Common Depth Point

Scott Simon

2 min read

·

12-01-2025

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The Common Depth Point (CDP) method is a cornerstone of modern seismic reflection surveying. Understanding its principles is crucial for interpreting subsurface geological structures accurately. This post will explore what CDP gathers are, how they're created, and why they're so important in seismic data processing and interpretation.

Understanding the CDP Gather

A CDP gather is a collection of seismic traces that share a common midpoint between the source and receiver locations. Imagine a seismic survey: sound waves are emitted from a source (e.g., a vibrator truck), travel downwards, reflect off subsurface interfaces, and are recorded by an array of geophones or hydrophones. Each trace represents the recorded signal from a specific receiver. A CDP gather assembles traces that share the same midpoint, regardless of the source-receiver offset.

Why the Midpoint Matters

The midpoint is crucial because it represents a specific location beneath the Earth's surface. By stacking traces with the same midpoint, we improve the signal-to-noise ratio. This is because the reflections from the subsurface layers are coherent and will align, whereas the random noise will be largely incoherent and will cancel out during stacking. The result is a clearer image of the subsurface.

The CDP Process: From Acquisition to Stack

The creation of a CDP gather involves several steps:

1. Data Acquisition: Seismic data is acquired along a survey line using sources and receivers spaced at predetermined intervals.
2. Geometry Calculation: The precise location of each source and receiver is recorded, allowing the computation of the midpoint for each trace.
3. Sorting and Binning: Traces are sorted and grouped according to their common midpoints, forming the CDP gathers.
4. Normal Moveout (NMO) Correction: Before stacking, NMO correction compensates for the travel-time differences caused by the varying source-receiver offsets. This alignment process is crucial for accurately summing the traces in a CDP gather.
5. Stacking: The NMO-corrected traces within each CDP gather are summed together, improving the signal-to-noise ratio and creating a stacked section. This stacked section provides a clearer representation of subsurface reflectors.

Importance of CDP gathers in Seismic Interpretation

CDP gathers are fundamental to several aspects of seismic data processing and interpretation:

• Velocity Analysis: The shape of the reflections within a CDP gather provides information about the velocity of seismic waves in the subsurface. This information is critical for accurate depth conversion.
• Multiple Suppression: CDP gathers help identify and mitigate the effects of multiple reflections, which can complicate the interpretation of the primary reflections.
• Amplitude Analysis: Analyzing the amplitudes within CDP gathers can reveal information about the properties of the subsurface layers, such as lithology and fluid content.

In conclusion, the Common Depth Point method is essential for obtaining high-quality seismic images. Understanding the principles of CDP gathers is therefore crucial for anyone involved in seismic exploration and interpretation. The process, from acquisition to the final stacked section, relies heavily on precise measurements and sophisticated data processing techniques to reveal the secrets hidden beneath the Earth's surface.