Common Midpoint (CMP) Reflection Profiling

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CMP Reflection Profiling.

As a rule of thumb most seismic reflection surveying are aiming at having a seismic trace at which when this section is processed and  interpreted the subsurface picture (image) of the surveyed area can be revealed. In a previous post, titled as 2D and 3D seismic reflection surveying we have already gained a general insight on how seismic reflection surveying can be done. Let us today through this post, expand our talk specifically about 2D seismic reflection (profiling). I would like to talk about the basic principle regarding Common Mid-Point (CMP) surveying, as the title of this post described.

Ok let us be together until the end of this post and I hope you will gain a more good insight on 2D - seismic reflection.


CMP is an acronym (short letters for), "Common MidPoint" 

From a simple theoretical view, during acquisition of seismic reflection data, the shot of seismic source in form of seismic energy generate seismic wave into the subsurface, this seismic ray is moving down until it reach the subsurface reflecting horizon due to change in lithology where it reflected back to the surface where it is recorded by various seismic receivers (either geophones or hydrophone). It is that simple! In a layman term.

When it comes to real practice as the source (shot) is moving forward during survey and also the position of detectors also changes over time. This condition causes no two (2) reflected rays to sample the same point on the subsurface. Just think like you move forward while you bounce your ball down on the ground and pick it up. The ball bounce position will vary as you move forward.

Likewise when seismic source is located at the front of array of receivers. The spacing of receivers (geophones) is Δx along the entire array. After each shot is fired, the array is moved forward a distance nΔx. In a marine survey this would consist of the whole airgun and hydrophone streamer being towed forward by the survey ship. See figure below.

On land the last geophone would be brought up to the front of the array, a new hole drilled, loaded with explosives and fired.

However in real practice during the survey progress (from right to left direction) the source and receiver positions are pulled in steps of 1 interval towards the left. Think of a marine ship with a shot (source) towed at the back end moving to the left with its streamer floated and connected back at a certain distance. See the figure (1 and 2), below to get the insight.

Figure 1: Marine survey ship on end spread shot-detector layout.

Figure 2: A simple on end spread source (S) - receiver (R) layout

Then the challenge is that each seismic trace would represent the unique sampling of different points on the subsurface reflector (at points 1, 2, 3...8 see figure (1). At the end of the day the geophysicist would not have a clear insight about the position of the subsurface reflector before seismic processing.

Then the good strategy is to assume the position of the subsurface reflector is at a certain depth vertically down from the distance at midpoint between shot (source) and receiver on the surface, see figure (3) below. This would be possible as the source to receiver spacing is known from your seismic survey design layout. Then the midpoint can also be determined. Hope now you grasped a basic concept about of what is going on about CMP seismic profiling.

Common Mid Point (CMP) Reflection profiling is the 2D reflection profiling at which reflections from the same point are recorded by different source station pairs.

It occurs when seismic traces from different shots have a common point on the surface midway between the source and receiver pairs. See figure (3) below,

Figure 3: A simple representation of CMP and CDP positions

Common Depth Point (CDP)

Is defined as halfway point when a wave travels from a source to a reflector then back to a receiver. It is also known as Common Reflecting Point, at depth on a reflector. See figures (2 and 3) above.

Fold

The Fold refers to the number of times a particular subsurface point has been sampled. This quantity is also called the coverage (in percent). It is equal to the number of traces in the CMP gather and is numerically evaluated by

Fold (F) = N/2n

where n is the moveup rate in units of receiver spacing, N = number of receivers per shot

Moveup rate (n) = shot (source) spacing/receiver (geophone) spacing. 

For example, if geophones are 4 meters apart and shots are employed every 8 meters, then the moveup rate is n=8/4=2. 

For the survey with 6 geophone channel array will give 3 rays for each mid-point, the same as to say it has 3-fold CMP coverage or 300% coverage. 

Since, F = N/2n = 6/2×1

F = 3 or 300% coverage.

n = 1, For shot spacing = receiver (geophone) spacing. 

Example for simple survey.

A land based seismic survey is carried out using a shot interval of 20 m and a cable containing  200 non-overlapping geophone groups with a spacing of 10 m. The geophones on land are at a depth of 8 m. Calculate the maximum fold of the survey

Solution

1. Fold of the survey

N = number of receivers (geophones) per shot = 200

Shot (source) spacing = 20 m

Receiver (geophone) spacing = 10 m

Recall, Fold = N/2n

But n = Shot (source) spacing/receiver (geophone) spacing. 

n = 20/10 

n = 2.

Fold = N/2n

Fold = 200/(2 × 2)

Fold = 200/4 = 50

The maximum fold of the survey = 50

Advantage of using CMP method

Since CMP surveying allows CMP gathering, such as collecting sets of seismic traces at different offset distances from a common Point on reflector.

Then through CMP gathering we can primarily determine and save the two purposes that may assist during seismic trace processing.

(i) It enhances signal to noise ratio (SNR) since more traces can be collected at the same point at different shots. When seismic traces at CMP gathers corrected for Normal move out, they can be added together by so called CMP - stacking, to increase the signal to noise ratio (SNR).

(ii) It facilitates determination of seismic velocities used in seismic trace processing and interpretation. Since different seismic traces have their travel time and the ray path is known, then the seismic velocity through subsurface can be determined.

All in all CMP seismic profiling through CMP gathers to CMP stacking will improve signal to noise ratio (SNR), and hence it assist seismic processing and interpretation.

Thank you everyone! 

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