Seismic Survey Geometry
Seismic survey geometry refers to the arrangement and spatial relationship between the source points (where seismic energy is generated) and the receiver points (where seismic energy is recorded) in a seismic survey.
The geometry plays an important role in seismic data processing and interpretation in determining the accurate image of subsurface.
3D seismic surveys for hydrocarbon explorations were prioritised based on regional trend of geological structure from existing data, in the area of interest.
Cost Optimization
The constrain triangle play an important role in planning of 3D survey, and cost optimization.
Plan your budget for 3D project, define time line for entire seismic acquisition, processing and data interpretations. The desired quality for the data entirely depends on scope of the work and other factors in constraint triangle like project cost and time line will govern the quality of the output data.
Inputs For Survey Designing
Full fold boundary coordinates, shallow and deeper targets in terms of depth, interval velocity and dominant frequency at target level.
Resources available for seismic data acquisition (total no. of channel available, cable length, type of instrument.
Other information like logistics informations about the area of operations.
Technical Requirements
Resolution and Bin Size
The recorded samples must allow the reconstruction of the original signal without ambiguity. A proper sampling is given by Nyquist condition (or Shannon theorem), which states that two samples per period are minimum to reconstruct a discrete signal.
The sampling interval may be written as
Δt ≤ T/2 or Δt ≤ 1/2 fmax
where, T is time period and f is frequency
The spatial sampling for shots and receivers can be written as
Δx(r,s) ≤ Vmin / 2fmax
Whereas the spatial sampling for the midpoint can be written as
Δxm ≤ Vmin / 4fmax
For the dipping layer case, the spatial sampling can be written as
Δx(r,s) ≤ Vmin / 2fmax sinθ
Δxm ≤ Vmin / 4fmax sinθ
where, θ is dip of the layer with respect to horizontal plane.
Designing Box Size or LMO
The Largest Minimum Offset(LMO) – is diagonal distance of box created with two consecutive receiver and shot lines, LMO pass through the central bin of the box, it should be kept as small as possible, ensuring that it is less than or equal to the depth of shallowest reflector to be imaged or shallowest target.
LMO ≤ Shallowest target depth
Practically it must be less than 500m
Designing Spread Length
Rule of Thumb – Spread length must be at least 1.2 to 1.25 times of deepest target depth.
Receiver Station and Shot Point Interval
The receiver interval is double of the inline bin size, and the shot interval is double of the crossline bin size.
No. of Receivers
No. of Receivers per Line = 2 x (Spread Length / Receiver Interval)
Note: No. of receivers must be in multiple of 2 x Inline Fold.
Total Number of Active Channels
TAC = No. of Channels per Receiver line x Toal No. of Receiver Lines
Shot Line Interval (SLI)
SLI = 1/2 (No. of Active Channels x Receiver Interval) / Inline Fold
Receiver Line Interval (RLI)
For orthogonal geometry receiver line interval is calculated from Pythagoras theorem.
RLI < √(Xmin2 – SLI2)
where, Xmin is largest minimum offset
Salvo
Salvo is defined as the number of shots in the unit template.
For half-swath rollover geometry
Salvo = 1/2 (No. of RL) x (RLI / SI)
Or
Salvo = Crossline Fold x (RLI / SI)
Rollover
Rollover is the movement of unit template.
Inline Rollover – the movement of unit template in the receiver direction (i.e. unit template jump to next shot line).
Inline Rollover = Shot Line Interval (SLI)
Xline Rollover – is template movement to the next swath
Xline Rollover = Salvo × SI
Fold
The fold refers to the number of times a cdp or bin in the subsurface is mapped by seismic waves. The fold for 3D seismic is calculated by summing the number of traces falls within the bin.
Fold (2D line)
Fold = 1/2 (No. of Active Channels x Receiver Interval) / Shot Interval
Fold (3D)
Inline Fold = 1/2 (No. of Active Channels x Receiver Interval) / Shot Line Interval
Crossline Fold = 1/2 (Total no. of Receiver Lines)
Aspect Ratio
Aspect ratio is defined as the rotio of crossline patch dimension to the inline patch.
AR = (Crossline Patch / Inline Patch)
Learn More About Seismic
Click 👉 Seismic Data Acquisition
Click 👉 Seismic Survey Geometry
Click 👉 Seismic Data Processing
Click 👉 Seismic Depth Imaging
Click 👉 Seismic Data Interpretations