Riding the Waves: Getting S- When Getting P-
For decades, seismic analysis of subsurface geology has been limited to information that can be extracted from compressional-wave (P-wave) seismic data – but numerous geophysicists are now becoming aware of the advantages of combining shear-wave (S-wave) data with P-wave data.
The advantage, simply stated, is this: A broader range of rock and fluid properties can be estimated than what can be estimated with P-wave data alone.
The purpose of this article is to explain that it may be easier and less costly than you think to acquire S-wave data across onshore prospect areas when conventional P-wave seismic data are being collected.
Seismic sources used to acquire P-wave data across land-based prospects always apply a vertical force vector to the Earth. This statement is true for vibrators (the most common land-based P-wave source), explosives in shot holes and the various types of weight droppers and thumpers that have been utilized to acquire P-wave data over the years.
When a vertical impulse is applied to the Earth, two types of wavefields radiate away from the impact point – a P wavefield, and an SV (vertical shear) wavefield.
(A minor amount of SH – horizontal shear – energy also radiates away from the application point of a vertical impact, but this S-wave mode is weak and will not be considered in this discussion.)
Two examples of the relative energy that is distributed between a downgoing P wavefield and a downgoing SV wavefield produced as the result of a vertical impulse are illustrated on figure 1. These P and SV radiation patterns correspond to different values of Poisson’s ratio for the Earth medium where the vertical impulse is applied.
A surprising principle to many people, including geophysicists, is that although a vertical-impact source is considered to be a P-wave source, the SV wavefield produced by such a source is often more robust than is its companion P wavefield.
For example, to determine the relative strengths of the downgoing P and SV wavefields at any take-off angle from the source station, one has to only draw a raypath, such as dash-line SAB on figure 1, oriented at take-off angle Φ. The points where this line intersects the P and SV radiation pattern boundaries define the relative strengths of the P and SV modes in that illumination direction.
For take-off angle Φ in this example, the strength (B) of the SV mode is larger than the strength (A) of the P mode.
A real-data example that illustrates this physics is displayed as figure 2. This example is a vertical seismic profile (VSP), which is one of the best measurements that can be made to understand seismic wave-propagation physics.
Here, both a downgoing P wave and a downgoing SV wave are produced by the vertical vibrator that was used as the energy source. Either wave mode, P or SV, can be used to image geology. Both modes are embedded in the data, but people tend to utilize only the P-wave mode.
How can we begin to take advantage of the SV-wave data that conventional land-based P-wave seismic sources produce? Only two alterations have to be made in conventional seismic field practice:
- Deploy three-component geophones rather than single-component geophones.
- Lengthen the data traces to ensure that SV reflections produced by the downgoing SV wavefield are recorded. Because SV velocity is less than P-wave velocity by a factor of two or more, SV data traces need to be at least twice as long as the traces used to define P-wave data.
These alterations can be done with minimal cost, and the potential benefits of acquiring two S-waves (P-SV or converted shear, and SV-SV or direct shear) rather than just P-wave data can be immense.
Our profession needs to utilize longer data traces when acquiring all land-based seismic data.