Borehole seismic tool

Details Borehole seismic tool Borehole seismic tool

2000-03-10

2001-01-09

Dang, Khanh (Department: 2831)

Acoustics

Geophysical or subsurface exploration

Well logging

C367S911000

Reexamination Certificate

active

06170601

ABSTRACT:

TECHNICAL FIELD
The present invention relates to borehole logging tools, and in particular to aspects of a sensor package for a seismic logging tool in which the sensor package is urged against the wall of a borehole.
BACKGROUND ART
Seismic logging tools are know for use in performing vertical seismic profile surveys from boreholes to evaluate the surrounding underground formations. The tools can be single sondes or multi-shuttle tools comprising a number of similar sondes (shuttles) linked together by means of a cable and logged through a borehole while seismic signals are generated at the surface. At each measurement level, each shuttle is anchored to the wall of the borehole during a measurement operation by means of an anchoring arm. The sensors (geophones or geophone accelerometers) are mounted in packages which are located in the tool body in such a way that they are isolated from the vibrational behaviour of the tool when anchored in the borehole. This is typically done by spring-mounting the sensor package in the tool and clamping the tool body securely to the borehole wall (by anchoring the tool as described above) when measurement are to be made. One such seismic logging tool is Schlumberger's Combinable Seismic Imager tool (CSI). Aspects of the CSI are described in U.S. Pat. No. 4,563,757; U.S. Pat. No. 4,575,831; U.S. Pat. No. 4,987,969; and U.S. Pat. No. 5,200,581. In particular, patents '969 and '581 relate in particular to the sensor package and deployment. Schematic figures from the patents is shown in FIGS.
1
a-b
and
2
a-b.
FIGS.
1
a
and
1
b
show a sensor package
30
which is housed in a cavity in the tool body
21
. The package is connected to the tool body by a resilient suspension formed from two coil compression springs
40
,
50
which locate around centring studs
40
a,
50
a,
40
b,
50
b,
on the package
30
and tool body
21
. Retraction actuators
41
,
51
pass through the coil springs and engage in bores in the package
30
. In the retracted position, the actuators are withdrawn such that the flared ends engage in the bores and pull the package into the cavity against the action of the springs. Thus the package is held within the cavity and is protected from mechanical damage when running in or pulling out of hole. When the tool is deployed and the tool body anchored against the borehole wall, the actuators are released and the package is pressed against the borehole wall by the action of the springs.
FIGS.
2
a
and
2
b
show an alternative system which still uses springs
140
,
150
and centring studs
140
a,
140
b,
150
a,
150
b,
but the actuators and bores are replaced by a system of a rod
160
and rockers
143
,
153
. Pulling on the rod causes the rockers to retract the sensor package into the tool, while releasing the rod allows the springs to force the package out of the cavity as before.
In both cases, the tool includes an active system for retracting the sensor package into the tool body. This adds to the size, weight and complexity of the sonde which can be significant where a number of sondes are to be linked together as in a shuttle tool.
Other prior art systems are described in U.S. Pat. No. 4,874,060 and U.S. Pat. No. 4,811,814; Borehole seismic shuttle tools currently used also include the SST500 of Compagnie General de Geophysique and the seismic shuttle tools of VNIIGIS/OZGA.
The present invention has as its object a tool in which the use of a retracting mechanism is avoided while still providing for secure location of the sensor package in the tool.
DISCLOSURE OF INVENTION
One aspect of the present invention provides a borehole tool comprising (a) a tool body having a cavity with an opening in the wall of the tool body; (b) an anchoring arm attached to the tool body and operable to anchor the tool body to the borehole wall; (c) a sensor package positioned in the cavity and mounted on resilient mounts which act to urge the sensor package in the direction of the opening, such that when the tool body is anchored to the borehole wall, the sensor package projects part way through the opening and is held against the borehole wall by the action of the resilient mounts. A tool according to this aspect of the invention is characterised in that when the tool body is not in contact with the borehole wall, peripheral portions of the sensor package is urged into contact with the tool body wall surrounding the opening.
The provision of resilient mounts for the sensor package provides acoustic isolation when the tool is anchored to the borehole wall such that vibrations of the tool body are not communicated to the sensor package and so do not interfere with the formation measurement.
The resilient mounts are typically compression springs although any other form of resilient mount can be used, such as silicone rubber, provided that its natural action is to urge the sensor package towards the opening. Unlike the prior art tools, there is no retraction mechanism. Thus, the normal, unanchored position for the sensor package is to project part way through the opening. The tool body surrounding the opening is preferably formed with shaped seats, the sensor package having correspondingly shaped peripheral portions to locate the package when urged against the tool body. The seats can be shaped such that the package normally assumes the same position when not in contact with the borehole wall.
The action of the mounts is to move the package radially with respect to the tool body. Axial movement can be prevented by providing a connecting member connected to the tool body and preferably flexible to ensure acoustic isolation from the tool body when anchored to the borehole wall.
An operable package backup pad can also be provided to hold the package in the seats, especially when the tool is run into the borehole or pulled out of the borehole at higher speeds that are normally encountered in logging activities. In one embodiment, the backup pad is located on the anchoring arm and bears on the package when the arm is retracted against the tool body. When the arm is extended but not anchored, the case when the tool is moved between measurement levels in the borehole, the package is held in the seats by the resilient mounts.
A further embodiment of the invention also has means for securing the package to the tool body around the opening so that the anchoring of the tool body to the borehole wall is sufficient to apply the sensor package to the borehole wall (i.e. the two are acoustically coupled instead of being isolated).
By avoiding the use of a retraction mechanism, the tool can be made much more compact and lighter


REFERENCES:
patent: 4563757 (1986-01-01), Decorps et al.
patent: 4575831 (1986-03-01), Decorps et al.
patent: 4811814 (1989-03-01), Staron et al.
patent: 4874060 (1989-10-01), Guerendel et al.
patent: 4987969 (1991-01-01), Boyle et al.
patent: 5200581 (1993-04-01), Boyle et al.
patent: 5864099 (1999-01-01), Wittrisch et al.

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