Spooky ghosts in seismic!

A ghost is in an unwanted part of the seismic signal. There are three different kind of ghosts, the receiver ghost and the source ghost and the combination of both ghosts. There are different methods to suppress it in the first place and to remove it from data.

As discussed earlier, the perfect seismic signal would be a peak with a white spectrum (white, as in: white sunlight contains all visible wavelengths).

Spooky ghosts in seismic. (B) Source ghost, (C) receiver ghost, (D) source/receiver ghost

Spooky ghosts in seismic. (B) Source ghost, (C) receiver ghost, (D) source/receiver ghost

Due to technical and physical limitations we can’t produce that, neither onshore nor offshore. A typical source used in offshore seismic acquisition is a device that fires air bubbles in regular intervals which produces a wave, hence an “airgun”. In an ideal world the waves would only travel downwards and illuminate the subsurface directly under us. Raypath A shows this ideal scenario. But water is isotropic and transports energy in all directions equally. Energy that travels upwards first gets reflected back down at the sea surface and effectively duplicates the signal. The targeted horizon gets illuminated twice, which is shown in raypath B and represents the source ghost. The reason for this is an almost perfect reflection coefficient of -1 at the water-to-air interface at the sea surface.

R = {Z_1 - Z_2\over Z_2 + Z_1}^2

Where Z is the product of acoustic velocity and density. If you put in Z2=1490*1.027 and Z1=0.001225*343 for seawater and air it gives us a reflection coefficient of 0.998902 that is almost a perfect reflector.

Physics doesn’t care where a wave comes from, it is all treated the same way in an isotropic media such as water. That means the energy that comes back and gets recorded at a receiver, continues and gets reflected again by the near-perfect water-to-air interface. It travels downwards and gets recorded a second time. This is called the receiver ghost because it happens at the receiver end, shown in raypath C. If that wasn’t complicated enough there is the combination of both effects, the source/receiver ghost combo shown in raypath D.

What does that mean? It means that in the worst case scenario, the signal you send down comes back four(!) times. We would prefer it to be only once.

There are a couple of innovative technologies out there that try to suppress the source ghost. Other technologies, mostly in data processing, focus on the receiver ghost by splitting up the wavefields with specialized receivers.

to be continued…. 🙂

Suppressing ghosts will be a new topic, coming soon.

Posted in Geophysics, Seismics.

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