February 28, 2022 NCS Admin

What is the difference between 2D vs. 3D seismic data?

To answer this question, we first need to define a few terms. To begin with, 2D refers to two-dimensional seismic data and 3D refers to three-dimensional seismic data. The term “dimension” refers to a measurable extent of some kind, such as length, width, or height.
In physics and mathematics, the dimension of a space is defined as the minimum number of coordinates needed to specify any point within it. For example, a line is one-dimensional since only one coordinate is needed to specify a point on it. Similarly, a surface such as a plane is two-dimensional because two coordinates are needed to specify points on a plane, and a volume such as a cube is three-dimensional because three coordinates are needed to specify a point within a volume.

So, at the simplest level, we can think of the difference between 2D vs. 3D seismic data in terms of the minimum number of coordinates needed to specify any point in that space.

To get a better handle on what this means, let’s first look at a 2D seismic section to get a better understanding of what it represents:

Figure 1 – A 2D (Two-Dimensional) seismic section showing the two coordinate axes of horizontal distance (labeled CMP) at the top of the figure and vertical time (labeled TWT) along the sides of the figure.
(Image from: http://www.sub-surfrocks.co.uk)

This 2D seismic section has two dimensions. In the horizontal direction (equivalent to the x-axis) which is labeled with CMP numbers along the top of the figure, we are traversing the horizontal surface of the earth, equivalent to walking in a straight line along the earth’s surface (for land seismic data) or sailing in a straight line along the ocean’s surface (for marine seismic data). In the vertical direction (equivalent to the z-axis), which is labeled with TWT (ms), we are traversing down into the earth, with the top of the section being the earth’s surface and the data below that representing the subsurface structure of the earth at progressively deeper depths. The only added complication to this picture is the observation that for a seismic section processed using conventional time-based processing the z-axis is measured in terms of two-way travel time (hence the label TWT, which stands for Two-Way Travel time) expressed in milliseconds (abbreviated ms, with 1 millisecond equal to 0.001 seconds). So for the sample 2D seismic section above the vertical axis ranges from 0 milliseconds at the top to 1200 milliseconds (equivalent to 1.2 seconds) at the bottom.

The following P-Cable data example from a survey in the Barents Sea, offshore Norway, dramatically illustrates the ability of P-Cable data to provide extremely detailed images of complex, small-scale fault systems. Referring to the vertical seismic sections in Figure 2, the P-Cable dataset (bottom) clearly indicates a series of small-scale faults in the Cretaceous section just below the Upper Regional Unconformity (between 640 and 700 msecs TWT). This series of faults is completely absent in the conventional dataset, primarily due crossline spatial aliasing of the steeply dipping fault planes as a result of the 25 meter crossline bin. This difference in resolution is even more strikingly seen in a time slice through the polygonal fault system (Figure 3).

The above explains the coordinate axes for the
2D seismic section, but what about the image itself?

The seismic imaging method uses sound waves to image the subsurface structure of the earth. Everyone is familiar with the use of sound waves to image structures beneath the surface as this is the same type of technology employed in the ultrasound scans doctors use to image a developing fetus beneath the skin of the mother’s abdomen:

Figure 2 – Examples of ultrasound images of a developing fetus. This type of imaging using sound waves is completely analogous to the seismic technology we use to image the earth’s subsurface. The three images above show a 2D (planar) ultrasound image on the left, a 3D (volumetric) ultrasound image in the center, and High Definition (HD) 3D volumetric ultrasound image on the right. The HD ultrasound image is analogous to the UHR3D (Ultrahigh Resolution3D) image that P-Cable produces of the earth’s subsurface.
(HD Ultrasound Image Courtesy of Baby Love Ultrasound.)

For seismic imaging, instead of creating images of objects a few inches beneath the skin’s surface, we are creating images of geologic features thousands of feet beneath the earth’s surface. A good example of what the 2D seismic image in Figure 1 represents can be seen if we compare that image to a photograph of a road cut in a mountainous region with a similar type of geology:

Figure 3 – Comparison of a photograph of a road cut in a mountainous region with faulted geology (left) with the 2D seismic section shown in the previous Figure 1 (right), which also exhibits faulted geology.

Not surprisingly, just as we can produce volumetric ultrasound images, we can also produce volumetric seismic images of the earth’s subsurface, and that is precisely what a 3D seismic image is:

Figure 4 – Perspective view of images from 3D seismic data volumes that provide a volumetric image of the earth’s subsurface. The vertical sides of these images are referred to as vertical seismic sections and the horizontal surface of these images are referred to as time slices (or depth slices if the seismic data have been imaged in depth).

The closest analogy to what these volumetric seismic images of the earth’s subsurface reveal can be seen in photographs of erosional features such as the Grand Canyon, which reveal the beauty and complexity of geologic features normally hidden from our view:

Figure 5 – The Grand Canyon, a massive erosional feature in Arizona that exposes both the beauty and the complexity of geologic features beneath the surface of the earth that are normally hidden from our view. The seismic imaging method uses low-frequency sound waves to produce images of the subsurface geologic structures without the need for erosional exposure.

In essence, 3D seismic imaging allows us to reveal
structures beneath the earth’s surface that are hidden from view.

Hopefully, these seismic imaging examples and photographic analogs give you a basic idea of what 2D and 3D seismic images represent in terms of subsurface geologic structures.

There is, however, an important difference between 2D and 3D seismic images that we have not covered in this article, and that is because this topic requires a deeper understanding of how seismic imaging (imaging with sound) works. We will cover that topic in the next installment of this series of articles, but as a teaser to that next article, a key difference between 2D seismic images and 3D seismic images is that 2D seismic images are always distorted images of the earth’s subsurface.

How and why 2D seismic images are distorted is a fascinating topic that we will take up in a future article.