JPS5853876B2 - Method of ground investigation using elastic waves using the boundary surfaces of each layer in the hole as the focal point - Google Patents

Description

[Detailed Description of the Invention] This invention provides elasticity for the structural state of the ground that is the subject of underground civil engineering work, particularly in urban areas, such as the installation of underground sewer pipes and electrical cables, subway construction, and other building foundation construction. It concerns a method for surveying by waves.

In general, the ground that makes up urban areas and their surrounding areas is often complex and diverse, but this is due to the fact that houses and buildings are densely packed, public roads are paved and have a lot of foot traffic, and there is frequent car traffic. Therefore, it is difficult to conduct a satisfactory ground investigation.

Especially when it comes to shield construction along long routes along public roads,
Since we have no choice but to stop at the so-called ``Bori/Gue'' survey, the ground conditions between the polls remain completely unknown, which often causes various problems during construction.

The present invention was researched and developed in order to solve these difficult problems, and makes it possible to investigate long continuous so-called F lines or planes without being affected by ground obstacles, even in urban areas. It has a major advantage in that.

Traditionally, elastic wave exploration methods include the refraction method, which uses refracted waves, and the reflection method, which uses reflected waves.
In most cases, the refraction method, which was successfully put into practical use and patented by Mintrop (Germany) in 1919, is applied, and is now widely used as a powerful method for geological surveys.

This refraction method is usually called general elastic wave exploration method,
An elastic wave that is emitted by an explosion of dynamite, a falling weight, etc. at the surface or shallow depths, enters the underground, is critically refracted at the boundary with the lower layer, propagates through the layer surface, and is critically refracted again until it returns to the surface. The basic principle is to read time, or transit time, from wave records observed by seismic receivers placed in a straight line on the earth's surface, and to create and analyze transit time curves.

However, in the observation method based on this principle, the fine waves of upper layers where the wave propagation speed is higher than the lower layers, such as concrete or asphalt paved roads, buildings with concrete foundations/basements, etc. If there is an obstacle that causes distortion in the propagation of waves, theoretically the wave motion would not propagate regularly to the lower ground layer, making it impossible to apply this method.

The method of the present invention solves this problem, and utilizes poling holes conducted at appropriate intervals for ground investigation, and determines the location of each stratum boundary surface within the confirmed holes, that is, the depth of the ground at a predetermined depth. The principle is to emit an earthquake and receive and observe the waves that reach the earth's surface through direct and layered propagation and critical refraction.

Therefore, it differs greatly from the general elastic wave exploration method in terms of the ejection format, the propagation and appearance position of refracted waves, and the form of the travel time curve.

The details are described below. Figure 1 shows v1 speed layer, v2
Schematically represents the case of a horizontal three-layer structure in which the velocity layer and the v3 velocity layer are located at depths of zl and z2, respectively, Figure A is based on the principle of the present invention, Figure B is the case of general elastic wave exploration, The wave propagation path (lower figure) and travel time curve (upper figure) are shown.

Regarding diagram A according to the principle of the present invention, the wave emitted at the position of the interface h□ between the V1 layer and the V2 layer in the hole at point 0, and the wave generated by vl becomes a wave path with a critical angle θ1□28In”- at a distance of 2 Up to (critical distance)

Therefore, the emitted wave of hl advances to the 73rd layer surface in the lower layer, undergoes critical refraction, travels along the layer surface, is critically refracted again, and appears on the surface at a distance X2, during which time the travel curve (time t is plotted on the vertical axis).

The horizontal axis is a straight line (distance The refracted waves that are refracted at the critical refraction angle θ1□= at the boundary surface, propagated along the layer surface, critically refracted again, and returned to the earth's surface are each observed as initial waves.

In this case, the travel time curve &L is v1 up to the distance (critical distance) where the direct wave and the refracted wave traveling through the v2 layer arrive at the same time (critical distance)
A straight line indicating the velocity of the layer (distance (critical distance) until the inclined waves arrive at the same time) There are the following differences and features compared to exploration.

(2) In the case of the present invention, unlike general elastic wave exploration, there is no direct wave that propagates horizontally near the ground surface, but only direct waves and refracted waves that reach the ground surface from below.

Therefore, even if a high-velocity layer exists near the ground surface, it will hardly be affected.

■ As can be seen from Figure 1, the critical distance for each layer is extremely short compared to general elastic wave exploration, so the travel time curves of refracted waves propagating through each layer from very close to the focal hole can be obtained over long periods.

Taking the critical distances X1 and Xcl as an example, from the theoretical formula,
■ Since the critical distance is significantly shortened, in the case of the present invention, even when surveying deep foundations, a long survey line that takes into account the critical distance like general elastic wave exploration (normally required when the depth to the foundation is 2) is required. The length of the survey line to be used is L≧6 to 7Z), and it is not necessary to install the cable, and it can be applied even in a narrow area.

■ By placing the epicenter at the boundary between each layer in the hole, the attenuation of the wave path corresponding to the critical distance of the seismic energy is eliminated, the wave transmission distance becomes longer, and good records can be obtained, making it possible to reduce the sheath intensity. I hope for improvement.

■ For waves incident from the earth's surface, if there is a high-velocity layer or a low-velocity layer as an intermediate layer, if it is dropped under certain conditions, it will not be possible to record the existence of the lower layer on the travel time curve. In the case of &, it is possible to grasp its existence because it emanates from below.

■ Also, theoretically, in the general elastic wave exploration method, if the thickness of the lower layer exceeds a certain thickness determined from the velocity value of the upper layer, it will not appear in the travel time curve (it will become a blind layer). In the case of the present invention, in which vibrations occur at the boundary surfaces of each layer, it is also possible to track such thin layers.

■ Dangerous materials such as dynamite cannot be used in urban areas for security reasons, but developments include the concussion method of dropping a weight based on standard penetration tests, and "air guns" that release compressed gas (air or nitrogen gas). Since the method is carried out in the borehole,
You can work safely without any problems.

In addition, experimental and trial exploration methods have been carried out in the past in which the power is generated at the surface of the earth by generating vibrations inside the poling hole, the bottom of the hole, or in the tunnel, but these methods do not require the generation of vibrations at the interfaces of the strata. Then, we systematically capture the direct propagation and critical refraction waves in the layer plane,
This method is fundamentally different from the method of the present invention, which investigates and analyzes the ground structure in detail from the upper layer to the lower layer.

Next, an example in which the method of the present invention is actually applied is schematically shown in FIG. 2.

Regarding Fig. 2, ■ and ■ (A) show poling holes for ground investigation conducted at certain intervals (100 to zoom in the investigation of shield construction).

■ is an "air gun" used as a non-explosive source, and a high-pressure gas cylinder (usually 80 ~
140 atm/) air or nitrogen gas).

■ is a cable line with a wire. In addition to passing the current to operate the "air gun" using the button on the operation panel marked with ■,
It is used to hang and hold an "air gun."

3 is a shot wire that extracts the current at the moment of oscillation, and is connected to the amplifier 0.

■ is a group of power receivers deployed on the ground, each connected to the main line called [phase] and input to the amplifier at 0 via the relay line called 0.

In addition, an underwater seismic receiver (@) may be inserted at a depth of +3 in the same stratum into the other end of the polling hole (which will be used as the launching hole next time) corresponding to the originating polling hole, and the travel time may be checked.

(2) is a recorder that records the output from 0 as a wave, and an example of that recording is shown in [Phase].

Now, after deploying a group of power receivers (usually 24 to 48) on the ground (usually at intervals of 5 to IOm) and completing each connection and instrument adjustment, if you deploy with an "air gun", the generated elasticity The waves propagate along the layer surface, undergo critical refraction, reach the earth's surface, are converted into electrical signals by seismic receivers at various locations, and are amplified and recorded.

Therefore, by reading the time from the time base (T, B,) that indicates the moment of eruption to the initial movement of each wave, a travel time curve that shows the relationship between distance and time can be obtained, and by analyzing this, it is possible to Know the condition.

Figure 3 shows an example of an investigation into shield construction along a public road in an urban area.

■ is a ground investigation and seismic polling hole, which was constructed on the sidewalk or vacant lot on the side of the asphalt-paved public road, and ■ is a group of power receivers deployed on the sidewalk facing ■.

■, [phase] indicates a building with an underground foundation or basement along a public road.

Now, if we look at the case where an earthquake is emitted at the interface between layer 1 and V2 in the poling hole, or at a depth of h2 at the interface between layer V2 and V3, those waves will directly hit the strata surface under the public road. It propagates, undergoes critical refraction, and reaches a ground-based seismic receiver.

In plan, the propagation of this wave takes on the shape of a fan that spreads to the receiver group with the focal point as the center.

Therefore, according to the method of the present invention, asphalt pavement existing on the ground surface (normally the propagation velocity of elastic waves is 800 to 12
00 m/s), and by selecting the observation unit length appropriately, there is almost no need to consider the influence of buildings on both sides of the public road, and it is possible to connect the polls under the public road. You can learn about continuous ground conditions and underground structures.

In addition, based on the same idea, if we place seismic receivers radially or at arbitrary positions around the poling hole of the ground survey and observe the waves propagating on each layer, we can detect waves within a planar area. Since the depth of each layer can be determined and contour maps etc. can be created, it can also be applied to surveys of the foundation ground of buildings, bridges and other structures in narrow areas, and the ground of dam sites.

[Brief explanation of the drawing]

Figure 1 is a diagram explaining the principle of the present invention. Figure A shows the elastic wave propagation path when using the method of the present invention, and Figure B shows the elastic wave propagation path when using the general elastic wave exploration method and the travel time curve obtained in that case. This is a comparison of the differences in form. FIG. 2 shows an embodiment in which the method of the present invention is applied. Figure 3 shows the relative relationship between the oscillation hole and the power receiver array, and the state of the underground wave propagation path when applied to a ground investigation for shield construction along public roads in an urban area.

Claims (1)

[Claims] 1. Elastic waves are emitted by an air gun or other device at each strata interface within a poling hole drilled underground, and the waves that propagate through the strata interface, undergo critical refraction, and appear on the ground surface are sent straight to the ground surface or in an appropriate direction. A method of ground investigation using elastic waves that enables deep exploration in urban areas with many ground obstacles and in narrow geographical conditions by conducting tracking observations with seismic receivers deployed at widely spread locations.