GB2211610A

GB2211610A - Improved seismic source of the explosive type for generating shear waves in strata

Info

Publication number: GB2211610A
Application number: GB8725250A
Authority: GB
Inventors: Paul Michael Taylor; Peter James Jackson
Original assignee: Coal Industry Patents Ltd
Current assignee: Coal Industry Patents Ltd
Priority date: 1987-10-28
Filing date: 1987-10-28
Publication date: 1989-07-05
Also published as: GB8725250D0

Abstract

A seismic shear wave source is created by placing an explosive charge (3) in an aperture (2) of a steel bar (1) and inserting this in a hole (4) in a strata, such as coal, to be monitored. The aperture is orientated so that upon detonation of the charge the resulting shear wave is polarised in a desired direction. There may be a number of such apertures along the bar. In another arrangement, the bar is replaced by a hollow cylinder with one or more apertures adapted to hold a charge carrier. The resulting shear waves propagate through the strata and they can be detected by geophones. <IMAGE>

Description

IMPROVED SEISMIC SOURCE This invention relates to a method and also an apparatus for providing an improved form of seismic source for generating shear waves in a seismic detection system.

When prospecting and analysis of rock strata are being undertaken by seismic means it is usually the practice for a seismic source of an explosive nature to be detonated at one point and for geophones or other detectors to pick up the waves created by the source as they pass through the intermediate strata and for these waves to be recorded and subsequently analysed in order that the nature of the strata may be determined. The use of normal explosive charges means that a point source of energy is used and the energy from that source is propagated in all directions through the strata without any control over the polarisation of that energy. A number of recently developed exploration techniques require the use of shear waves, in-seam seismic exploration being one example.In-seam seismic exploration requires a seismic source to generate seismic pulses in one part of the seam which can then be propagated as horizontally polarised shear waves, known as channel waves, through the seam to detectors in another part thereof. Clearly a source which can preferentially release its energy in the plane of the seam will be more efficient for in-seam seismic exploration than an uncontrolled source as it will produce more of the desired energy and less seismic noise for a given charge size.

It is an object of this present invention to provide an improved seismic source which enables shear energy to be released by design and which is suitable for many applications including in-seam seismic -work.

According to a first aspect of the present invention a method of producing a seismic source comprises drilling a hole in the strata, loading an explosive charge into the aperture of a carrier member, inserting the member so that the aperture is aligned perpendicular to the desired direction of propagation of the shear waves to be produced by the detonation of the charge. The member is preferably a longitudinal bar with a single aperture. This aperture may be in the centre of the bar and there may be a plurality of apertures along the bar. The bar may be made of any material that can withstand the proximity of the explosive charge, steel for example. The member may, instead of being a bar, be formed from a tube with the aperture or apertures containing a charge carrier or carriers.

According to another aspect of the invention apparatus for producing an improved seismic source comprises a longitudinally extending bar of material having an aperture (s) in one side thereof, provided to receive the explosive charge(s).

One example of the invention will now be described with reference to the single diagramatic drawing accompanying the specification which shows in Figure 1 a perspective view of a bar for use with the invention and in Figure 2 a plan view of the bar in place in a hole in strata.

Referring now to the drawings the seismic source is camprised by a steel bar 1 in which an aperture recess 2 has been formed and in Which is loaded a charge 3 canprising, for example, a detonator and 100g of mine approved explosive. In this example the bar is 1.5m long and is circular in cross section having a diameter of 60mm. A hole 4 is drilled in strata such as a coal seam and the hole 4 extends parallel with the lay of the seam. The dimensions of the hole are slightly in excess of the dimensions of the bar 1 so that it fits easily into the hole 4.As in this example it is required to produce shear waves with their displacement amplitude in the direction of the arrows 6 and with their propagation in the direction of the arrow 10 as shown in Figure 2, the bar is orientated in the hole 4 so that the mouth of the aperture 2 is rotated 180deg fran the direction of the arrows 6. When the charge 3 is detonated by sending a current pulse over the wires 7 to the charge 3, the reaction of the force of detonation in the direction of arrow 8 is such as to force the whole length of the bar 1 against the edge 9 of the hole 4 with such a force that shear waves are radiated with their displacement amplitude in the direction of the arrows 6 and they are propagated in the direction of arrow 10.

It will be appreciated that the necessary asymmetry required for shear wave generation is provided by the unequal distribution of forces on the borehole subsequent to the detonation. The reaction to the detonation is applied to the borehole over the whole of the length of the bar in the direction of the arrows 6 whereas the force applied to the borehole directly in the direction of the arrow 8, due to the gaseous expansion of the explosive charge, is over a much shorter length. The resulting shear waves propagate through the strata and can be detected by geophones or similar detectors located within the strata.

The use of the invention enables shear waves to be produced and propagated with a desired polarisation in any strata by the use of a member which is simple and easy to construct and the method requires no great skill in use.

Claims

aA'Ms

1. A method of producing a seismic source comprising drilling a hole in strata, loading an explosive charge into an aperture of a carrier member, inserting the member into the hole and orientating the aperture and detonating the charge to produce shear waves of a desired polarisation.

2. The method as claimed in claim 1, wherein the member is a longitudinal bar with a single aperture.

3. The method as claimed in claim 1, wherein the member is of tubular construction with a charge carrier in the aperture.

4. The method as claimed in any preceding claim, wherein the member has a plurality of apertures.

5. The method as claimed in any preceding claim, wherein the member is of steel.

6. The method of producing a seismic source substantially as hereinbefore described and with reference to the acccmpanying drawings.

7. Apparatus comprising a longitudinal bar having at least one aperture therein to receive an explosive charge.