Seismic Velocity Model

Thrust Belt Imaging Inc.

By Nancy Eaton

This image is a cross section of a thrust belt, a geological formation caused by compressional tectonics, a natural process that ultimately results in the formation of large mountain ranges.

Thrust belts present enormous potential for tapping into oil and gas deposits, but it can be difficult for energy companies to determine optimal drilling sites due to the complexity of the underlying geology. As a result, drilling in thrust belt areas can present a significant financial risk, as each attempt can cost up to ten million dollars.

Today, companies interested in drilling in these areas can decrease their risk by turning to Calgary’s Thrust Belt Imaging (TBI) to analyze seismic data using their proprietary imaging technology, called Structures. The image shown here was generated by TBI and represents a cross section of a thrust belt in Alberta, Canada.

“We’ve developed our own system for imaging seismic data,” says Dr. Rob Vestrum, a partner at TBI. “One thing that’s required to get an image of the sub-surface is to understand the rock velocity. The seismogram records the time it takes for a seismic wave to echo or bounce off a rock formation in the sub-surface, but what we really want is to get an image of the sub-surface depths. The difference, of course, between distance and time is velocity. So we need a velocity model of the sub-surface in order to accurately image our seismic reflectors in depth. Using Structures, our interpretive model-building tool, we interpret the major and velocity boundaries on a seismic image, and use that interpretation to further refine the seismic image.”

TBI applies a series of diagnostics to its model to determine and correct various velocity effects that may obscure or mis-position seismic reflectors. Vestrum explains that velocity can vary depending upon such things as lithology (the type of rock), and depth of burial, since rocks under pressure tend to have higher velocity. The TBI application is also unique in including an algorithm for correcting anisotropic effects, which is when seismic waves move at higher or lower velocities depending upon whether they move in directions along or across rock bed layers.

In this image, the colors represent a rainbow scale of rock velocity. The cooler colors—the purples and blues—represent the lower velocities in the range of 3,000 to 3,500 meters per second. The hotter colors—the reds, yellows and oranges—are in the higher velocity range of about 6,000 meters per second. The colors in between, the green and the blue, are in the range of 4,000-5,000 meters per second.

To gather the original data, the exploration company drills several hundred ten-meter shot holes in a linear path and places dynamite charges within them. Receivers are also laid out along this path that record the seismic reflections as each charge is exploded. “So you have several hundred shots, and for each shot you’ve got several hundred receivers listening to the reflections. So that ends up being tens of thousands of seismic traces recorded for a simple 2D line. In 3D seismic data, we often receive millions of seismic traces,” says Vestrum.

The exploration company sends the raw field data, typically on tape, to TBI, which loads it into WesternGeco’s Omega seismic data processing application. These processes prepare the data for importing into their Mac-based seismic imaging system. “There are two parts of our system that run on the Mac.,” Vestrum says. “The first is Structures, the model-interpretation system we’ve been talking about, which is where we draw the interpretive seismic velocity model and then display the seismic results.”

In the zoomed section of this image, Vestrum’s annotations are shown as blue boxes with hairlines running through them. “That’s where I’ve interacted with my image and estimated where the dip, or tilt, of the layers should be in order to define the orientation of the high velocity direction,” says Vestrum. The blue hairlines without squares represent the computer’s interpolation between his points, or ’picks,’ of where the dip should be. “And so I have real-time updating of what the dip in my model is going to look like given the dip picks that I make on the seismic image,” he says.

“The second place we use the Macs is for the imaging algorithm, which takes the pre-processed seismic data and processes it into a seismic image using our interpreted velocity model,” Vestrum explains. “We run that on an Xgrid cluster of Macs that do the processing. And because we want to be experimentally interactive, one thing we’ve done is made our system quite efficient as far as getting data in and out of the visualization package. Once we have a new velocity model, we send the job off to our 28-node Xgrid cluster to make all the calculations required to create a new seismic image, and then we can import that seismic image back onto our interpretive model builder. And that whole process takes about ten minutes.”

The red lines on this image that converge to a single point in the subsurface represent multiple seismic raypaths that image the same point in the subsurface. The two lines on the far left and right of this fan of rays represent the raypath from a shot and receiver several kilometers apart. The yellow line running up the middle of the ray fan illustrates the raypath for a shot and receiver at the same location. “We use this diagnostic to see the rock layers through which our seismic energy has travelled and we can also see how the rays bend or refract when crossing rock boundaries,” says Vestrum.

Displaying the final deliverable images on two 30-inch Apple Cinema displays, TBI confers with geologists and geophysicists from the exploration companies to help them better understand their targeted geological settings. “From our images, they learn a lot about the seismic data while they’re working through the process with us and they also get a more accurate view,” Vestrum says. “We’ve imaged structures in areas where other companies have gone before us and could not get a usable seismic image.”

Vestrum says that TBI is working on new version of their application that can do three-dimensional rendering, enabling them to extract two-dimensional ’slice’ images from a three-dimensional model. “We’re thinking we might want to use a third display,” he says. “Right now we look at two different directions in the survey on the two displays: one screen shows an east-west slice and one shows a north-south slice. The third screen, perhaps displayed above the other two, would show a map view or a three-dimensional view of the model surfaces.”

“We see room for improvement in handling 3D data volumes and visualizing 3D structures in our velocity model,” adds Vestrum. “As graphics cards become faster and computer memory capacity increases, we anticipate that we will soon be as interactive and experimental with our 3D models as we are with our 2D models today.”