Astronomy versus Seismic Exploration: More Similar Than You Thought
I compare some interesting aspects of Astronomy and Seismic Exploration. The most obvious differences related to the scale of observations, and the fact that astronomers rely upon 'passive' measurements and geophysicists typically create their own 'active' sources of energy. Both pursuits must overcome incredibly weak signal strengths, and both deploy a variety of sensors in creative ways to enhance the level of information in their data.
Mapping the Heavens
In this photo a camera in a fixed location takes a beautiful photo of the heavens on a clear night. In the background you can see a radio telescope dish pointing upwards. By precise mechanical control, such a dish can scan the sky, and accounting for the rotation of the earth, can also produce an image of the heavens – but over the range of frequencies and wavelengths corresponding to radio waves, rather than corresponding to visible light.
As we will see, the resolution and insights gained by geophysicists deserve to be placed on the same level of achievement as the insights from astronomers and astrophysicists…
Both the camera and the radio telescope struggle to provide much three-dimensional insight into the universe around us, although an apparent ‘shift’ in the color of visible light from each source can tell us information about the relative motion between us and that object, as well as provide some insights into our relative separation. For example, a shift towards the wavelengths of red light is indicative of objects moving apart—which is the basis for our belief that the universe is still expanding.
If several radio telescopes are placed a large distance apart we can exploit the platform of interferometry to build a three-dimensional understanding of the universe, and perhaps the grandest example is the EHT, or Event Horizon Telescope, an international collaboration of installations that can use facilities distributed over the full diameter of the earth. The GPS clock calibration of all locations is good to less than one-millionth of one second, and this collaboration produced the first-ever image of a black hole—or at least the event horizon around the black hole.
Why go to such a massive expense and effort to build technology such as this?
Part of the reason is that the atmosphere of the earth absorbs much of the electromagnetic energy spectrum, as we can see across the top of the figure above.
Visible light can obviously pass through the atmosphere, allowing us to see beyond the atmosphere, and longer wavelengths, between centimeters and several meters corresponding to radio waves, are also detectable from the surface of the earth. But pretty much all other wavelengths of electromagnetic energy can only be detected by placing appropriate sensors above the atmosphere in earth orbit.
You can see across the top of the figure that the range of detectable wavelengths varies from fractions of a nanometer, where a nanometer is one-billionth of a meter, to hundreds of meters.
Our eyes can see objects as small as tens of micrometers, and for comparison, a strand of DNA is a couple of nanometers across, which is also roughly the wavelength of gamma rays.
In the top row of the figure above we see a collection of images produced from different electromagnetic wavelengths. A large galaxy is being consumed by a supermassive black hole somewhere near its center.
In the lower left image we see an entirely different image of this galaxy. Super high velocity gas is being ejected into space as the black hole consumes the galaxy.
The version of the same figure above has various telescopes and arrays superimposed over the images they contributed too. Some instruments are placed above the earth's atmosphere and some instruments are configured as vast arrays of sensors on the terrestrial surface.
Overall, we see complementary measurements from a variety of observations at the surface of the earth and from above the atmosphere.
When we record such a complementary range of wavelengths we see in the lower center image that one can build composite images containing information that cannot be derived from any of the contributing narrow-band observations in isolation.
Key Takeaways from Astronomy
Some key messages from astronomy are as follows…
- It is essential record data over a large range of frequencies / wavelengths.
- To record incredibly weak signals requires incredibly sophisticated sensors.
- Some instruments can be placed on the ‘wrong side’ of the atmosphere (surface).
- Other instruments need to be placed above the absorbing atmosphere (in space).
- The larger the area of the antennae and the larger the number of sensors within the antennae the higher the sensitivity, accuracy and resolution of measurements.
But first, let’s identify a few important differences.
Differences Between Electromagnetic Waves and Seismic Wavefields
Electromagnetic energy travels directly from points in space to observers on earth
There is almost complete loss of amplitude as light travels through the vast distances of space due to spreading (1/𝑑^2 )
In contrast, seismic wavefields are highly scattered acoustic pressure (sound) energy that propagate in highly complex ways throughout the geology of the Earth’s subsurface
There is almost complete loss of energy as seismic wavefields travel only 1 km (!) through the Earth’s subsurface due to a combination of spreading, transmission and absorption effects.
Seismic Exploration Requires 'Active' Sources
Overhead photo of a Ramform vessel towing an array of sources (circled) and streamers. The cartoon on the right is a schematic underwater perspective. These configurations of sensors represent the largest moving objects in the world. Up to 18 x 8 km streamers with groups of sensors every 12.5 m. Each sensor location is in fact a multisensor location (collocated pressure and particle velocity sensors). The streamer spread width can be as large as almost 2 km.
If I limit myself to the marine environment where PGS operates, the images above are an example of our own vast seismic sensor arrays used to survey the subsurface of the earth.
The photo on the left is taken from above, and the rendering on the right is of the same setup, but taken from a perspective below the water, looking upwards to the surface. The line of yellow objects along the surface of the water are floats, or ‘head buoys’, below which are suspended each seismic streamer at a depth of about 20 meters. As many as 18 streamers may be towed, each typically 8 to 10 km long, and there is a group of sensors placed every 12.5 meters along every streamer. In the PGS case we actually have two groups of different sensors every 12.5 meters, so we refer to these as multisensor streamers.
The multisensor platform records different and complementary information about the seismic wavefield produced by the acoustic sources towed in front of the streamers. You can see air bubbles from compressed air released into the water as often as every few seconds. The associated pressure wavefields travel downwards into the earth, and some of that acoustic energy is weakly scattered back up to the surface from acoustic contrasts in the geological strata. We need this vast array of extremely sensitive sensors to coherently record those very weak signals that do find their way back to the surface.
Seismic Imaging
3D seismic image displayed in a manner so that the highly faulted and deformed geological interface between two layers of geological strata is visible in the middle. The reddish images are 'vertical sections' through the same data. Sedimentary layers within the Earth resolved as acoustic contrasts within seismic images. Such seismic ’volumes’ may cover over 100,000 square kilometers and form the base product for geoscientists to search for candidate drilling locations.
Seismic Absorption
If we think back to the absorption of electromagnetic energy by the atmosphere of the earth, a similar principle applies to the absorption of seismic wavefield frequencies within the earth…
In the series of seismic panels above, the vertical scale is a depth between about 1 and 10 kilometers, and the numbers annotated across the top are the frequencies of the resolvable seismic reflection signals. We see that, as is quite commonly the case, frequencies above about 50 Hz can only be recovered down to depths of a few kilometers before the returning signals become too weak and are overwhelmed by noise. We can improve this situation somewhat with appropriate computer processing, but the fact is that by comparison to the electromagnetic spectrum used by astronomers, geophysicists have to work with a tiny range of frequencies.
Most target geology seismic wavelengths are in the order of tens to hundreds of meters, which is at the very long end of radio astronomy wavelengths. Seismic wavelengths of about 30 to 40 meters only penetrate about 2 to 3 kilometers into the earth. Longer wavelengths penetrate deeper. The resolution of geological features in seismic images is proportional to one-quarter of the 'dominant wavelength'.
It therefore follows that our highest resolution will typically be worse than about 10 meters.
A Problem of Scale
A big challenge to seismic exploration is the fact that reservoir engineers want reservoir property information at sub-millimeter scale, despite our seismic image resolution being no better than several meters. In the figure above, we see that many layers in the spectacular geological outcrop on the left are compressed into one ‘seismic layer’ on the right.
Computer rendering of a 'digital rock' showing the external view on the left and the pore spaces on the right. from an oil and gas perspective, we are interested in the amount of porosity (the higher, the better) and permeability (interconnectivity of the pore spaces: also the higher, the better) in the reservoir rock fabric. Note the 5 nanometer (0.000000005 meter) scale bar.
The computer rendering of a sedimentary rock sample above shows the faces of a small cube on the left. The scale bar is 5 nanometers long, which is between the width of DNA chains and the size of proteins.
We can see some indication of the porespace in the face of the cube that might be saturated with fluids, and if we had the magic power to see only the equivalent porespace we would see the image on the right. This is the type of reservoir information desired: what is the volume of porosity, how are the pores interconnected, what fluids are hosted within this porespace, under what pressure and temperature conditions, how might fluids move through the rock under producing well conditions, and so on.
We clearly need a bridge between the scale of seismic images and the grain scale, and we need a bridge between the elastic properties of seismic data and the geological properties of the reservoir rocks and fluids.
Wireline Logging
Just as the solution to the absorption of electromagnetic energy by the atmosphere is to place appropriate sensors above the atmosphere, if we want to make much higher resolution geophysical measurements than the resolution achieved by surface seismic experiments, we must place sensors deep into the earth at the reservoir level.
When an exploration well is drilled it is common to deploy a variety of tools down the borehole and make measurements of the rock and fluid properties between the face of the borehole and the geology up to several meters from the face of the borehole. Wireline logging may include sonic recordings in the kilohertz range, the resistivity measured by electrical currents transmitted between sensor locations within the borehole, the natural energy of gamma rays emitted by certain rocks, and so on. Core samples will also be recovered from the borehole and analyzed even more comprehensively in the laboratory.
Wireline logging has significantly higher resolution than surface seismic, but the information gathered is limited to the discrete borehole location.
Subsurface Characterization of Rock and Fluid Properties
Remember also how the measurement of a complementary range of frequencies and wavelengths of the electromagnetic energy spectrum allows astronomers to produce composite images.
The same logic applies to the quantitative interpretation of seismic data.
In the upper left and upper right images above we see 'elastic' properties of the earth derived by building models that explain the observed seismic data. This information on its own tells us little about the subsurface geological properties.
The lower left image above, is a compilation of complementary insights derived from the borehole and laboratory measurements—the insights gained by making measurements within the earth where we can derive much higher resolution information.
Collectively, the surface seismic measurements calibrated to rock physics models—enable us to use the seismic data in different ways and estimate geological reservoir properties.
In the center image, the colors represent the variation in the density of the rocks, which is one useful and better way to view images of the subsurface. The black ellipse shows the location of a low-density feature that appears to be gas hosted with a sedimentary formation. Further investigation is required, but now we have some greater insights in our quest for commercial oil and gas.
Summary
Geophysicists deal with a range of particle displacements (at the sensors) and observable seismic wavelengths that span the scale of the entire electromagnetic spectrum used by astronomers, and the scale of matter from DNA to the largest animals.
Both astronomers and geophysicists must use incredibly sophisticated instruments and software to record and analyze our data. For geophysicists, multisensor recordings provide the most informative data.
Ideally, we complement our surface-based observations with various data recorded within the reservoir geology, and together these enable us to predict the distribution of reservoir properties at the grain scale deep in the earth
Interested in Hearing More?
PGS have a collection of short Technology Webinars at https://www.pgs.com/techbyte.
Disclaimer
The content discussed here represents the opinion of Andrew Long only and may not be indicative of the opinions of Petroleum Geophysical AS or its affiliates ("PGS") or any other entity. Furthermore, material presented here is subject to copyright by Andrew Long, PGS, or other owners (with permission), and no content shall be used anywhere else without explicit permission. The content of this website is for general information purposes only and should not be used for making any business, technical or other decisions.
Comments
Post a Comment