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Some fundamentals of geophysical exploration

There are several means of exploring the Earth by geophysical methods. Each of these techniques exploits fundamental physical aspects of Earth materials such as electrical, magnetic, acoustical, or gravitational properties. While these techniques do not allow detailed examination of the rocks beneath us, they often enable geologists and geophysicists to infer the most likely properties of large volumes of rocks. The physical fundamentals of various geophysical exploration techniques are discussed in the following paragraphs.

Gravity methods

The Earth's gravitational attraction varies slightly from one place to another on the Earth's surface. Some of this variation occurs because the Earth is not a perfect sphere, and some is related to differences in elevation on the Earth's surface. While these variations in gravity are predictable and can be calculated for each spot on the Earth's surface, other variations in gravity, such as those caused by unknown geologic features are not predictable.

For example, in north-central Kansas, there is an anomaly known as the Midcontinent Gravity High where the Earth's gravity is about 0.006% greater than normal. In other words, it would be slightly more difficult for a track and field athlete to high jump or pole vault in north-central Kansas than in other parts of the state.

Gravity measurements are made with an instrument known as a gravity meter, and maps can be produced that show differences in the pull of gravity across the state. These variations are useful in locating geologic faults and ancient volcanoes, for example. They can also indicate the presence of geologic basins that are filled with unusually large thicknesses of sedimentary rocks.

Magnetic methods

The Earth's magnetism varies from place to place, much as the gravity varies. The variation in strength of the magnetism is caused primarily by concentrations in rocks of a magnetic mineral called magnetite. Rocks such as granite and sandstone have a high magnetite content relative to such rocks as limestone and shale.

These variations in magnetism have been measured for the state of Kansas by towing an instrument known as a magnetometer behind an airplane. The resulting magnetic maps are useful in finding geologic faults and geologic basins that are filled with unusually large thicknesses of sedimentary rocks or buried mountains that arc covered with unusually thin sediments.

Electrical methods

Variations in the electrical properties of Earth materials can be measured at the Earth's surface and within drill holes. These measurements are very often made in holes at the time of drilling, but are not often made at the Earth's surface in Kansas. The presence of oil and gas in rocks in a drill hole is indicated by unusually high electrical resistance.

Concepts of seismic-reflection prospecting

Seismic reflection, a powerful technique for underground exploration, has been used for over 60 years. The purpose of this short discussion and the accompanying figures is to describe basic principles and features of seismic reflection. This discussion is for those who have heard of seismic reflection but do not know how it works. Seismic waves are essentially sound waves that travel underground at velocities of 2 to 4 miles per second (3 to 6 km per second), depending upon the type of rock through which they pass.

Seismic-reflection techniques depend on the existence of distinct and abrupt seismic-velocity and/or mass-density changes in the subsurface. These changes in either density or velocity are known as acoustical contrasts. The measure of acoustical contrast (formally known as acoustic impedance) is the product of mass density and the speed of seismic waves traveling within a material. In many cases, the acoustical contrasts occur at boundaries between geologic layers or formations, although manmade boundaries such as tunnels and mines also represent contrasts.

The simplest case of seismic reflection is shown in fig 16. A source of seismic waves emits energy into the ground, commonly by explosion, truck-mounted vibrators, mass drop, or projectile impact. Energy is radiated spherically away from the source. One ray path originating at the source will pass energy to the subsurface layer and return an echo to the receiver at the surface first In the case of a single flat-lying layer and a flat topographic surface, the path of least time will be from a reflecting point midway between the source and the receiver with the angle of incidence on the reflecting layer equal to the angle of reflection from the reflecting layer.

Figure 16--Reflection from one layer.

energy moves from source to receiver, and can be reflected off the 
interface between two layers

The sound receivers at the surface are called geophones and are essentially low-frequency microphones. Signals from the geophones are transmitted by seismic cables to a recording truck, which contains a seismograph. The seismograph contains amplifiers that are very much like those on a stereo music system. The sounds returning from the Earth are amplified and then recorded on digital computer tape for later processing and analysis. The purpose of computer processing is to separate echoes from other sounds, to enhance the echoes, and to display them graphically.

In the real world, several layers beneath the Earth's surface are usually within reach of the seismic-reflection technique. Fig. 17 illustrates that concept. Note that echoes from the various layers arrive at the geophone at different times. The deeper the layer, the longer it takes for the echo to arrive at the geophone. Because several layers often contribute echoes to seismograms, the seismic data become more complex.

Figure 17--Reflection from three layers.

echoes from each layer arrive at different times based on the kinds of rocks the seismic energy moves through

In the case of a multi-channel seismograph, several geophones detect sound waves almost simultaneously. Each channel has one or more geophones connected to it. Reflections from different points in the subsurface are recorded by various geophones. Note in fig. 18 that the subsurface coverage of the reflection data is exactly half the surface distance across the geophone spread. Hence, the subsurface-sampling interval is exactly half the geophone interval at the surface. For example, if geophones are spaced at 16 m (52 ft) intervals at the Earth's surface, the subsurface reflections will come from locations on the reflector that are centered 8 m (26 ft) apart.

Figure 18--Schematic drawing of seismic-ray paths for a single shot with a six-channel reflection seismograph.

the width of the coverage at depth is half that of the geophone spread

In fig. 19, we have placed source locations and receiver locations in such a way that path S1-R2 reflects from the same location in the subsurface as path S2-Rl. This is variously called a common-reflection point (CRP) or a common-depth point (CDP), depending upon the preference of the author. The power of the CDP method is in the multiplicity of data that come from a particular subsurface location. By gathering common midpoint data together and then adding the traces in a computer, the reflection signal is enhanced. Before this addition can take place, however, the data must be corrected for differences in travel time for the reflected waves caused by the differences in source-to-geophone distance. The degree of multiplicity is called CDP fold. A seismograph with 24 channels, for example, commonly is used to record 12-fold CDP data.

Figure 19--The concept of the Common Depth Point (CDP). Note that ray paths from two different shots (S1 and S2) reflect from a common point in the subsurface.

the CDP helps by reinforcing signal and cancelling out noise

The seismic-reflection method is used to determine the spatial configuration of underground geological formations. Fig. 20 shows conceptually what we are trying to accomplish with such a survey. Note that the peaks of the seismic reflections have been blackened to assist in the interpretation. This example is a very simple version of typical near-surface geology that depicts a buried sand lens in a river valley. The deeper the sand lens below the surface, the more difficult it is to detect, but the physical principles remain the same.

Figure 20--Schematic showing a seismic section relating to real-world geology.

The seismic reflections help delineate rock structures and rock type changes in the subsurface

Earlier in this discussion, we touched on the analogy between a seismograph and a stereo music system. A stereo music system has control knobs to enhance high frequencies (like a flute) or low frequencies (like a bass drum). A seismograph has similar capabilities in choosing the sound frequencies that are recorded. A seismologist selects the frequencies to be enhanced depending on the depth and size of underground geologic features of interest.

To detect small geologic features, it is necessary to use a seismograph that can record and enhance high-frequency sound waves. The use of high-frequency seismic waves in reflection seismology is known as "high-resolution" seismic exploration. As research and instrumentation developments allow recording higher and higher seismic frequencies, it is becoming possible to prospect for progressively smaller geologic targets.

Compressional waves, or P-waves, are the most common type of seismic wave used for reflection prospecting. P-waves propagating through the Earth behave similarly to sound waves propagating in air. P-waves generate echoes (reflections) when they come in contact with an acoustical contrast in the air or under the ground. In the underground environment, however, the situation is more complex because energy that comes in contact with a solid acoustical interface can be transmitted across the interface or converted into refractions and/or shear waves as well as reflected waves.

Seismic reflection is sensitive to the physical properties of Earth materials and is relatively insensitive to chemical makeup of both Earth materials and their contained fluids. The seismic-reflection technique involves no assumptions about layering or seismic velocity. However, no seismic energy will be reflected back for analysis unless acoustic impedance contrasts are present within the depth range of the equipment and procedures used. This is identical to the observation that sound waves in air do not echo back to an observer unless the sound wave hits something solid that causes an echo. The classic use of seismic reflections involves identifying the boundaries of layered geologic nits. It is important to note that the technique can also be used to search for anomalies such as isolated sand or clay lenses and cavities.

Fig. 21 depicts a single explosive charge fired in a drilled hole to provide a source of seismic waves for a seismic-reflection survey. The seismic waves (which are really sound waves) echo from underground rock layers. These echoes are then detected at the Earth's surface by geophones (which are really low-frequency microphones). The signals are transmitted to the recording truck via cables. The seismograph in the recording truck is much like a multi-channel stereo music system. The seismograph's amplifiers condition and amplify the data and send the data to a digital tape recorder. After the data are placed on computer tape by the recorder, they are ready for processing. The signals are processed in a computer to produce a final display called a seismic section. The seismic section displayed here shows echoes from rock units a few hundred feet below the Earth's surface (the Lawrence, Stanton, and Wyandotte formations).

Figure 21--Schematic cross section of geology, seismic-ray paths, and processed seismic data.

overall look at seismic acquisition and display

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Kansas Geological Survey, Education
Placed online April 2001
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