Seismic Refraction Interpretation 2 and 3 Layers Case

Refraction Seismology Definition A method that maps geologic structure using the travel times of head waves. Ground surface

Refraction Seismology Head Waves Head waves are elastic waves that enter a high-velocity medium (refractor) near the critical angle and travel in the high-velocity medium nearly parallel to the refractor surface before returning to the surface of the Earth. S R Layer 1 Velocity = V1 ic Layer 2 Velocity = V2 V2 > V1

Refraction Seismology Objectives The objective in refraction surveys is to measure the arrival times of head waves as a function of source-receiver distance so that the depth to and velocity of the refractors in which they traveled can be determined. X T

2 Layer Case V2 > V1 Z x S R ic A B Layer 1 Velocity = V1 Layer 2 Refracted time from S to R is given by

2 Layer Case

Straight line equation 2 Layer Case Straight line equation A is the slope B is intercept with vertical axis at x = 0

Traveltime Curve Refracted waves Direct waves Time (s) Offset (m)

to is the intercept time Depth to Refractor to is the intercept time

3 Layer Case Horizontal Refractor B D C 1 2 x Layer 1 Velocity = V1 Z1 Layer 2 Velocity = V2 Z2 Layer 3 Velocity = V3 V3 > V2 > V1 Refracted time from S to R is given by

3 Layer Case Horizontal Refractor

n Layer Case Horizontal Refractor

Definitions Critical distance: Critical distance (xc) is the minimum horizontal distance from the shot point at which the first refracted pulse can be recorded. Critical refraction has same travel time as reflection Angle of reflection same as critical angle Cross-over distance: Cross-over distance (xco) is the horizontal distance from the shot point where the direct wave reaches the receiver simultaneously with the refracted wave. xco xc ic

Shot – Geophone Relation Definitions Shot – Geophone Relation Forward shooting Reverse shooting Split shooting Offset shooting

2 Layer Case Dipping Refractor Time (s) Offset (m) Reciprocal time Slope=1/Vd Slope=1/Vu t1d t1u Slope=1/V1 S R  Zd  Zu A  B

2 Layer Case Dipping Refractor Downdip shooting Updip shooting

2 Layer Case Dipping Refractor If  is small enough so that, cos  = 1 and sin  = , then If  is very small then

Recording Instrument (Seismograph) 120 channels Bison from 1980’s Up to 64 channels Stratavisor NZ from Geometrics 24 channels Geode from Geometrics

Receivers (Geophones)

Seismic Sources (Land) Sledgehammers Find trapped miners experiment (AZ. USA) Find sinkholes (Utah, USA)

Seismic Sources (Land) Weight drop (Nevada, USA) Vibroseis Explosive

Setup a Seismic Survey Seismograph Source cable Source point Geophone cable Geophones

Seismic Sources (Land)

Picking First Arrivals Data example, Park City, UT. Profile # 1

Our First Field Test Why? Introduction to field work Where? Here on campus What? 2D profile When? 48 Channel 4 shots (2 forward and 2 reverse) Targets: find layer velocity, thickness and dip Survey layout: two perpendicular receiver lines, 24 channel each Number of stacks and geophone interval will be determined in the field

Summary Seismic refraction can be used to find layer velocity Depth to refractor and its dipping can also be found Layer velocity is equal to 1/slope Layer thickness is found from the value of the intercept time Layer dip can be found if the depth at two points is known