Patent Description:
Seismic survey technologies are commonly used to explore and develop resources for the petrochemical industry. Petrochemical products are ubiquitous in the modern economy, and may be found in everything from oil and gasoline to medical devices, children's toys, and a wide range of other everyday household items. To meet the continuing demand for these products, oil and gas reserves must be accurately located and surveyed, so that these important resources can be effectively managed. As a result, there is an ongoing need for new seismic sensor systems and more advanced exploration technologies.

Scientists and engineers typically utilize seismic wave-based exploration techniques to locate new oil and gas reservoirs, and to survey and manage existing reserves over time. Seismic surveys are performed by deploying an array of seismic sensors or receivers over the region of interest, and monitoring the response to controlled emission of seismic energy via seismic sources such as vibrators and air gun arrays, or explosive detonations. The response depends upon the seismic energy reflected from subsurface structures, allowing the corresponding geological features to be imaged, including mineral reservoirs and other underground structures of interest.

Marine seismic surveys may proceed by towing an array of seismic sensors or receivers behind a survey vessel, with the receivers distributed along one or more streamer cables. A set of air guns or other seismic sources is used to generate seismic energy that propagates down through the water column to the ocean floor (or other bottom surface). A portion of the seismic energy penetrates the ocean floor and is reflected from subsurface structures, then returns back through the water column to be detected in the streamer array. Seismic receivers can also be disposed along ocean- bottom cables, or provided in the form of individual, autonomous seismic nodes distributed on the seabed.

Typical seismic receivers include pressure sensors and particle motion detectors, which can be provided as individual sensor components, or combined together with both sensor types located in close proximity within a receiver module or seismic node. For example, a set of pressure sensors can be configured in a hydrophone array, and adapted to record scalar pressure measurements of the seismic wavefield propagating through the water column or other seismicmedium. Particle motion sensors include accelerometers and geophones, which can be configured to provide single-axis or three-dimensional vector velocity measurements that characterize motion of the medium in response to propagating seismic waves.

Geophysical data pertaining to the subsurface structures is acquired by observing the reflected seismic energy with an array of such receiver components. The observed seismic signals are used to generate a seismic image that describes the subsurface geology and composition in and around the survey area. The overall image quality depends not only on signal sensitivity but also on noise effects, contributing to the constant demand for more advanced sensor and receiver technologies.

The information included in this Background section of the specification, including any references cited herein and any description or discussion thereof, is included for technical reference purposes only and is not to be regarded as subject matter by which the scope of the invention as defined in the claims is to be bound. <CIT> discloses a seismic sensor comprises a central mass having three principal axes and disposed within a frame. Transducers are affixed to the central mass and to sensor beams that are connected to the frame. <CIT> discloses a sensor comprising: a mass element; a frame surrounding the mass element; a connecting body having flexibility, and connecting the mass element to the frame; a pressure detecting unit; and an acceleration detecting unit. <CIT> discloses a micro electro mechanical systems sensor.

This application is directed to a multi-axis, single mass particle motion sensor or accelerometer, suitable for use in seismic surveys as well as other applications in which accelerometers may be used, including vehicle navigation (e.g., aviation, marine vessels, and motor vehicles), technical device sensors, consumer device sensors, etc. In an example implementation, a plurality of sensor components may be coupled to a single central mass, providing sensitivity to translational motion in three independent directions, and sensitivity to rotational acceleration about three orthogonal axes. The individual sensor components may be configured to sense the corresponding translational and rotational motions based on dynamic coupling to the mass, including, but not limited to, force-sensitive translational couplings and rotational couplings. The translational and rotational couplings may be preferentially sensitive to either acoustic waves or shear effects, providing for a combined sensor signal with improved vector fidelity, as described herein.

In some embodiments, a seismic sensor comprises a housing or frame and a central mass disposed within the frame. The central mass may be defined by three principal axes. A plurality of transducers may be mechanically coupled between the frame and the central mass, and arranged in pairs positioned on opposing sides of the central mass, along each of the three principal axes. Depending on application, piezoelectric crystal transducers may be used as the sensing element, with crystals aligned to provide signal sensitivity to shear stress. Electronics may be provided to combine signals from the transducers in each pair in order to generate output characterizing translational and rotational acceleration of the frame.

In one exemplary implementation, a multi-axis, single mass acceleration sensor includes a three-dimensional frame, a test mass, a plurality of transducers, and a plurality of struts. The test mass may be disposed within and spaced apart from the frame. The transducers are mechanically coupled to the frame at a plurality of respective locations on the frame. The struts are configured to couple to the central mass at a plurality of respective positions, respectively, and to couple with respective sets of the transducers at the plurality of respective locations, thereby suspending the test mass within the frame. The sensor is thus responsive to translational motion in multiple independent directions and to rotational motion about multiple independent axes.

In another exemplary implementation, a multi-axis, single mass acceleration sensor includes a three-dimensional frame, a hollow, spherical test mass with three orthogonal principal axes, six pairs of shear crystals, and six suspension blades. The test mass is disposed within and spaced apart from the frame. The shear crystals are mechanically coupled to the frame. The suspension blades are each coupled at a center portion to the central mass normal to opposite ends of each of the three orthogonal principal axes, respectively, and coupled with a respective pair of shear crystals at each lateral end of the suspension blades, thereby suspending the test mass within the frame. The sensor is oriented with respect to three orthogonal principal axes that each pass through a center of mass of the test mass. The sensor is thus responsive to translational motion parallel to the three orthogonal principal axes and to rotational motion about the three orthogonal principal axes.

A more extensive presentation of features, details, utilities, and advantages of the present invention as defined in the claims is provided in the following written description, including various representative embodiments of the invention, and as illustrated in the accompanying drawings.

In this disclosure, reference is made to examples and embodiments of the invention. However, it should be understood that the invention is not limited to these or any other specifically described embodiments. Any combination of the disclosed features and elements, whether related to different embodiments or not, is contemplated to implement and practice the invention. Furthermore, in various examples and embodiments the invention provides numerous advantages over the prior art. Although certain embodiments may achieve these and other advantages over other potential solutions, and over the prior art, whether or not a particular advantage is achieved by any given embodiment is not limiting of the invention as claimed. The following examples, aspects, features, embodiments and advantages of the disclosure are merely illustrative, and should not be considered elements or limitations of the claims, except where explicitly specified in the claim language. Likewise, references to "the invention" shall not be construed as a generalization of any inventive subject matter that is disclosed, and shall not be construed to represent an element or limitation of any claim except where expressly recited therein.

An implementation of a multi-axis, single mass accelerometer <NUM> is depicted in accompanying <FIG>. The accelerometer <NUM> is configured to detect both translational and rotational acceleration. The accelerometer <NUM> is principally composed of a test body or test mass <NUM> housed by, suspended within, and connected to a frame <NUM> through a plurality of sensors <NUM>. (Note: in the figures the sensors are labeled 106a1-106f2; however, indication of all the sensors collectively in this discussion may be made merely by reference to "sensors <NUM>. ") The frame <NUM> provides a reference structure for measurement by the sensors <NUM> of the inertial resistance of the mass <NUM> to acceleration of the frame <NUM> when subjected to external stimulus or energy. The frame <NUM> may define six flat faces or sides <NUM> forming a cube around the mass <NUM>. Such a frame <NUM> may be formed as a monocoque or similar structural shell component, machined from a suitable metal material, ceramic (e.g., a MACOR material), or from another material with suitable rigidity and other mechanical properties. The frame <NUM> should be extremely stiff and should withstand deformation under typical forces and loads to be experienced by the accelerometer <NUM>. In one exemplary implementation the frame <NUM> may be machined out of brass. In the embedment shown in <FIG>, the frame <NUM> is of two-part construction consisting of a first half 112a and a second half 112b, shown to better advantage in <FIG>. Each of the first and second halves 112a, 112b may be machined with threaded shafts to receive screws or other fasteners to fix the first and second halves 112a, 112b.

A central aperture <NUM> may be formed in each of the side faces <NUM> of the frame <NUM> defined and surrounded by a plurality of lateral pillars or beams <NUM> extending between corner structures <NUM> to provide a perimeter framework for a cubic frame <NUM>. The central apertures <NUM> may be oblong in shape as shown, but may be formed in any other shape (e.g., square, octagonal, etc.) sufficient in size (area) to expose a surface of the test mass <NUM> for structural connection with the sensors <NUM>. In symmetric embodiments, the beams <NUM> and corner structures <NUM> may be similarly formed to be substantially identical in size, thickness, and construction. One exterior surface of each of the beams <NUM> may be recessed to form a pocket <NUM>. Opposing beams <NUM> may define a pair of pockets <NUM> on each face <NUM> of the frame <NUM>. The orientation of each pair of pockets <NUM> on a face <NUM> is orthogonal to pairs of pockets <NUM> on each adjacent face <NUM> and parallel to the orientation of the pockets <NUM> on the opposing face <NUM> of the frame <NUM>. Interior surfaces of the frame <NUM> may be machined to match a curvature of the form of the test mass <NUM> in order to provide a close tolerance between the frame <NUM> and the test mass <NUM> to maintain a uniform separation distance between the interior surfaces of the frame <NUM> and the test mass <NUM> suspended therein. Alternatively, the interior surfaces of the frame <NUM> need not be machined for close tolerances and the separation distances between various of the interior surfaces of the frame <NUM> and the test mass <NUM> suspended therein can be different. However, the test mass <NUM> is typically centered within the frame <NUM>.

In some implementations, the test mass <NUM> may be a hollow sphere, for example, made of a relatively dense or "heavy" material, i.e., a material with a high specific gravity to provide a significant moment of inertia relative to the size of the test mass <NUM>, particularly when the accelerometer <NUM> is small in size, e.g., less than <NUM>. In exemplary embodiments, the test mass <NUM> may be made of brass or titanium formed as hollow spheres with uniform wall thickness comprising the mass. In some embodiments, areas on a spherical mass may be flattened to aid in attachment of the test mass <NUM> to the frame <NUM>. In such embodiments in which the density of the test mass <NUM> is not uniform (e.g., due to variations in surface form or wall thickness, or both), it is desirable that the mass distribution of the test mass <NUM> be symmetrical. As rotational movement is to be measured, the polar moment of inertia of the test mass <NUM> is an additional factor to consider in the design of the test mass <NUM>. The size, mass, and density of the test mass <NUM> may thus be selected to optimize the response characteristics for both translational and rotational acceleration sensitivity. Depending upon the type of acceleration to be measured, the characteristics of the test mass <NUM>, as well as the frame <NUM>, may be tuned to be sensitive in desired frequency bandwidths while attenuating undesired bandwidths.

Other configurations of the test mass <NUM> are also possible; for example, a solid sphere or multifaceted shape (e.g., a symmetrical polyhedron) having symmetry about axes X, Y, and Z. Generally, shapes that are symmetrical about one or more axes of translation or rotation are suited for achieving the desired sensory results in an accelerometer, as described herein. In other implementations, the test mass <NUM> may be solid rather than hollow, and formed either of a single substantially uniform material or from one or more different types of core materials and additional layers or components that build out to the outer layer or outer surface of the test mass <NUM>. The determinations of whether the body of the mass <NUM> should be hollow or solid, and homogenous or made of multiple materials and layers, are design choices made to achieve the dynamic requirements of both translational and angular acceleration measurements for a particular environment or application.

Depending on embodiment, one or both of the test mass <NUM> and the outer surface thereof may be formed of an electrically conductive material in order to provide a conductive path for connection with an electrode of a sensor or transducer as described herein. For example, the test mass <NUM> may be formed as a solid or hollow sphere of aluminum, brass, copper, beryllium copper, or other conductive material, or the test mass <NUM> may be formed of a ceramic or other insulating material with a conductive coating on the outer surface (e.g., in a gold, aluminum, or other conductive metal plated configuration).

Alternatively, one or both of the test mass <NUM> and the outer surface of the test mass <NUM> may be made of a nonconductive or electrically insulating material, e.g., to reduce the effect of stray capacitance on the signal-to-noise (S/N) ratio of the accelerometer <NUM>, e.g., caused by the conductive area of the test mass <NUM>. For example, the test mass <NUM> may be formed as a solid or hollow sphere of a machinable ceramic (e.g., a MACOR material), or from an injection molded plastic.

As shown in <FIG>, the test mass <NUM> may be attached to a number of sensors <NUM> by a number of braces or struts. In an exemplary implementation, these struts may be formed as flat, blade-like structures, hereinafter referred to as suspension blades 120a-120f, that attach to the sensors <NUM> at lateral ends and to the test mass <NUM> at a center location along the length of each suspension blade 120a-120f. The sensors <NUM> are mounted at various locations on the frame <NUM>. In the embodiment shown in <FIG>, in order to aid in attachment to the suspension blades 120a-120f, the spherical test mass <NUM> may be formed with a number of planar coupling surfaces <NUM>, wherein each attachment surface <NUM> is at a radial distance from the center of the spherical test mass <NUM> less than the radius of the spherical test mass <NUM>. If determination of translational and rotational accelerations is desired with respect to more than one independent direction, the transducers may be disposed along independent (e.g., transverse or orthogonal) axes X, Y, and Z. In the embodiment of <FIG>, six planar coupling surfaces <NUM> are formed in opposing pairs on the outer surface of the test mass <NUM>, aligned with opposing ends of the three orthogonal axes X, Y, and Z having an origin at the center of the test mass <NUM>; and thus an axis between each opposing pair is orthogonal to axes extending between each of the other opposing pairs. The coupling surfaces <NUM> provide a bonding and alignment surface for attachment of the suspension blades 120a-120f. Additionally, an insulator <NUM>, e.g., a ceramic pad, may be placed between the coupling surfaces <NUM> and respective suspension blades 120a-120f in order to provide electrical isolation between the test mass <NUM> and the sensors <NUM>. The suspension blades 120a-120f may be fixed to respective coupling surfaces (with or without an insulator <NUM>) via an adhesive or other bonding agent (e.g., a metal weld if both the test mass <NUM> and suspension blades 120a-120f are metals capable of welding and electric isolation is not necessary).

In the embodiment shown in <FIG>, for example, twelve (<NUM>) individual sensors 106a1 -106f2 are arranged in six sets of laterally opposing pairs coupled to attachment surfaces in respective pockets <NUM> within the frame <NUM>. An insulator <NUM>, e.g., a ceramic pad, may be placed between the attachment surfaces of each of the pockets <NUM> and respective sensors 106a1-106f2 in order to provide electrical isolation between the sensors <NUM> and the frame <NUM>. In one embodiment, the insulators <NUM> may be one-sided such that an outer side coupled to the sensors <NUM> is conductive to provide a connection for an electrical lead while the inner side connected to the attachment surfaces of the pockets <NUM> of the frame108 are nonconductive. The test mass <NUM> is oriented within the frame <NUM> such that the planar coupling surfaces <NUM> are each respectively parallel to a respective face <NUM> of the frame <NUM> and, consequently, to a corresponding pair of pockets <NUM> on the respective face <NUM>. However, as shown in <FIG> and <FIG>, the depth of each pair of pockets <NUM> may be greater than the depth of the coupling surfaces <NUM> with respect to a corresponding parallel axis A centered within the test mass <NUM> (i.e., one of the axes X, Y, or Z) such that a distance D<NUM> between the coupling surface <NUM> and the axis A is greater than a distance D<NUM> between the attachment surfaces of the pockets <NUM> and the axis A.

As shown in <FIG>, for example, twelve acceleration sensors 106a1, 106a2, 106b1, 106b2, 106c1, 106c2, 106d1, 106d2, 106e1, 106e2, 106f1, and 106f2 are affixed to laterally opposed pockets <NUM> on each face <NUM> of the frame <NUM>, in order to sense translational and rotational motion of the frame <NUM> with respect to the test mass <NUM> in the three orthogonal directions defined by perpendicular axes X, Y and Z. In particular implementations, piezoelectric crystal transducers 106a1-106f2 may be used to measure the locally-defined stress response due to translation and rotation of the frame <NUM> about the test mass <NUM>, e.g., in response to external torques and forces due to incident energy (e.g., physical motion, acoustic waves, seismic energy, etc.). The force transducers or acceleration sensors 106a-106f may be chosen to be highly sensitive or tuned to measure specific types or bandwidths of incident energy imparted to or experienced by the acceleration sensor in order to suitably characterize the response of the test mass <NUM> to the incident energy to be measured.

In such a configuration, it may be advantageous to select a piezoelectric transducer such as an acceleration sensor 106a-106f that is configured to react to shear stress on opposing sides of the piezoelectric crystal device. Suitable transducer configurations include, but are not limited to, lead magnesium niobate-lead titanate (PMN-PT) shear mode crystals (e.g., available from CTS Corporation) and piezoelectric accelerometer sensor (PAS) elements. Such a transducer registering or responsive to shear stress may provide greater sensitivity than alternatives due to the configuration of the test mass <NUM> within the frame <NUM> and the goal of measuring relative acceleration between the two. In some exemplary implementations, the transducer may be a single relaxor ferroelectric crystal, for example, a PMN-PT crystal, or other suitable transducer. The crystal responses are substantially flat with respect to frequency. This occurs because the crystal resonance is significantly out of the frequency bands of interest.

Piezoelectric elements fabricated from single crystals can exhibit superior sensing properties, relative to polycrystalline elements. Single crystal transducers can also be relatively insensitive to temperature effects, and may have high mechanical energy to electrical energy conversion factors. If a single crystal is cut transverse to the orientation of the crystal alignment, at an appropriate angle, e.g., as done by CTS Corporation in the manufacture of their <<NUM>> poled transverse shear single crystals, a transducer highly responsive to preferred shear loads and stresses (as opposed to tension or compression loads and stresses or off-axis shear loads and stresses) may be formed. In some particular embodiments, the sensors <NUM> may be shear-mode piezoelectric transducers, for example, oblong, rectangular, or similarly-shaped single crystal transducers with dimensions of about <NUM> × <NUM> × <NUM>. More generally, the shape and dimensions of the crystals forming the sensors <NUM> can vary, either individually or as a set, depending on application.

Three sets of two pairs each of piezoelectric transducer sensors 106a1-106f2 can each be mounted (e.g., using a conductive adhesive) onto one-sided insulating material previously fixed to the pockets <NUM> of the frame <NUM>. Pairs of the sensors <NUM> on a single face <NUM> may be connected together and to the test mass <NUM> by suspension blades 120a-120f. The suspension blades 120a-120f are shown in greater detail in <FIG>. Each suspension blade <NUM> may be formed (e.g., stamped or cut) out of a flat sheet of beryllium copper or similar material with good conductivity and a high ratio of tensile modulus to tensile yield. The suspension blades <NUM> may be formed as shown as elongate, rectangular blades with two apertures <NUM> formed therein. The apertures <NUM> may be oblong or elongate. The apertures <NUM> separate two lateral sensor contact areas <NUM> from a center mass coupling area <NUM>. Each of the sensor contact areas is affixed (e.g., with conductive adhesive) to one of the sensors <NUM> in each pair of sensors on a face <NUM>. The mass coupling area <NUM> is similarly connected to one of the coupling surfaces <NUM> on the test mass <NUM>.

It may be preferable that the suspension blades <NUM> be electrically isolated from the test mass <NUM>. Therefore, a rigid insulating material <NUM> (e.g., a ceramic pad) may be placed and adhered between the suspension blades <NUM> and the test mass <NUM>. The insulating material <NUM> is preferably rigid in order to transfer energy between the frame <NUM> and the test mass <NUM> without attenuation. The sensor blades <NUM> are designed to be stiff or rigid along their longitudinal direction, parallel to the sensing axis <NUM> between each pair of sensors <NUM> on a respective face <NUM> of the frame <NUM>. However, the suspension blades <NUM> are also designed to be relatively softer or less stiff along the non-sensing axes <NUM> and <NUM>. This configuration results in greater response to translational forces along the sensing axes <NUM> of each respective sensing blade <NUM>. This structural response is effectuated by the removal of material to form the apertures <NUM>, leaving narrow beams <NUM> of material along the edges of the suspension blades <NUM>.

The result of this design is the generation of relatively high mechanical shear stress on the sensors <NUM> in the principal or primary sensing direction, e.g., along the longitudinal dimension <NUM> of the corresponding suspension blades <NUM>. Conversely, there may be lower shear stress in the cross-axis directions relative to the primary sensing direction, reduced by the absorption of the shear stress by the suspension blades <NUM> in the transverse directions. As noted above, the sensors <NUM> mounted between the suspension blades <NUM> and the frame <NUM> are responsive to shear stress, which manifests in a plane defined by the sensing axis <NUM> and the non-sensing axis <NUM> extending outward from a center of mass of the test mass <NUM> normal to the longitudinal sensing axis <NUM> of the respective suspension blade <NUM>. Thus, as a force impacts the frame <NUM>, a relative acceleration with respect to the test mass <NUM> arises and the crystal sensors <NUM> on each end of the suspension blade skew in the plane defined by axes <NUM>, <NUM>, which is the manufactured shear sensing plane of the crystal sensors <NUM>, and a voltage is generated.

The configuration of the suspension blades <NUM> also reduces the impact of "crosstalk" from the principal sensor axis <NUM> to the other independent axes <NUM>, <NUM>. The construction of accelerometer <NUM> thus increases sensitivity by augmenting the effects of the crystal orientation properties of the individual sensors <NUM> in order to produce relatively more voltage when stressed in their preferred sensing direction, and to reduce off-axis effects in order to achieve a desired level of vector fidelity. More specifically, the accelerometer <NUM> is configured to generate a consistent representation of the acceleration or impulse along each independent measurement axis, regardless of sensor orientation. The accelerometer <NUM> is also configured to substantially reduce the response to an impulse along one particular sensor axis, as observed by the other two independent (orthogonal) axes. This improves the vector fidelity by providing a more accurate representation of the actual (physical) vector wavefield observed at the accelerometer <NUM>, while reducing non-physical sensor response effects.

As described above, the test mass <NUM> may have parallel planar coupling surfaces <NUM> disposed at opposite ends of the orthogonal axes X, Y, Z, and each face <NUM> of the frame <NUM> has pockets <NUM> positioned on lateral sides of the coupling surfaces sized to receive the sensors <NUM>, which may be mounted therein using an adhesive or other mechanical attachment. A first pair of sensors 106a1, 106a2 (e.g., piezoelectric crystal transducers) are mounted on opposing sides of the mass <NUM> and spaced along the linear sense axis <NUM>, in this case the horizontal X-axis <NUM>, on a first face <NUM>; and a second pair of sensors 106b1, 106b2 are mounted on opposing sides of the mass <NUM> and spaced along the horizontal X-axis on a second, opposing face <NUM>. The piezoelectric crystals in each pair of sensors are mounted in opposite crystal orientations such that, for example, the positive output for shear sensitivity of the first sensor 106a1 is along the X-axis but in the opposite direction (not orthogonal) to the positive output for shear sensitivity of the second sensor 106a2. On the opposite face <NUM> of the frame <NUM>, the positive output for shear sensitivity of the third sensor 106b1 is along the X-axis in the opposite direction (not orthogonal) to the positive output for shear sensitivity of the fourth sensor 106b2. In addition, the orientations of the third and fourth sensors 106b1, 106b2 are arranged parallel to and mirror the orientations of the first and second sensors 106a1, 106a2, such that the positive output of the first sensor 106a1 is in the same direction as the positive output of the third sensor 106b1, and the positive output of the second sensor 106a2 is in the same direction as the positive output of the fourth sensor 106b2. This set of four sensors 106a1-106b2 jointly create a translational accelerometer along the horizontal X-axis and a rotational accelerometer with sensitivity to rotation about the Y-axis.

A second set of sensors 106c1-106d2 may be similarly mounted on faces <NUM> of the frame <NUM> on opposing ends of the X-axis and oriented parallel to the Y-axis <NUM>, such that the sense axis <NUM> of the sensors 106c1-106d2 is rotated perpendicular to that of the first set of transducers 106a1-106b2. The piezoelectric crystals in each pair of sensors are mounted in opposite crystal orientations such that, for example, the positive output for shear sensitivity of the fifth sensor 106c1 is along the Y-axis but in the opposite direction (not orthogonal) to the positive output for shear sensitivity of the sixth sensor 106c2 (not visible in figures). On the opposite face <NUM> of the frame <NUM>, the positive output for shear sensitivity of the seventh sensor 106d1 is along the Y-axis in the opposite direction (not orthogonal) to the positive output for shear sensitivity of the eighth sensor 106d2. In addition, the orientations of the seventh and eighth sensors 106d1, 106d2 are arranged parallel to and mirror the orientations of the fifth and sixth sensors 106c1, 106c2, such that the positive output of the fifth sensor 106c1 is in the same direction as the positive output of the seventh sensor 106d1, and the positive output of the sixth sensor 106c2 is in the same direction as the positive output of the eighth sensor 106d2. This set of four sensors 106c1-106d2 jointly create a translational accelerometer along the Y-axis and a rotational accelerometer with sensitivity to rotation about the Z-axis.

A third set of sensors 106e1-106f2 may be mounted on faces <NUM> of the frame <NUM> on opposing ends of the Y-axis. A fifth pair of sensors 106e1, 106e2 are mounted on opposing sides of the mass <NUM> and spaced along the linear sense axis <NUM>, in this case the Z-axis <NUM>, on a fifth face <NUM>; and a sixth pair of sensors 106f1, 106f2 are mounted on opposing sides of the mass <NUM> and spaced along the Z-axis on a sixth, opposing face <NUM>. The piezoelectric crystals in each pair of sensors are mounted in opposite crystal orientations such that, for example, the positive output for shear sensitivity of the ninth sensor 106e1 is along the Z-axis but in the opposite direction (not orthogonal) to the positive output for shear sensitivity of the tenth sensor 106e2. On the opposite face <NUM> of the frame <NUM>, the positive output for shear sensitivity of the eleventh sensor 106f1 is along the Z-axis in the opposite direction (not orthogonal) to the positive output for shear sensitivity of the twelfth sensor 106f2. In addition, the orientations of the eleventh and twelfth sensors 106f1, 106f2 are arranged parallel to and mirror the orientations of the ninth and tenth sensors 106e1, 106e2, such that the positive output of the ninth sensor 106e1 is in the same direction as the positive output of the eleventh sensor 106f1, and the positive output of the tenth sensor 106e2 is in the same direction as the positive output of the twelfth sensor 106f2. This set of four sensors 106e1-106f2 jointly create a translational accelerometer along the Z-axis and a rotational accelerometer with sensitivity to rotation about the X-axis. Thus, the orientation of each set of four sensors 106a1-106b2; 106c1-106d2; 106e1- 106f2 may be orthogonal to that of the other sets, e.g., where the pairs of respective crystals are mounted with their sense axes <NUM> oriented in parallel to the X, Y, and Z axes (<NUM>, <NUM>, and <NUM>), respectively.

In other embodiments, the pairs of sensors <NUM> on each side of the mass <NUM> may be oriented in the same direction. In this configuration, the piezoelectric crystals in each pair of sensors are mounted in the same crystal orientations (i.e., the crystal sensitivities are oriented in the same direction) such that, for example, the positive output for shear sensitivity of each pair of sensors is along the same axis and in the same direction. The pairs of sensors <NUM> on opposing sides of the test mass <NUM> are oriented such that the sensors <NUM> on opposing sides along the same axis and in the same direction, i.e., they are arranged as mirror images of each other.

<FIG> is a schematic diagram illustrating an external stimulus applied to the accelerometer <NUM>. As shown in <FIG>, a translational external stimulus applies force to the housing or frame <NUM> in the positive direction of the X-axis. The individual sensors <NUM>, e.g., piezoelectric crystal transducers, are mounted to the frame <NUM> with one-sided insulators <NUM> sandwiched in between. As shown in the schematic view of <FIG>, the "+" and "-" symbols overlaid on each of the transducers 106a1-106b2 are indicative of the shear stress sensitivity alignment of the sensors <NUM> and are oriented in opposing directions for each sensor pair. They are not necessarily indicative of the location or polarity of the electrodes connected to the transducers <NUM>. The surface or side of the insulators <NUM> opposite from the frame <NUM> and adjacent to the sensors <NUM> may be conductive to act as an electrode for connection of wires from processing circuitry to the sensors <NUM>. The suspension blades 120a, 120b mechanically connect the test mass <NUM> to the sensors <NUM>. The suspension blades 120a, 120b preferably are conductive and provide a ground connection to the negative electrode of the sensors <NUM>. The suspension blades 120a, 120b are mechanically connected to the coupling surfaces <NUM> of the test mass <NUM> preferably via a rigid insulator <NUM> to electrically isolate the test mass <NUM> from the sensors <NUM> to reduce the potential noise source of the test mass <NUM>.

For a translational accelerometer sensor <NUM>, when a suitable external impulse travels along a designated positive sensing axis, the sensor <NUM> generates a positive (or negative/inverted) signal responsive to the resulting shear stress on the sensor <NUM>. The inertial resistance of the test mass <NUM> resists the external acceleration or stimulus and a shear stress develops across each pair of sensors 106a1-106b2 positioned to respond to the external acceleration. The body of the test mass <NUM> and the frame <NUM> push and pull on the opposing faces of the sensors <NUM> via the suspension blades <NUM>, generating shear stress. The sensors 106a1 and 106a2 are oriented opposite to each other in polarity of output such that when a force is applied to the accelerometer <NUM> in the indicated direction, the output of the first sensor 106a1 will be a positive value and the output of the second sensor 106a2 will be negative as the force is opposite the crystal orientation. Similarly, the output of the third sensor 106b1 will be a positive value and the output of the fourth sensor 106b2 will be negative. Alternatively, the sign convention may be reversed, without loss of generality. If the sensor response (piezoelectric crystal voltage output) were to merely be summed for each pair of sensors 106a1, 106a2 and 106b1, 106b2, the positive and negative values of each pair would cancel each other out and provide a voltage response of substantially zero.

However, as shown, the voltage output of the pairs of sensors 106a1, 106a2 and 106b1, 106b2 is transmitted to a pair of difference amplifiers 150a, 150b. The difference amplifiers 150a, 150b subtract the second voltage signal input from the first voltage signal input before amplification. Therefore, in view of the opposite orientation of crystal sensitivity for each pair of sensors 106a1, 106a2 and 106b1, 106b2, the result is additive. Thus, the input signal received from the sensors <NUM> is potentially twice the magnitude of the output from each individual crystal. As indicated in <FIG>, each pair of sensors 106a1, 106a2 and 106b1, 106b2 generates a signal which is amplified by the difference amplifiers 150a, 150b and output as V1 and V2. These voltage outputs may be summed, taking into account DC offset removal, sensitivity corrections, and orthogonality corrections, to generate a translational acceleration response value for a particular axis.

As described above, the mechanical design of the suspension blades <NUM> minimizes response of the sensors <NUM> to off-axis forces, which further helps attenuate off-axis noise. In addition, noise attributable to off-axis force components, which manifest as common-mode signal voltage, is largely rejected and filtered from the final signal, due to the common-mode rejection filtering inherent in differential amplifiers. As an example, consider the isolated suspension blade <NUM> in <FIG> subject to pure translational acceleration in direction <NUM>. The acceleration of the frame <NUM> to which the suspension blade is mounted is in a direction <NUM> normal to the length of the suspension blade <NUM>, which extends in direction <NUM>. Even though the openings <NUM> in the suspension blade <NUM> limit the stiffness of the suspension blade <NUM> perpendicular to direction <NUM> (in order to maximize crystal response to motion in direction <NUM>), some tension occurs against the crystals attached to the suspension blade <NUM> as the test mass <NUM> moves inward or outward in response to movement of the frame <NUM> in direction <NUM>. The charge/voltage response of the two crystal sensors <NUM> of the suspension blade <NUM> to this motion is of the same polarity, though, and is effectively rejected by the common-mode rejection of the difference mode amplifiers 150a, 150b. Similarly, for a rotation of the frame <NUM> around the test mass <NUM> about an axis parallel to direction <NUM>, the force induced on the suspension blade <NUM> causes tension against the crystals <NUM>, again causing a common-mode signal rejected by the difference mode amplifier 150a, 150b. The overall effect of the dual crystal/difference mode amplifier is to further reject unwanted signal (contamination) in the longitudinal axis direction <NUM> from translational motion in the perpendicular axes, directions <NUM> and <NUM>, and from rotational motion about directions <NUM> and <NUM>, promoting high-fidelity transduction.

As noted above, in an alternative embodiment, when the pairs of piezoelectric crystal sensors <NUM> are aligned in the same orientation, the output of the pairs of sensors may be transmitted in parallel to a corresponding circuit for filtering and amplification. A person of skill in the art will understand this alternate wiring arrangement of the sensors and it is thus not described in further detail here. Notably, in this arrangement, the input charge received from each pair of sensors <NUM> aligned on one side of the test mass <NUM> is potentially twice the magnitude of the output from each individual crystal. In yet a further embodiment, each sensor <NUM> about the test mass <NUM> may be wired individually and directly to a corresponding amplifier within a sensing circuit. In such a configuration, the sensors <NUM> may be electrically insulated from the suspension blades <NUM> in addition to the test mass <NUM> by placing additional insulating pads between the sensors <NUM> and the suspension blades <NUM>. Again, a person of skill in the art will understand this alternate configuration and wiring arrangement of the sensors and it is thus not described in further detail here. Each of these wiring configurations may provide unique advantages that may be preferable depending upon the output data desired.

The three sensor axes X, Y, Z may be orthogonally arranged, e.g., with the positive polarity orientation of each axis satisfying the right-hand rule (see <FIG>). For a rotational acceleration sensor, a coupled set of pairs of piezoelectric crystal transducers <NUM> as disclosed herein will generate voltage of a positive polarity for acceleration in a clockwise direction around their common axis, viewed through the plane of the motion and along the positive axis normal to the plane. Described another way, looking down from the positive end of the axis normal to the plane of rotation, acceleration of a coupled set of pairs of sensors in a counter-clockwise direction around the axis will generate voltage of a positive polarity. Alternatively, the sign convention may be reversed, without loss of generality. From the same sensors that measure translation acceleration along the X-axis, rotational acceleration may be calculated for rotational movement about the Y-axis. From the same sensors that measure translation acceleration along the Y-axis, rotational acceleration may be calculated for rotational movement about the Z-axis. And from the same sensors that measure translation acceleration along the Z-axis, rotational acceleration may be calculated for rotational movement about the X-axis.

<FIG> is a representative wiring diagram for a set of piezoelectric transducers or sensors in a multi-axis, single mass accelerometer as described herein. <FIG> is a simplified schematic denoting the wiring of the preamp boards and twelve channels Xa1-Zf2 corresponding to the sensors <NUM> in embodiments of the multi-axis, single mass accelerometer disclosed herein. In some embodiments, an integrated preamp/digitizer may be placed behind the buffer preamp. In additional embodiments, a post-digitization processer for gain adjust and offset removal may be used before forming the final translational and rotational motion output signals, e.g., via digital summation and differencing.

In some configurations, the multi-axis, single mass accelerometer may have twelve sensor outputs, pairs of which from the same face of the frame are combined by the difference amplifier to result in six axial voltage outputs: VXa, VXb, VYc, VYd, VZe, and VZf. These voltage outputs can then be used to generate three orthogonal translational acceleration responses and three rotational acceleration responses. The translational and rotational acceleration responses for each axis (which may be designated Xt, Xr; Yt, Yr; and Zt, Zr, respectively) are formed from the sum and difference of the six voltage outputs from the difference amplifiers. A sign convention determines whether the translational and rotational outputs are derived by summing or differencing the axial voltage outputs. For particular design purposes, the translational response signal may be generated by summation of the opposing voltage outputs for an axis. As a consequence, the difference of the opposing voltage outputs along an axis may be used to generate the rotational acceleration response. The sign convention is however arbitrary, and may be reversed without loss of generality.

For example, in an alternative embodiment as described above, when the pairs of piezoelectric crystal sensors <NUM> are aligned in the same orientation, the voltage output of pairs of sensors on opposing sides of the test mass (oriented parallel to the same sensing axis) wired in parallel may be differenced in order to detect rotational motion about a second axis orthogonal to the sensing axis. A person of skill in the art will understand this alternate wiring arrangement of and computational output for the sensors and thus such is not described in further detail here.

<FIG> is a schematic representation of an exemplary method for configuring a multi-axis, single mass accelerometer as disclosed herein to significantly increase the sensitivity of the crystal sensor response. A simple two-dimensional disk of radius R<NUM> is presented in plan view; consider a rotational sensor, similar to a single axis sensor in the three-dimensional sensor described herein, in which a piezoelectric crystal connects such a disk to an external frame. A force F (delivered by the frame) and acting on the circumference of the disk at Recreates a torque (τ=FR<NUM>). In the context of a rotating body, torque may be expressed in terms of angular acceleration (α), i.e., τ = Iα, where I is the moment of inertia of the rotating body. For a thin disk of radius R<NUM> rotating about an axis Z: <MAT>.

Presuming that the crystal area is small relative to the disk as a whole, the force that generates the torque can be represented as the product of the average shear stress (σ) on the crystal, which largely determines the voltage it generates, and its area (A), i.e., F = σA. Upon substitution, the equation for torque then becomes: <MAT> Presuming the mass of the test mass <NUM> is small relative to that of the frame <NUM> to which it is attached and to that of any equipment to which the frame <NUM> itself is attached, then a given external rotational acceleration stimulus will generate a given angular acceleration (α) of the test mass <NUM> and of the frame <NUM>, no matter the radius at which that stimulus acts. Accordingly, the right hand side of the above equation may be viewed as a relative constant; and the stress on a piezoelectric sensor may be viewed to scale as: <MAT> where R<NUM> is the radius of the crystal sensor from the center of the test mass, at which it serves as a torque arm connecting the frame <NUM> to the inertial mass <NUM>.

For example, as shown in <FIG>, if the radius R<NUM> of the torque arm can be reduced to half of the radius R<NUM> of the test mass, then the shear stress on the crystal sensors may be significantly increased and thereby improve the sensitivity of the sensors, relative to a sensor in which the crystal connects at the circumference of the test mass. In other words, when R<NUM> = <NUM>R<NUM>: <MAT> In this example, the sensitivity could potentially be doubled.

Returning to <FIG>, it should now be apparent that by creating the coupling surfaces <NUM> on the test mass <NUM>, the length of the torque arm at the attachment point of the suspension blades <NUM>, which further attach to sensors <NUM>, is shortened by a distance D<NUM>, as shown in <FIG>. Thus, by reducing the distance D<NUM> between the point at which the struts couple with the test mass 104and the center of the mass (in this example, the interface between suspension blades <NUM> and the coupling surfaces <NUM>), the sensitivity of the sensor <NUM> responses to the forces acting on the accelerometer <NUM> is enhanced by several percent, relative to a design in which the sensors are attached at the full radius of the test mass <NUM>.

When a seismic sensing node is coupled to the sea floor (or other sedimentary bottom of a body of water), translational and rotational measurements will reflect the corresponding movement of the sediments (which are considered to be a solid) at that location. Unlike in a fluid where only p-waves (also known as longitudinal or compressional waves) propagate, in a solid one encounters propagation of both p-waves and s-waves (also known as shear or transverse waves). Further, because the sea floor is the notional interface separating the solid sedimentary floor from the fluid body of water, sometimes referred to as a "free boundary," the sea floor often gives rise to other types of waves, so-called interface waves, such as the Scholte wave. In existing industry practice to discern direction of propagation for a seismic wave one has to use a spatially-distributed array of seismometers that measure translational motion so that the spatial derivative of the measured vertical acceleration in the X direction (dAz/dx) and the spatial derivative of the measured vertical acceleration in the Y direction (dAz/dy) can be calculated as a finite difference approximation.

Because fluids do not support s-waves, a unique aspect of free boundaries is that the shear stress in the (X, Z) and (Y, Z) vertical planes vanishes at the boundary (i.e., the solid/fluid interface). This, in turn, leads to the following result: rotation measured about the X-axis (Rx) equals dAz/dy, the spatial derivative of the measured vertical acceleration in the Y direction and, similarly, rotation measured about the Y axis (Rv) equals dAz/dx, the spatial derivative of the measured vertical acceleration in the x direction. Note that the spatial derivative of a propagating wave characterizes its direction of propagation. Unlike prior sensors, the multi-axis, single mass accelerometer disclosed herein is a <NUM>-component sensor and can record all of these waves in combination with its translational and rotational components. The multi-axis, single mass accelerometer is thus able to measure propagation direction at the free boundary interface for a certain component of the seismic wavefield (Vz) and acts as a "point seismic array. " (See, REF: Investigating the Point Seismic Array concept with seismic rotation measurements, D. Aldridge and R. Abbott, Sandia National Lab. , Report SAND2009-<NUM>). The full set of <NUM>-component measurements may thus be sufficient to algorithmically determine the type (compressional or shear), speed, and direction of any incident plane wave, thus dispensing with the need to deploy spatially-extended receiver arrays.

Thus, as it relates to the vertical acceleration, by virtue of recording both the value of the function Vz and the values of its spatial derivatives via Rx and Rv (slopes of the function) at the same point, one can exactly interpolate a regularly sampled grid by a factor of two, subject to the aliasing frequency limit of the subsampled grid. As an example, one can use a <NUM> × <NUM> rather than a <NUM> × <NUM> sampling grid and interpolate exactly from the former to the latter at a substantially reduced cost, which at the theoretical limit equals <NUM>%. This in turn has a profound effect on reducing the cost of acquiring ocean bottom seismic data.

A multi-axis, single mass accelerometer may thus be implemented as a twelve-component seismic sensor configured for measuring three degrees of freedom in translational motion along three independent directions and three degrees of freedom in rotation about three independent axes. In some of these embodiments, the accelerometer is provided with three sets of piezoelectric sensors arranged along independent or orthogonal axes and coupled in opposing pairs to a frame and to a test mass suspended therein.

The resonance of such a multi-axis, single mass accelerometer may be one or more orders of magnitude higher than other sensors (e.g., a typical particle acceleration sensor), which enables a reasonable implementation of a high-cut mechanical shock system while maintaining flat response within a selected frequency band of interest. The accelerometer may incorporate directional sensing by configuring the crystal sensors and beams in unique orientations, and/or by attaching the crystal sensors to the test mass with supports that are "stiff" along the principal axis of sensing and "soft" along the cross axes. One objective that may be met by such a configuration is to reduce or minimize distortion in the frame, while enhancing or maximizing shear stress in the crystal along the primary sensing direction.

Each of the transducers or sensors may have a crystal orientation defining a shear sensitivity, with the shear sensitivity of the transducers in each pair being oriented generally parallel to one another, but in opposing directions of sensitivity, and substantially perpendicular to the respective principal axis. In other words, each crystal may be sensitive to shear in the plane defined by the sense direction <NUM> and the normal direction <NUM>, with one crystal in a pair generating positive voltage for a positive rotational distortion about the normal direction <NUM> and the other generating positive voltage for a negative rotational distortion about the normal direction <NUM>. Furthermore, the shear sensitivities of the three sets of opposing pairs of transducers may be oriented in mutually perpendicular directions. The mutually perpendicular directions can correspond to the principal axes.

The frame or housing may comprise a plurality of sides arranged in pairs on the opposing sides of the central mass, as defined along the three principal axes, with each pair of transducers coupled to a respective one of the sides of the frame. A suspension blade may be disposed along a central portion of each side of the frame. Each lateral end of each suspension blade is configured to couple to a respective sensor, and a middle portion of each suspension blade is configured to couple to the test mass. Each suspension may have a non-uniform cross section defining a relatively higher stiffness along a primary sensing direction <NUM> of the respective transducer (e.g., as defined by the crystal orientation, with shear sensitivity in the plane defined by the sense direction <NUM> and the normal direction <NUM>), and a relatively lower stiffness perpendicular to the primary sensing direction (e.g. in the normal directions <NUM> and <NUM>).

In any of these examples and embodiments, the suspension blades may be conductive. Each of the sensors may be electrically coupled to the conductive suspension blades, in order to define a common terminal potential. For example, each of the transducers may be oriented with a negative electrode side disposed toward the suspension blades, and all of the suspension blades may be electrically connected to define a common ground.

Electronics may be configured to combine signals of the sensors in each pair using differential amplification, in order to generate an output characterizing acceleration of the frame in a direction parallel to the corresponding principal axes. Similarly, the electronics may be configured to combine signals of the transducers in each set of pairs to generate an output characterizing rotational acceleration of the frame about the respective principal axes. Depending on electrode and response convention for the selected transducer crystal orientation, the amplified signals from sensor pairs in each set may be combined by summing to generate the (translational) acceleration output, and combined by subtraction to generate the rotational acceleration output, utilizing suitable preamplifier electronics, gain factors, and other parameters.

Claim 1:
A multi-axis, single mass acceleration sensor (<NUM>) comprising: a three-dimensional frame (<NUM>);
a test mass (<NUM>) disposed within and spaced apart from the frame (<NUM>);
a plurality of transducers (<NUM>) mechanically coupled to the frame (<NUM>) at a plurality of respective locations on the frame (<NUM>); and a plurality of struts configured to
couple to the test mass (<NUM>) at a plurality of
respective positions and to couple with respective sets of the transducers (<NUM>a1,f2) at the plurality of respective locations, thereby suspending the test mass (<NUM>) within the frame (<NUM>); wherein the sensor (<NUM>) is responsive to translational motion in multiple independent directions and to rotational motion about multiple independent axes.