Haunted Seismic Data, Ghostbusted by Science
USGS-designed capability removes "ghost reflections" from seismic data
Removing ghostly artifacts from sparker seismic reflection data doesn’t require a phone call to a certain group of parapsychologists in matching jumpsuits. The so-called “ghostbuster” capability developed by USGS provides the highest resolution and most accurate sub-bottom images for scientists to make the best possible interpretations and seismic hazard assessments.
When Research Geophysicist Jared Kluesner and Geophysicist Alicia Balster-Gee open new seismic reflection imagery on a computer, they expect to see a few ghosts in the data. Not Sixth Sense-type ghosts, but shadowy apparitions in the layers of sub-bottom imagery that obscure the overall picture, making it difficult to interpret seismic faults, submarine landslide deposits, and other geologic features.
Are Kluesner and Balster-Gee concerned that all these hard-won seismic data are possibly haunted? Not particularly, because they are, in fact, ghostbusters when it comes to removing so-called ghost reflections from seismic data.
Seismic Reflection
The USGS and partners rely on low-energy marine seismic sources to conduct offshore research on geologic structures beneath the seafloor. Just as sonar uses sound to detect objects and measure distances underwater, seismic reflection can “see” through the seafloor by reflecting sound off the layers below, which travels back towards the sea-surface to be recorded by a long string of underwater microphones called a streamer. Scientists then process this data into an interpretable image in which earthquake-related features such as faults and submarine landslide deposits can be mapped.
Over the last decade, USGS has invested millions of dollars, hundreds of days at sea, and thousands of person-hours in acquiring, processing, and interpreting seismic reflection profiles. Until recently, a large portion of these datasets did not fully exploit the high-resolution potential of newer seismic sources, effectively limiting scientists' ability to best interpret the data.
Haunted Sparker Data
One low-energy seismic source that is used for offshore surveying is the sparker. A sparker is a sound source that uses a high-voltage bank of capacitors, charged to several thousand volts, that then discharges through a spark-gap system submerged in a conductive liquid (in this case, saltwater). The initial spark creates an outgoing pulse, but also vaporizes the surrounding water, creating a bubble with a sound pulse of its own.
These pulses of sound don’t just travel down to the seafloor—they also travel up and reflect off the sea-surface to create ghost reflections, which appear as short-period multiples in the data with reversed polarity. This polarity switch is similar to inverting colors on a photograph, except that with seismic data the positive and negative amplitude values are reversed. These unwanted ghost reflections also occur on the recording side of the system. Sound reflects off the seafloor and layers below, travels up to the streamer and past it, reflects again off of the sea surface, then travels back down to the streamer.
As the data are processed, this “complex source signature” creates artifacts in the seismic reflection profile: fuzzy overlays, bubble pulses, or ghost reflections, that reduce the overall resolution of the image, potentially masking critical geologic information.
Busting Ghost Reflections
To tackle this problem, USGS and partners set out to develop approaches that could directly characterize the outgoing sound pulse in an effort to best remove these spurious reflections. The goal was to effectively “collapse” the sparker’s complex source signature, eliminate ghosts, and produce a more accurate and higher-resolution image of the sub-surface. The result: A towed setup called the “ghostbuster” that accurately records the outgoing complex source signature and a series of processing workflows to remove the bubble pulses and ghost reflections from the data.
By placing underwater microphones called hydrophones below the sparker source, scientists can record each individual pulse that is generated by the sparker sound source. This not only captures the down-going primary pulse and bubble pulse, but it also records the sea-surface ghost reflections of the same pulses. These recordings are then used during the data processing stage to deterministically remove the unwanted reflections from the data. Ghost reflections on the streamer side of the system are removed using predictive or modelling approaches.
“While low-energy sparker sources can provide high-resolution imagery and are relatively simple and inexpensive, their complex and unpredictable source signature requires careful processing to remove artifacts like ghost reflections from offshore sub-bottom data,” said Kluesner.
Who You Gonna Call?
Removing ghosts from sparker seismic reflection data, or de-ghosting, doesn’t require a phone call to a certain group of parapsychologists in matching jumpsuits. The “ghostbuster” capability, laid out in the publication Practical approaches to maximizing the resolution of sparker seismic reflection data, provides the highest resolution and most accurate images for scientists to make the best possible interpretations and hazard assessments. Ultimately, science using these data contributes to offshore fault databases, and helps inform earthquake probability and seismic hazards maps.
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Coastal and Marine Geohazards of the U.S. West Coast and Alaska
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Coastal and Marine Geohazards of the U.S. West Coast and Alaska
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