Flexible 3 D seismic survey design Gabriel Alvarez

Flexible 3 -D seismic survey design Gabriel Alvarez Stanford University Victor Pereyra, Laura Carcione Weidlinger Associates Inc. Alvarez, Pereyra, Carcione 1

Goal Show with a simple 3 -D example how to optimize the design of a seismic survey such that it is: Locally optimum to illumination. Require many fewer shots than a standard design. Does not compromise the logistics. Alvarez, Pereyra, Carcione 2

Characteristics Flexible: allow survey parameters to change in a systematic way. Exhaustive: exploits all subsurface information as well as logistic and economic constraints. Dips, depths, velocities, presence of fractures, etc Available recording equipment, maps of surface obstacles, etc. Illumination-based: uses target illumination as the primary design consideration. Alvarez, Pereyra, Carcione 3

Design Example Single depth-variable target: 300 -3000 m Land prospect. Sources are expensive. Alvarez, Pereyra, Carcione 4

Subsurface model View from the inline direction Alvarez, Pereyra, Carcione 5

Subsurface model View from the cross-line direction Alvarez, Pereyra, Carcione 6

Target reflector Inline direction Cross-line direction Alvarez, Pereyra, Carcione 7

The standard approach Alvarez, Pereyra, Carcione 8

Target parameters • Minimum depth: 300 m • Maximum depth: 3000 m • Maximum dip: 60 degrees • Minimum velocity: 2000 m/s • Maximum frequency: 60 Hz • Minimum trace density: 240000 tr/km 2 Alvarez, Pereyra, Carcione 9

Survey recording patch 500 m 400 m X X X X X X 20 m 8 receiver lines 20 shot salvo 24 fold Alvarez, Pereyra, Carcione 10

Other parameters Max offset inline=2990 m Max offset xline=1590 m Shot density: 100 shots/km 2 Aspect ratio: ~2 Fold: 24 (6 x 4) Number of receiver lines: 8 Number of channels/line: 300 Alvarez, Pereyra, Carcione 11

Problem: Maximum-minimum offset MMO=640 m Some bins have minimum offset larger than the target depth. Alvarez, Pereyra, Carcione 12

Alternatives to solve the problem 1. Halve the receiver- and the source-line intervals. 500 m 250 m 400 m 200 m MMO=320 m. Good. But … Receiver and shot density are doubled. The fold is doubled: 12 x 4 The aspect ratio is doubled: 4 The salvo is halved: 10 Alvarez, Pereyra, Carcione 13

Alternatives to solve the problem 2. Halve the receiver and source-line interval and use a rectangular bin X X X 20 m 40 m X X X 20 m Good. Now the source density doesn’t change. But … The fold is doubled: 12 x 4 The aspect ratio is doubled: 4 The salvo is now one-fourth: 5 Alvarez, Pereyra, Carcione 14

Why the need to compromise? Because we are using the same parameters for the entire survey area. We can use different parameters in different parts of the survey: the target is shallow only in a small region. Alvarez, Pereyra, Carcione 15

The proposed approach: subsurface-based design Alvarez, Pereyra, Carcione 16

The method in a nutshell Use a subsurface model to trace rays to the surface at uniform opening and azimuth angle. Record the emergence position of the rays at the surface. Compute locally optimum spatially-varying geometry. ray tracing Alvarez, Pereyra, Carcione 17

Spatially-varying geometry 1. Maintain a standard geometry but allow changes in the parameters (line intervals). 2. Maintain a standard receiver template but allow sources in “arbitrary” positions. 3. Allow sources and receivers to be in “arbitrary” positions. Alvarez, Pereyra, Carcione 18

Model space dimensionality Fixed orthogonal geometry: Only six parameters describe each geometry. Receiver interval Source interval Receiver line interval Source line interval Number of receivers/line Number of receiver lines/patch. Each parameter has a limited number of acceptable values (integer optmization). Alvarez, Pereyra, Carcione 19

Assign source-receiver positions For each geometry: based on ray emergence position being closer to a source or receiver line. Alvarez, Pereyra, Carcione 20

Preprocessing For each trial geometry: • Compute total distance that the rays were moved. • Compute shot and receiver density, fold, aspect ratio, offsets, etc. Alvarez, Pereyra, Carcione 21

Fitness function f: fitness value i: index of trial geometry λ: to balance objectives vs. constraints δ: relative weight of each objective ε: relative weight of each constraint O: objectives (illumination and cost) C: constraints (fold, aspect ratio, MMO, etc) Alvarez, Pereyra, Carcione 22

Objectives and constraints Objectives (to minimize): • total distance to adjust the ray emergence positions • total number of sources • receiver- and source-line cut Constraints: • Equipment availability • Minimum fold (trace density) • Maximum-minimum offset (MMO) • Aspect ratio Alvarez, Pereyra, Carcione 23

Splitting the survey area 10 km Shallow zone: depths <400 m (<5 km 2) 10 km Mid zone: depths (400, 700) m (<10 km 2) Deep zone: depths >700 m (>85 km 2) Alvarez, Pereyra, Carcione 24

Shallow zone Weights-objectives δ 1 δ 2 δ 3 Weights-constraints ε 1 ε 2 ε 3 ε 2 Illumination Sources Line-cut MMO Channels A-ratio Fold 0. 7 0. 25 0. 05 0. 4 0. 2 0. 1 C 2 C 3 C 4 < 400 2000, 3000, 5000 1 -3 24 -36 Alvarez, Pereyra, Carcione 25

Results for shallow zone 320 m Max offset inline=1590 m 180 m Max offset xline=1070 m X X X 20 m 9 shots X X X 20 m Shot density: 156 shots/km 2 Aspect ratio: ~1. 5 Fold: 30 (5 x 6) Number of receiver lines: 12 Number of channels/line: 160 Alvarez, Pereyra, Carcione 26

Mid zone Weights-objectives δ 1 δ 2 δ 3 Weights-constraints ε 1 ε 2 ε 3 ε 2 Illumination Sources Line-cut MMO Channels A-ratio Fold 0. 6 0. 3 0. 1 0. 3 0. 2 C 1 C 2 C 3 C 4 500 -600 2000, 3000, 5000 1 -3 24 -36 Alvarez, Pereyra, Carcione 27

Results for mid zone Max offset inline=2640 m 440 m 360 m Max offset xline=1790 m X X X 20 m 18 shots X X X 20 m Shot density: 114 shots/km 2 Aspect ratio: ~1. 5 Fold: 30 (6 x 5) Number of receiver lines: 10 Number of channels/line: 260 Alvarez, Pereyra, Carcione 28

Deep zone Weights-objectives δ 1 δ 2 δ 3 Weights-constraints ε 1 ε 2 ε 3 ε 2 Illumination Sources Line-cut MMO Channels A-ratio Fold 0. 6 0. 3 0. 1 0. 4 0. 2 0. 3 C 1 C 2 C 3 C 4 800 -900 2000, 3000, 5000 1 -2 24 -32 Alvarez, Pereyra, Carcione 29

Results for deep zone 720 m Max offset inline=3590 m 720 m Max offset xline=3590 m X X X 20 m 36 shots X X X 20 m Shot density: 70 shots/km 2 Aspect ratio: ~1 Fold: 25 (5 x 5) Number of receiver lines: 10 Number of channels/line: 360 Alvarez, Pereyra, Carcione 30

Summary of optimum geometry Zone dr Shallow 20 Mid 20 Deep 20 ds 20 20 20 drl 180 360 720 dsl 320 440 720 nrl 12 10 10 dr: receiver interval ds: source interval drl: receiver-line distance dsl: source-line distance nrl: number of receiver-lines Alvarez, Pereyra, Carcione 31

Stats of optimum geometries Zone 1 2 3 Fold 5 x 6 6 x 5 5 x 5 Aspect ratio 1. 5 1. 0 Maxmin offset (1590, 1070) (1790, 2630) (3590, 3590) Alvarez, Pereyra, Carcione Source density 156 114 70 32

A look at the logistics Logistics are not compromised because: • for each source (salvo) the receiver template is standard orthogonal, • the receiver-line interval in zone 2 is half that in zone 3 and in zone 1 is half that in zone 2, • the sources are along continuous lines as usual. Alvarez, Pereyra, Carcione 33

The bottom line • The geometry is locally optimum from the illumination point of view. • The average source density is about half than with the standard approach. • Logistics are not compromised. Alvarez, Pereyra, Carcione 34

Additional remarks 1. We emphasized reflector depth, but we can also use reflector dip, curvature, etc. 2. Different geometries may be combined to form the final geometry. 3. Can estimate the local acquisition effort. This will help in dealing with surface obstacles. 4. Surface maps should be used at the design stage to further constrain the position of sources and receivers. Alvarez, Pereyra, Carcione 35

Conclusions The standard seismic survey design is too rigid because of the assumption that the subsurface is featureless. Relaxing this assumption allows the design to be flexible, illumination based, locally optimum in terms of the required acquisition effort. Alvarez, Pereyra, Carcione 36

Thank you for your attention. I will be happy to entertain your questions. Alvarez, Pereyra, Carcione 37