many h R obtained.

9 Offsets in the travel time-distance plot for head waves from opposite sides of a fault Δt Thin layers and low-velocity layers are difficult to detect Default case, unproblematic. A low-velocity layer does not generate a refracted wave (head wave) at all. A thin faster layer generates a head wave, but it may not be the first arrival at any distance.

10 Marine refraction seismics using Ocean Bottom Seismometers (OBS) 4-channel: hydrophone + 3 component seismometer Data logger + batteries + GPS clock Ballast weights (for coupling with seabed) Hydro-acoustic release Titanium tubes for > 6000 m Operation: days OBS Data Reduced-time versus distance plots, plotting t x/(6 km/s) on the y-axis (a 6 km/s refractor will appear flat).

11 Generalized velocity structure of continental and oceanic crust Oceanic crust: Velocity profile for different crustal ages Moho White et al. (1992) Velocities increase gradually through the oceanic crust (difficult to fit straight lines on Time Vs. distance plots). Moho is usually marked by a velocity jump to > 8.0 km/s

12 The Continental/Ocean Transition (COT) at conjugate rifted margins. Example: Baja California Peninsula western Mexico Data from OBS refraction/reflection surveys Analysis and interpretation of shallow seismic field data ti1 ti2

13 0 164_processed Trace number Time (ms) May :04:27