Classification of geophysical methods

 

Geophysical methods may be broadly classified under two headings: major and minor.There are four major geophysical methods: gravitational, magnetic, seismic, and electrical.

In the gravitational methods, measurements are made of anomalies in gravity attraction produced by differences in densities of formations and structures. In the magnetic method, measurements are made of anomalies in the earth's magnetic field due to geologic bodies of different degrees of para- or dia-magnetism. In either case, the reactions of geologic bodies are permanent, spontaneous, and unchangeable; the operator cannot control the depth from which they are received.

 

In the other two major methods, energy is applied to the ground for the purpose of producing a measurable reaction of geologic bodies. This gives the possibility of spacing transmission and reception points in such a manner that the depth range can be controlled. In the seismic method, energy is supplied by dynamite explosions, and the travel times (time interval between firing of the shot and reception of elastic impulses) of refracted and reflected waves are measured.

 

In one group of the electrical methods energy is applied galvanically, and the distribution of the potential or the electromagnetic field resulting from conductive bodies is measured. These are known as potential and electromagnetic (electromagnetic-galvanic) methods. In another group, known as electromagnetic-inductive methods, the primary energy is applied inductively to the ground and the distortions of the electromagnetic field are determined.

Memorize the words:

measurement – вимір attraction – тяжіння density – густота, щільність degree – ступінь, міра apply – прикладати spacing – розставляти з проміжками transmission – передача reception – сприймання point – точка range – ряд explosion – вибух refract – заломлювати(промені) reflect – відбивати(світло, тепло) wave – хвиля galvanic – гальванічний distribution – розподіл conductivity – провідність primary – первинний, первісний distortion – спотворення, викривлення determine – визначати, встановлювати

Answer the questions:

1. What are four major geophysical methods?

2. What is measured in gravitational methods?

3. What are anomalies in the earth's magnetic field caused by?

4. Can the operator in gravitational and magnetic methods control the depth from which the reactions of geologic bodies are received?

5. What is the purpose of applying energy in seismic and electrical methods?

6. How is energy supplied in seismic methods?

7. What is measured in seismic methods?

8. What are the two groups of electrical methods?

Seismic Methods

Seismic method is based on observation of trends of seismic waves and their velocity of propagation. Seismic waves in the rock environment are reflected, refracted or diffracted. The higher velocity of seismic wave propagation is the more solid medium we may expect. Seismic wave propagation velocity can be converted to module of elasticity, or other physical properties of the rock can be derived.

Memorize the words:

trend – напрям, відхилення velocity – швидкість propagation – поширення solid – тверде тіло medium – середовище convert – перетворювати module – модуль, коефіцієнт elasticity – еластичність, гнучкість derive – виводити, встановлювати походження

Answer the questions:

1. What are seismic methods based on?

2. What happens with seismic waves in the rock environment?

3. What does the higher velocity of seismic waves’ propagation mean?

4. What can seismic wave propagation velocity be converted to?

Basic principles

A simple seismometer that is sensitive to up-down motions of the earth can be understood by visualizing a weight hanging on a spring. The spring and weight are suspended from a frame that moves along with the earth’s surface. As the earth moves, the relative motion between the weight and the earth provides a measure of the vertical ground motion. If a recording system is installed, such as a rotating drum attached to the frame, and a pen attached to the mass, this relative motion between the weight and earth can be recorded to produce a history of ground motion, called a seismogram. Any movement of the ground moves the frame. The mass tends not to move because of its inertia, and by measuring the movement between the frame and the mass, the motion of the ground can be determined.

 

 

Memorize the words:

sensitive – чутливий, сприйнятливий motion – рух visualize – уявляти, spring – пружина frame – рамка relative – відносний drum – барабан rotate – обертатися attach – прикріпляти

 

Ancient era

Replica of Zhang Heng's seismoscope Houfeng Didong Yi

In AD 132, Zhang Heng of China's Han dynasty invented the first seismoscope (by the definition above), which was called Houfeng Didong Yi (literally, "instrument for measuring the seasonal winds and the movements of the Earth"). The description we have, from the History of the Later Han Dynasty, says that it was a large bronze vessel, about 2 meters in diameter; at eight points around the top were dragon's heads holding bronze balls. When there was an earthquake, one of the mouths would open and drop its ball into a bronze toad at the base, making a sound and supposedly showing the direction of the earthquake.

Modern designs

The principle can be shown by an early special purpose seismometer. This consisted of a large stationary pendulum, with a stylus on the bottom. As the earth starts to move, the heavy mass of the pendulum has the inertia to stay still in the non-earth frame of reference. The result is that the stylus scratches a pattern corresponding with the Earth's movement. This type of strong motion seismometer recorded upon a smoked glass (glass with carbon soot). While not sensitive enough to detect distant earthquakes, this instrument could indicate the direction of the pressure waves and thus help find the epicenter of a local earthquake – such instruments were useful in the analysis of the 1906 San Francisco earthquake.

Memorize the words: pendulum – маятник stylus – грамофонна голка inertia [ɪ'nɜːʃə] – інерція of reference – згаданий scratch – дряпати pattern – малюнок

Milne horizontal pendulum seismometer. One of the Important Cultural Properties of Japan. Exhibit in the National Museum of Nature and Science, Tokyo, Japan.

· After 1880, most seismometers were descended from those developed by the team of John Milne, James Alfred Ewing and Thomas Gray, who worked in Japan from 1880 to 1895. These seismometers used damped horizontal pendulums. After World War II, these were adapted into the widely used Press-Ewing seismometer. The horizontal pendulum seismograph was improved greatly after World War II. The Press-Ewing seismograph, developed in the United States for recording long-period waves, was widely used throughout the world. This device employed a Milne-type pendulum, but the pivot supporting the pendulum was replaced by an elastic wire to avoid friction.

The basic damped horizontal pendulum seismometer swings like the gate of a fence. A heavy weight is mounted on the point of a long (from 10 cm to several meters) triangle, hinged at its vertical edge. As the ground moves, the weight stays unmoving, swinging the "gate" on the hinge.

Memorize the words:

descend – походити, успадковувати adapt – пристосовувати pivot – стрижень avoid – уникати friction – тертя damped – амортизований swing – гойдатися, коливатися gate – ворота fence – огорожа mount – монтувати, встановлювати point – кінець, вістря triangle – трикутник hinge – завіса, навішувати