Seismic Measurements Of Waves In Seams As A Geophysical Tool In The Extraction Of Raw Materials

Inhaltsverzeichnis

1 Introduction

2 Theoretical Basics

3 Measuring Principles
3.1 Reflection Measurements
3.2 Through-transmission Measurements

4 Measuring Equipment And Implementation Of Measurements

5 Susceptibility To Interference Of The Measuring- System
5.1 In The Apparatus
5.2 In The Measuring-System

6 Measurement Results In The Acquisition Of Disturbances And Measurements With Special Objective

7 Evaluation Of Measurement Results

8 Future Development

9 Summary

10 Bibliography

1 Introduction

German studies have found that a significant proportion, between 16% and 20%, of all the raw materials deposited in the form of fluff are geologically disturbed. /1/^[1]

Due to the extreme mechanization of the mining companies in the German coal industry (approx. 99.3% in 1980) and the associated capital tie-up in the technology, geological disturbances in the seams, which may hinder or bring to a standstill, adversely affect the production results. So it costs depending on the nature of the fault and the longwall equipment 300,000 DM up to 2.5 million DM to pass through a fault in the seam. If even a mining operation has to be abandoned because it can not be passed through, the costs increase by almost 3 times as much as production, rental of machinery, construction of a new facility and relocation. / 2 /

From the above figures, it immediately becomes clear how important it is to explore in advance the homogeneity of the raw material in the seam of planned mining operations.

There are various methods available for exploring the apron, which have different levels of security and statements.

As one of the safest methods of forecasting of underground situations, seismic measurements of waves in seams have emerged, which are to be dealt with in this work.

The work is an interdisciplinary bridge between geophysics and practical application in the extraction of flooded raw materials; In the present specification and introduction coal is chosen because of its importance to the economy.

The basic system is for all seamy ones Deposits are equal and of general importance.^[2]

2 Theoretical Basics

The prerequisite for the application of the Flözwellenseismik is the presence of a seismic channel, in which there are between the above and below the seam layers and himself sufficient density differences. An example is a coal seam between two rock layers.

The representation is given because the density contrast between coal and adjacent rock is approximately 1: 1.75 to 2, and the coal is a medium in which seismic waves propagate more slowly than in the surrounding rock. Basically, the wave velocity increases with the density of the medium.

The ratio of the propagation velocities of the waves is equal to that of the density contrast. The emergence of Flözwellen is systematically since r provided in Figure 1.

Abbildung in dieser Leseprobe nicht enthalten

Figure 1: Formation of seam waves by mining by reflection of seismic waves at the interfaces of a seam / 13 /

Seismic waves, which are excited in the seam, can not radiate spherical, as it really is their nature, but are partially reflected at the two interfaces to the hanging and lying and thus form a special wave, the "seam-wave". These waves are excited in the seam, guided through the seam and received in the seam again.

Meet these waves on a fault ^[3], which cuts the sea this technique is based on the measuring method of the seismic of seam-waves in such a way that behind the interface rocks of different density than coal are present, they are reflected. If there is no disturbance, the flute waves spread unhindered. This technique is based on the measuring method of the seam seismic/ 7 /.

To describe seam-waves in more detail, it requires the following differentiation:

1. compression waves - P waves

2. shear waves - S waves

a) vertically polarized - SV waves

b) horizontally polarized - SH waves

Depending on the geometry and particle movement, flute waves are differentiated into those of the Rayleigh-type and the Love-type. The Rayleigh type consists of P- and SV-waves, those of the Love-type exclusively of SH-waves.

Symmetrical waves of the Love type have proven to be well-suited for measurement practice, and can be easily separated from other types of waves by targeted orientation of the vibration pick-ups / 10 /. Like all guided waves, flute waves have a dependence between the propagation velocity and the wavelength, they disperse^[4]. In the first place, the dispersion is bound to the different shear wave velocities of coal and secondary rock, but it also depends on the channel width, means seam height.

The effects of the dispersion are in detail:

l With increasing dispersion, the energy of the waves is more strongly bound to the soft medium, so that at higher frequency, the wave can not leave the medium.

l Even with an excitation of the wave over short temporal influence of the pulse, the flute wave appears as a long, time-extended wave trains due to the dispersion; this causes a differentiation between the faster phase velocity and the slower group velocity. The relationship is shown in Figure 2:

Abbildung in dieser Leseprobe nicht enthalten

Fig. 2: Dispersion curve of the symmetrical flute waves of the Love-type / 10 /.

with: C = phase velocity

U = group speed

E R = ratio of energy in the seam to total power

x - axis = frequency [Hz] * Seaminess [m]

The group velocity curve has a pronounced minimum at which the kinetic energy carried in the medium is nearly equal to the total energy. This minimum is called the Airy phase, which stands out at high frequencies of almost 500 Hertz.

- The seam-wave is the longer by the dispersion, the greater the distance between the excitation and reception.

The Airy phase is used in subsurface seismic as a useful signal, while all other wave types represent noise that can be separated as low frequency by filtering. The principle is shown visually clearly in Figure 3:

Abbildung in dieser Leseprobe nicht enthalten

Fig. 3: Theoretical Flözwellenseismogramme / 6 /

with: a) Registered Flözwellenzug

b) filtered out Airy phase

c) calculated envelope^[5] the Airy phase

The seismogram time in part a) the flooding of the flooding waves depending on the way, and at the end of the trains the Airy phase is easily recognizable. Part b) clearly shows the separated Airy phase by filtering. Since the digital processing of the pure Airy phase is only possible through the formation of an enveloping envelope in order to be able to neglect the phase shift in the filtering, it is necessary to dispense with a part of the information obtained in the seismogram. The envelopes are reproduced separated in part c) / 2 /, / 3 / and / 9 /.

[...]

^[1] The given numbers refer to the numerical sequence in the bibliography as reference

^[2] Seam, mining name for the accumulation of sedimentary formed, usable minerals in the form of a layer that has in relation to their thickness a large length and width and is limited by almost parallel surfaces" from: The Little Mining Dictionary, Verlag Glückauf Gmb H, Essen, 1983. In the following, explanations of mining technical terms are taken from all of this cited source, without this being stated again.

^[3] A geological change of the original form of deposition of a mineral or rock. Disturbances can be caused by tectonic forces, which manifest themselves in the form of layer tears (distortions).

^[4] In physics, dispersion is defined as the dependence of wave properties on wavelength.

^[5] mathematically a curve that envelops a group of curves