1 Chapter 16: Geophysical Techniques: Seismic Waves and Interpretation
This book contains exercises for a physical geology lab class. It is under development, with a full 1st edition release planned for Fall 2024.
The goals of this chapter are:
- Evaluate seismic velocities of common Earth materials
- Understand how to set up a seismic survey
- Interpret a seismic section to understand regional geology and tectonics
Seismology is a branch of geophysics that studies earthquakes and how seismic waves move through the earth. You have already studied earthquakes, so now you need understand how waves move in the Earth. Wave movement is the basis for seismic surveys, which can tell us a great deal about the earth’s subsurface structures. We can learn about the large layers in the earth, such as the mantle and core, or much more locally, like the layers of rock at a building site.
You already know about P- and S-waves from studying earthquakes in Chapter x. The rate these moves (velocity) depends on rock type as well as the temperature, pressure, and orientation of the fabric. Figure 16.1 summarizes the velocity of many common rock types measured at the Earth’s surface.
Exercise 16.1 – Investigate Seismic Velocities
Measure lengths of all sides of your rock sample using the caliper (Zero the tool). Record three measurements per side and calculate the average distance in mm.
Set-Up Equipment (V-Meter MK IV) Be sure not to twist the cables.
Connect the transducer (source) cable
Connect the receiver cable
Connect charger
Use 2 Transducers (Source and receiver). First apply CIP Lube to the sample/rock surfaces (a little goes a long way)
Turn on the V-Meter and set acquisition parameters (Setup Menu). Press the “ESC” button to go to Main Menu and use these settings:
- Amplifier gain (50)
- Picture rate (2 mHz)
- Pulser voltage (Low)
Calibrate transducers (Setup Menu > Calibrate Transducers). Transducers should be aligned with light pressure. Wait until it says “Calibration is done”
How often do we calibrate? We should calibrate every time we change the voltage, number of pulses, and gain.
- Set P-distance in the V-Meter
- Convert the rock sample distance into meters
Measure rock velocity (Test Menu)
- Measure the velocity of Granite and one other rock
- Analyze results
- Does the obtained velocity value agree with published velocities?
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Seismic surveys are used to determine what lies beneath our feet. These surveys use energy from various sources which is either reflected by or refracted through layers in the Earth (Figure 16.z). The energy is recorded by geophones which is an instrument for detecting vibrations passing through rocks, soil, or ice. The signals recorded are called the seismic amplitudes, a measure of the difference in rock properties between two layers. Seismic data is commonly converted to impedance or hardness (this is not the same as Mohs hardness). The relative hardness can be positive, negative, or the same. Thus, many seismic sections will use three colors to distinguish different strata.
Exercise 16.2 – Conduct a Seismic Survey
Exercise 16.3 – Interpret a Seismic Survey
The geologic history of the North Sea, between Norway and Great Britain, involves several tectonic events as well as significant sedimentation. In 1976, significant quantities of oil and gas were discovered with center of the North Sea now having many oil and gas wells. Since it is underwater, the only way to figure out what happened is to look at seismic surveys and/or drill holes. As of 2023, there were over 800 wells drilled just in the Norwegian section of this marine basin. Some of these were just for exploration and others have yielded significant quantities of oil and gas.
First look at the stratigraphic column that shows the rock units for this region (Figure 16.3). Each blue, white and red line represents different rock layers with different seismic velocities or amplitudes. using a color scheme with blue for positive amplitudes, red for negative amplitudes, and white for zero amplitude. Notice whether these layers are continuous, discontinuous as well as how bright is the color. These combinations of these features are called seismic attributes or characteristics.
- Describe characteristics of the Neogene units compared to the Cenozoic units.
- Describe how you would identify the base of the Cenozoic:
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How many distinctive layers are there in the section from Cretaceous to Jurassic? How can you identify the base (bottom) of this sequence?
Figure 16.3 – Stratigraphic column with data from well 31/6-6. This well data was used to construct a seismic stratigraphic section for the different rock types. Geologic time is subdivided into P = Period, E = Epoch. In the right-hand column are the boundaries between different units as identified by seismic data. The blue color is used for positive amplitudes, red for negative amplitudes, and white for zero amplitude. Image credit: Bell, Jackson, Whipp and Clements (2014) CC BY - Now that you can identify the different types of rocks, mark the boundaries between these units on Figure 16.4. If the layers are not continuous, there may be faults or folds.
- Identify and mark at least 3 faults on this figure. What type of faults? What is the tectonic environment?
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- Are there any unconformities in this seismic section? If so, what type?
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- Critical Thinking: Why are there so many drill holes in this area?
Figure 16.4 Uninterpreted 2D seismic reflection profile across the Horda Platform in the North Sea between Norway and Great Britain. The vertical axis represents the time it took for the seismic energy to be recorded at the geophone array. The white area at the top is the ocean. The colored lines represent different seismic amplitudes with blue for positive amplitudes, red for negative amplitudes, and white for zero amplitude. The vertical lines are locations of drill holes. VE = vertical exaggeration. Since this is vertically exaggerated, dipping lines will appear steeper than they actually are. Image credit: Bell, Jackson, Whipp, and Clements (2014) CC BY
Additional Information:
References
Bell, R.E., Jackson, C. A.-L., Whipp, P.S., and Clements, B., 2014, Strain migration during multiphase extension: Observations from the northern North Sea, Tectonics, v. 33, p. 1936-1963 DOI: 10.1002/2014TC003551 CC BY
