Decoupled borehole sensor

A sensor module having tri-axial spatial sensitivity is mounted in a closed compartment of an acoustic logging tool. The sensor module is resiliently mounted with three degrees of freedom with respect to the body of the tool. The compartment includes a volume of damping fluid to suppress parasitic vibrations transmitted to the sensor module from the tool body. In the inactive condition, a contact shoe, associated with the sensor module, projects through a window in the closed compartment. In the operating configuration, the sensor module and its contact shoe are caused to retract into the compartment against a compressive force when the logging tool is pressed against the wall of a borehole.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention is concerned with down-hole acoustic logging tools for use 
in vertical seismic profiling. 
2. Discussion of the Prior Art 
In vertical seismic profiling, a seismic sensor is lowered into a bore hole 
to a plurality of predetermined depths. At each depth station, a surface 
source generates a seismic wavefield. The seismic waves detected by the 
sensor at the respective depths are transmitted by suitable means to data 
storage and processing means of any well known type. 
Typically, the sensor is mounted on or associated with some sort of sonde 
which may include other tools such as an inclinometer to measure the slant 
of the bore hole. The sonde may be one or two meters long and six to ten 
centimeters in diameter and may weigh on the order of 100 kilograms. The 
sonde is maneuvered up and down the borehole by a cable connected to a 
winch located at the surface. At each depth station, the sonde is locked 
firmly in the hole by a mechanical caliper device of known type that is 
usually electrically operated through control lines integral with the 
hoisting cable. 
In the hole, the sonde itself is perturbed by not only seismic waves 
propagating through the formation to which it may be locked at some depth 
station, but also by random noise such as rattling of the supporting cable 
and tube waves that travel through the borehole fluid. The various 
spurious wave fields set up resonances in the sonde in accordance with its 
parameters of mass, length and moduli of elasticity. Such resonances, if 
transmitted to the associated seismic sensor, obscure or badly distort the 
desired seismic signals. Therefore, the sensor must be acoustically 
isolated from the sonde itself. 
In U.S. Pat. No. 4,578,785, assigned to the assignee of this invention, the 
sensor is secured to one leaf of a spider. While the sonde is being 
lowered into the hole, the spider and the sensor are nested inside the 
bottom of the sonde. At a desired depth station the sonde is locked in 
place; the spider is ejected from the bottom open end of the sonde to 
expand and lodge itself against the borehole wall, completely isolated 
acoustically from the sonde. A slack umbilical line is provided to 
transmit seismic signals from the sensor to the sonde and thence to a 
processor at the surface via a wire line that is integrated with the 
hoisting cable secured to the sonde. The disadvantage of that device is 
the complexity of the ejection/recovery mechanism installed in the sonde 
plus the possibility that the spider might break loose from the sonde if a 
tight spot is encountered in the hole. 
U.S. Pat. No. 4,874,060 discloses a sensor probe that is mounted in a 
recess in a sonde that is, as usual, secured to the end of hoisting cable. 
At a desired depth level, an anchoring arm locks the sonde against one 
sidewall of the borehole. The sensor is mounted on an hydraulic piston 
that pushes the sensor laterally out of the side of the sonde to contact 
the same sidewall of the hole against which the sonde is locked. The 
sensor module is not acoustically isolated from the parasitic vibrations 
that may be set up in the sonde. The hydraulic piston and circuitry is 
complicated. In the case of a triaxial system, three separate piston units 
are needed. By reason of its configuration, the moving parts of the 
assembly, as disclosed in the patent, are open to the drilling fluid in 
the well bore; mud and solid particles suspended therein become lodged 
behind the mechanism to jam it. 
Another probe-type downhole sensor is taught by U.S. Pat. No. 4,811,814. 
Here, the sonde is locked against one sidewall of the well bore at a 
desired depth level. A sensor, spring mounted on a probe, is poked out 
laterally against the opposite side wall. Although the sensor is partially 
isolated from the sonde by springs, the probe extension mechanism (of 
undisclosed nature) is necessarily complicated and subject to jamming by 
particulate matter in the drilling fluid. Furthermore, if triaxial 
response is required, three separate mechanisms apparently are necessary 
according to the disclosure. Although the sensor is spring mounted and 
hence, is partially decoupled from the sonde, no damping is provided. 
Accordingly, harmonics and subharmonics of sonde resonances may corrupt 
the sensor signal. 
A sales brochure promulgated by Schlumberger advertises a Combinable 
Seismic Imager.TM.. This brochure illustrates a sonde that includes a 
spring-mounted sensor probe that can be pressed against the sidewall of a 
borehole by an undisclosed mechanism. As with any probe-mounted device 
that must be pushed out of a recess in a sonde, the probe-extension 
mechanism is necessarily complicated and is subject to contamination by 
drill-fluid-borne debris. 
It is an object of this invention to provide a downhole sonde-mounted 
seismic sensor that may be firmly pressed against the sidewall of a bore 
hole without use of complicated probe-extension mechanisms, that will have 
three degrees of acoustic and mechanical freedom with respect to the 
sonde, that will be acoustically decoupled from the sonde and properly 
damped, that will be protected from contamination by drill-fluid-borne 
debris and that will include triaxial capability within a single compact 
module. 
SUMMARY OF THE INVENTION 
In accordance with a preferred aspect of this invention, I provide an 
acoustic logging tool. The tool consists of a sonde having a closed 
compartment formed therein. A window is formed in the wall of the sonde 
opposite the closed compartment. A sensor module, having tri-axial 
sensitivity, is resiliently mounted within the closed compartment, the 
mounting arrangement of the module offers three degrees of freedom with 
respect to the sonde. A damping fluid fills the closed compartment. The 
purpose of the fluid is to damp the parasitic vibrations generated in the 
sonde from being transmitted to the sensor module through the resilient 
support means. The sensor module includes a contact shoe that is forced to 
project through the window by the compressive force of the resilient 
mounting means. In operation, the sonde is displaced against one wall of a 
borehole, forcefully pressing the exterior contact shoe against the 
borehole wall. The sensor module is thereby caused to retract into the 
closed compartment against the compressive force of the resilient mounting 
means. 
In accordance with another aspect of this invention, means are provided for 
equalizing the pressure in the closed compartment with respect to the 
ambient hydrostatic pressure of the borehole fluid.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
In FIG. 1, there are shown the essentials of the acoustic logging tool of 
this invention. A sensor module 10 is mounted in a compartment formed in a 
sonde 12. Sonde 12 is conventional; it may be a hollow tube 14, 1-2 meters 
long made of stainless steel or other suitable material, with an outer 
diameter, perhaps, of 6-10 centimeters. A bail at the top (not shown) is 
provided for attachment to a hoisting cable in a known manner. In addition 
to the sensor module 10, the sonde may include other types of sensing 
devices also not shown. A caliper arm 16 and clamping foot 18, shown here 
folded against the left hand outer wall of the sonde, is hingeably mounted 
on the sonde 12 to lock it against the wall of a borehole as will be 
discussed in connection with FIG. 2. Bumpers or standoffs 20 and 22 are 
mounted on the external wall of the sonde, on the side opposite to the 
caliper arm. 
A closed compartment 23 is formed in the sonde by bulkheads 24 and 26 which 
are sealed in place by any convenient means and which may be removable to 
allow access into the compartment. A window 28 is cut into the wall of the 
compartment, midway between bulkheads 24 and 26. Sensor module 10, which 
may be rectangular, circular or ovoid in outline when seen from the front, 
projects through window 28, urged in place by the compression springs 30 
and 32 along with top and bottom centering springs 34 and 36 to provide 
the sensor module 10 with three degrees of freedom relative to the sonde. 
A retaining shoulder portion 38 holds the module inside closed compartment 
23. Sensor module 10 may conveniently consist of two portions, 40 and 42 
which may be held together by bolts (not shown in FIG. 1, see FIG. 4). 
Portion 42 conveniently serves as an exterior contact shoe for 
transferring seismic signals to discrete sensor elements as will be 
discussed infra. A flexible seal 44 is clamped to the sensor module around 
the perimeter of the module, firmly gripped by the two halves 40 and 42 
when they are bolted together. The outer perimeter of seal 44 is secured 
to the outer wall of the sonde around window 28 by a retaining ring 46 
which may be held in place on the external wall of sonde 12 by screws (not 
shown in FIG. 1). Seal 44 may be of any suitable plastic that will survive 
the high temperatures of a deep borehole. It must be sufficiently pliable 
so that it will not transmit spurious vibrations from the sonde body to 
module 10. Sensor module 10 may be of stainless steel, aluminum, titanium 
or other material that is not subject to corrosion by the drilling fluids. 
Three separate seismic sensor elements 48, 50, 52 are mounted within sensor 
module 10. The sensor elements may be gimbal-mounted geophones or 
piezoelectric accelerometers at the option of the user in view of the 
field conditions and the types of measurements to be made. The sensor 
elements must be mounted such that their sensitive axes, as shown by the 
double-headed arrows, are mutually orthogonal to provide a sensor module 
having tri-axial spatial sensitivity. Electrical signal leads from the 
sensor elements are fished through passageways as shown, through a 
hermetic seal 54 at the top of sensor module 10 and hermetic seals 56 and 
58 in bulkhead 24, to form a conductor bundle 60. Conductor bundle 60 
extends on up the sonde to join the hoisting cable in a known manner. 
Closed compartment 23 is filled with a fluid 62 such as an inert silicone 
oil of any convenient type that will retain its desired viscosity under 
high temperature. The spring system, 30-36 provides three degrees of 
freedom and acoustic isolation from the sonde. The fluid 62 in compartment 
23 provides the damping that is essential to prevent undesired 
transmission of harmonic and subharmonic resonances from the sonde to the 
sensor module, through the spring system. The viscosity of the fluid is 
chosen after consideration of the area of that portion of module 10 that 
is exposed to the fluid, the spring constants of the resilient supports 
and the temperatures to be expected down the hole. Flexible seal 44 seals 
the fluid within compartment 23 yet permits sensor module 10 to freely 
project or retract into sonde 12. A fill plug 64 is furnished for filling 
the compartment with fluid. A flexible membrane or diaphragm 66, mounted 
on the side of compartment 23 and exposed to the ambient hydrostatic 
pressure in the well bore, provides pressure equalization with respect to 
the fluid pressures encountered in the borehole. 
The normal position for the sensor module 10, when inactive, is in the 
extended position as shown in FIG. 1, which is an arrangement that is just 
the opposite from that of the references. The corners of sensor module 10 
may be rounded so that it will not hang up on obstructions when it is 
lowered into the well bore. Because the module is resiliently mounted, the 
entire assembly it will easily slide through tight spots. 
FIG. 2 shows the sonde and the sensor module after having been lowered into 
a borehole 68, in operating configuration. At a selected depth station, a 
control signal from the surface causes caliper arm 16 to extend and press 
locking foot 18 against one side 70 of the borehole 68. The mechanism and 
control for that function are well known and conventional so there is no 
need to go into detail in the matter. With the caliper arm extended, sonde 
12 is forced against the opposite side 72 of borehole 68. That action 
causes sensor module 10 to retract inwardly as shown, pressing external 
contact shoe 42 forcefully against formation 74 (or the casing if the hole 
is cased) that makes up the borehole wall. Contact shoe 42, lodged firmly 
against formation 74, feels acoustic signals propagating through formation 
74 and transmits the signals to the respective sensor elements in sensor 
module 10. The preferred application force is a multiple of the mass of 
the sensor module, preferably about ten times the mass thereof, and is 
provided by compression springs 30 and 32 as well as by top and bottom 
centering springs 34 and 36. For a 0.5 -kilogram module, the preferred 
compressive force is about 5.0 kilograms. It is to be observed that the 
volume of fluid 62 that is displaced, when module 10 is caused to retract 
inwardly, causes pressure equalization diaphragm 66 to bulge outwardly. In 
effect, pressure equalization diaphragm acts as a reservoir to receive the 
displaced damping fluid. Alternatively, a spring-loaded piston arrangement 
might serve the same purpose. Seal 44 that surrounds sensor module 10 
prevents debris carried by the drilling fluids from interfering with the 
three degrees of freedom of the module with respect to the sonde. As 
before stated, fluid 62 inside compartment 23 damps spurious resonances, 
that might be generated in the sonde, from being communicated to the 
sensor module through the spring system. 
FIG. 3 is a plan view of the tool assembly along 3--3' of FIG. 1. In 
addition to the springs shown in the first two Figures, lateral centering 
springs 76 and 78 are furnished. It is preferable that a pair of 
compression springs such as 30 and 33 be provided on the back of sensor 
module 10, one pair near the top and one pair and near the bottom. The 
directional sensitivity of sensor element 48 is shown more clearly in this 
Figure. Otherwise the parts are as shown in FIGS. 1 and 2. 
FIG. 4 is a frontal external view of the tool assembly 12 as it would 
appear from 4--4', FIG. 1. Seal retaining ring 46 may be held in place on 
the outer wall 14 of the sonde by screws such as 80. If desired, the 
retaining ring 46 and the outer edge of the flexible seal 44 may be inset 
into the wall of the sonde for a smoother exposure. The two portions 40 
and 42 (FIGS. 1 and 2) of sensor module 10 are held together by 
countersunk bolts such as 82. The configuration of the sensor assembly is 
shown as rectangular but it may be square, round or ovoid as earlier 
mentioned. It is clear from the drawings that the sensor assembly presents 
a relatively clean outline to the environment within the well bore. In 
particular, there are no cavities or openings wherein debris can collect 
to jam free movement of the sensor module 10 relative to sonde 12. 
The configuration of the assembly of this invention as shown in the 
drawings is exemplary. Many variations in the design will be contemplated 
by those skilled in the art but which will fall within the spirit and 
scope of this disclosure which is limited only by the appended claims. For 
example, only one sensor module is shown in the drawings. Several 
independent sensor modules may be included in a single acoustic logging 
tool assembly. The distribution of the individual sensor elements within 
sensor module 10 may be altered from that shown. The construction of tool 
12 will be such as to allow the convenient emplacement of module 10 in 
closed compartment 23 and to permit the sealing of bulkheads 24 and 26 
inside tube 14, the details of which construction have not been shown 
since they are routine engineering design matters.