Patent Description:
Subterranean hydrocarbon deposits may be accessed by drilling a bore that extends from the surface of the earth to the hydrocarbon deposit, and then pumping hydrocarbons up to the surface through the bore. In some applications, a measurement tool may be run through the bore after the bore has been drilled to take measurements of the bore or the earth disposed immediately around the bore. However, because such systems are designed to measure formation properties along the axis of borehole they are inadequate for evaluating the formation far away from the borehole. Formation evaluation methods that depend on elastic wave propagation normally focuses on the refracted wave propagating along the walls of the borehole and other wavemodes that propagate within the bore along its axis. Microfractures and major fracture planes that extend tens of meters away from the borehole cannot be investigated with such conventional tools and methods. It would be beneficial to design a measurement tool capable of detecting small and large fractures that extend tens of meters away from the borehole that may act as storage or pathways for hydrocarbons. <CIT> discloses a method for creating three-dimensional images of non-linear properties and the compressional to shear velocity ratio in a region remote from a borehole using a conveyed logging tool.

Certain embodiments commensurate in scope with the claims are summarized below. These embodiments are not intended to limit the scope of the claims, but rather these embodiments are intended only to provide a brief summary of possible forms of the claimed subject matter. Indeed, the claims may encompass a variety of forms that may be similar to or different from the embodiments set forth below.

In one embodiment, a downhole measurement tool configured to be run through a bore according to claim <NUM> is provided.

In a third embodiment, a method according to claim <NUM> is provided.

Furthermore, any numerical examples in the following discussion are intended to be nonlimiting, and thus additional numerical values, ranges, and percentages are within the scope of the disclosed embodiments.

The disclosed techniques include utilizing a measurement tool that includes a source and a three-component receiver. As the measurement tool moves through a bore, the source emits a signal outward into the material surrounding the bore. The signal reflects off features in the material and back toward the bore. The receiver receives the compressive component and the two shear components of the reflected signal. The collected data may be used to create 2D and 3D images of the material surrounding the bore for performing formation evaluation.

<FIG> is a schematic of an embodiment of a mineral extraction system <NUM>. Oil and/or gas may be accessed from subterranean mineral deposits <NUM> via a well <NUM>. For example, a bore <NUM> may drilled using a drilling tool <NUM> (e.g., drill bit), extending from the surface <NUM> to the mineral deposit <NUM>. Though the bore <NUM> shown in <FIG> extends vertically from a drilling rig <NUM> at the surface <NUM> to the mineral deposit <NUM>, the bore <NUM> may extend at an angle oblique to the surface <NUM>. Similarly, the bore <NUM> may change directions as it extends from the surface <NUM> to the mineral deposit <NUM>. That is, the bore <NUM> may include portions that extend oblique to, perpendicular to, or parallel to the surface <NUM>. A measurement tool <NUM> may be inserted into the bore <NUM> behind the drilling tool <NUM> for taking measurements of, or imaging, a volume of material <NUM> surrounding the bore <NUM> for formation evaluation. The measurement tool <NUM> may be run down the bore <NUM> behind the drilling tool <NUM> and measurements taken as the bore <NUM> is drilled (logging while drilling, or LWD). In other embodiments, the measurement tool <NUM> may be run down the bore <NUM> after the bore <NUM> has been drilled and measurements taken as the measurement tool <NUM> is pulled back up through (e.g., retrieved from) the bore <NUM> (wireline logging). In further embodiments, the measurement tool <NUM> may be run down the bore <NUM> after the bore <NUM> has been drilled and measurements taken as the measurement tool <NUM> is pulled back up through the bore <NUM> while pipe is being removed from the bore <NUM> (logging while tripping, LWT).

The measurement tool <NUM> may include one or more sources <NUM> that emit a signal that propagates through the earth, and one or more receivers <NUM> that receive signals reflected off of features <NUM> (e.g., planar fractures, microfractures, faults, bedding planes, and other scatterers) within the volume of material <NUM> around the bore <NUM>. Data collected using the measurement tool <NUM> may be analyzed using a computing device <NUM> (e.g., computer, tablet, mobile device, etc.), or a combination thereof. The computing device <NUM> may include communication circuitry <NUM>, a processor <NUM>, memory <NUM>, communication ports <NUM>, and a user interface <NUM>, which may include a display <NUM>. While the measurement tool <NUM> is being passed through the bore <NUM> to take measurements, or following the measurement tool <NUM> being passed through the bore <NUM>, data may be passed to a memory component <NUM> (e.g., via cable <NUM>), which may be located at the surface <NUM>, or within the measurement tool <NUM>, for storage until the data is processed. In other embodiments, collected data may be passed to the computer <NUM> wirelessly (e.g., via the cloud <NUM>) or through a wired connection via communication ports <NUM>. The computer <NUM> may be located near the drilling rig <NUM> or remote from the well <NUM>. In some embodiments (e.g., the computer <NUM> is located remotely relative to the well <NUM>), the data may be passed to the computer <NUM> via the cloud <NUM> or over a network. In other embodiments, the computer <NUM> may be in wireless communication with the measurement tool <NUM> while the measurement tool <NUM> is traveling through the bore <NUM> and analyzing data in real time or near real time. In some embodiments, the operation of the measurement tool <NUM> may be adjusted based on analysis by the computing device <NUM> (e.g., dynamic software). The computer <NUM> may be outfitted with software stored on the memory component <NUM> and executed by the processor <NUM> to facilitate analysis of the collected data. For example, the computing device <NUM> may be capable of post-processing the data collected by the measurement tool <NUM>, and identify features <NUM> in the volume of material <NUM> surrounding the bore <NUM>. Based on reflected signals received by the receivers <NUM>, 2D and 3D imaging of the volume of material <NUM> surrounding the bore <NUM> may be performed.

<FIG> is an illustration of a signal <NUM> propagating through an isotropic material. As shown, the signal includes a compression component, P, and a shear component, S. The compression component, P, extends axially along the axis of travel <NUM>. The shear component, S, acts orthogonal to the axis of travel <NUM>. In the illustrated embodiment, the shear component, S, is oriented along axis <NUM>. However, it should be understood, that the shear component, S, may be oriented along axis <NUM>, or in any other direction. When the signal <NUM> propagates through an anisotropic or birefringent material, the shear component splits into first and second shear components, S1 and S2, which are generally polarized orthogonal to one another along a given raypath in subsurface earth formations.

<FIG> is an illustration of shear signal splitting in anisotropic materials. As illustrated, a signal propagates through an isotropic material <NUM>, passes through an anisotropic material <NUM>, and then exits the anisotropic material <NUM> back into the isotropic material <NUM>. As shown and described with regard to <FIG>, the signal propagates through the isotropic material <NUM> as a single signal with pressure component P, and a shear component, S, oriented along plane <NUM> intact as a single signal <NUM>. The signal <NUM> contacts a front plane <NUM> of the anisotropic material <NUM> and the shear component, S, splits into two polarized components, S1 and S2, because the refractive index of the anisotropic material <NUM> depends on polarization of the signal <NUM>. The first shear component, S1, polarized about a first plane <NUM>, propagates at a first speed according to a first refractive index of the anisotropic material <NUM>, while the second shear component, S2, polarized about a second plane <NUM>, propagates at a second speed according to a second refractive index of the anisotropic material <NUM>. The first shear component, S1, and the second shear component, S2, reach a back plane <NUM> of the anisotropic material <NUM> at different times and exit the anisotropic material <NUM> into the isotropic material <NUM>. Thus, what began as the shear component, S, of the signal <NUM> is received by the receiver <NUM> as first and second shear components, S1 and S2, which arrive at different times.

If the medium is isotropic the received shear waves are polarized in the plane of propagation containing the reflection point. To completely capture such arrivals an axially oriented receiver is used. It should be noted that the conventional cross-dipole geometry does not record this component of the wavefield. When stresses, planar fractures, microfractures are present in the volume of material <NUM> surrounding the bore <NUM>, as shown in <FIG>, the volume of material <NUM> becomes anisotropic and birefringent. As such, signals <NUM> emitted by the one or more sources <NUM>, reflected by features <NUM> in the volume <NUM>, and received by the one or more receivers <NUM> will be split into a pressure compressive component, P, and first and second shear components, S1 and S2, which arrive at different times. Thus, receiving the full wave field of the reflected signal P, S1, S2, and capturing them with a <NUM> component receiver results in the most complete formation evaluation of the volume of material <NUM> surrounding the bore <NUM>.

<FIG> is a schematic view of the measurement tool <NUM> of <FIG> disposed within the bore <NUM>, in accordance with an embodiment. For clarification, an axis <NUM> of the bore <NUM> is shown. Though the bore <NUM> is shown in <FIG> extending vertically, it should be understood that the bore <NUM> may extend horizontally or at an angle oblique to the surface <NUM> of the earth. Similarly, as the bore <NUM> changes directions, so too does the bore axis <NUM>. An X, Y, Z coordinate system is also shown in <FIG>. For the sake of simplicity, it should be understood that the coordinate system also mirrors changes in direction of the bore <NUM> such that the bore axis <NUM> always extends in the Z direction.

As shown, the measurement tool <NUM> includes a source <NUM> and a receiver <NUM>. As previously discussed, the measurement tool <NUM> may include multiple sources <NUM> and multiple receivers <NUM>. Similarly, the source <NUM> and the receiver may be a part of the same module or assembly, or part of separate modules or assemblies. The source <NUM> may be any device that can be excited electrically or mechanically to generate compressional and shear waves out into the volume of material <NUM> surrounding the bore <NUM>. <FIG> illustrates a particular instance of the source that is commonly used in cross-dipole geometries. However, other multi-mode systems having <NUM>, <NUM>, <NUM>, or more poles may also be possible. Here the source emits a signal in a plane parallel to the XY plane and orthogonal to the bore axis <NUM> into the volume of material <NUM> surrounding the bore <NUM>, as evidenced by arrows <NUM> and <NUM>. However, the source <NUM> may be any number of devices capable of emitting a signal into the volume of material <NUM> surrounding the bore <NUM>. According to the invention, the source <NUM> is any vibrational source that may be electrically or mechanically activated to generate compressional and shear waves. The source <NUM> may be capable of operating as a monopole, a dipole, or both, for example. The signal penetrates deep (e.g., <NUM>, <NUM>, or <NUM> meters or more) into the volume of material <NUM> surrounding the bore <NUM> such that fractures of a wide range of sizes may be detected. For example, the signal may have a frequency greater than <NUM>.

The receiver <NUM> is a sensor capable of receiving the P, S1, and S2 components of the reflected signal. For example, the receiver <NUM> may have first and second elements, indicated by arrows <NUM> and <NUM>, respectively, oriented in a plane parallel to the XY plane and orthogonal to the bore axis <NUM>. A third element, indicated by arrow <NUM>, may be oriented parallel to the bore axis <NUM>. As previously discussed with regard to <FIG>, the P component of the reflected signal travels along the axis of propagation. For isotropic media the shear component, S, is polarized in the plane of propagation. For anisotropic media the shear wave splits into two polarized components, S1 and S2. As such, the first and second elements <NUM><NUM>, oriented in the plane orthogonal to the bore axis <NUM>, receive primarily the P component and the S2 component of the reflected signal. The third element <NUM>, oriented parallel to the bore axis <NUM>, receives the S1 component. According to the invention, the receiver <NUM> is a three-component (3C) sensor (e.g., a three-axis magneto resistive sensor, a piezo electric sensor, magnetorestrictive, capacitive sensor, MEMS sensors, etc.). The receiver <NUM> may include one or more geophones or accelerometers. Further, the receiver <NUM> may include multiple sensors, either in a single package or in separate packages. As previously discussed, though the measurement tool <NUM> shown in <FIG> has a single source <NUM> and a single receiver <NUM>, it should be understood that embodiments having multiple sources <NUM>, multiple receivers <NUM>, or a combination thereof, are also envisaged.

The measurement tool <NUM> may be run through the bore <NUM> to take measurements of the volume of material <NUM> surrounding the bore <NUM>. Measurements may be taken as the measurement tool <NUM> moves through the bore <NUM> toward the hydrocarbon deposit and away from the surface, or as the measurement tool <NUM> moves through the bore <NUM> toward the surface and away from the hydrocarbon deposit. Data acquisition may be continuous as the measurement tool <NUM> moves through the bore <NUM>, or data acquisition may occur at discrete locations as the measurement tool <NUM> moves through the bore <NUM>. As discussed with regard to <FIG>, analysis of the collected data may occur aboard the measurement tool in real time or near real time, or the data may be collected and passed to an external computing device for analysis.

Existing systems typically utilize crossed-dipole sources (i.e., two dipole antennas positioned orthogonal to one another) to emit a signal and a two-component receiver with both elements aligned within a plane orthogonal to the bore axis <NUM>. As a result, the two-component receiver only receives the P and S2 components of the reflected signal. The S1 component, which travels perpendicular to the bore axis <NUM> and is polarized and a plane parallel to the bore axis <NUM>, and in most cases the most dominant arrival, is not fully captured. As such, the effective direction of measurement is only along bore axis <NUM> and measurements can only be taken a few meters into the volume of material <NUM> surrounding the bore <NUM>. Utilizing a three-component receiver <NUM>, as shown in <FIG>, allows the measurement tool <NUM> to capture all three components (i.e., P, S1, and S2) of the reflected signals, such that the effective directions of measurement are both along the bore axis <NUM> and radially outward from the bore axis <NUM>, enabling formation evaluation deep into the volume of material <NUM> surrounding the bore <NUM>. For example, using the disclosed techniques, formation evaluation may be performed a distance of up to <NUM> meters or more into the volume of material <NUM> surrounding the bore <NUM>. Once data is collected by the measurement tool <NUM>, 2D or 3D images may be generated for the volume of material <NUM> surrounding the bore <NUM>.

For cross-dipole sources, <FIG> shows the various planes used for 2D imaging once data has been collected. Though 3D imaging provides better quality images of the volume of material <NUM> surrounding the bore, 3D imaging may take more processing power than 2D imaging. According to the invention (e.g., when processing power is limited), 2D imaging is performed prior to 3D imaging or instead of 3D imaging. As shown, the bore <NUM> and the bore axis <NUM> extend along the line at the intersection of the XZ plane <NUM> and the YZ plane <NUM>. The XY plane <NUM> extends outward orthogonal to the bore axis <NUM>. As described with regard to <FIG>, it should be understood that as the bore <NUM> changes directions, so do the bore axis <NUM> and the coordinate system. In some embodiments, the measurement tool <NUM> may include a gyroscope or other sensor to help determine the orientation of the measurement tool. As shown, the source <NUM> emits a signal in a plane orthogonal to the bore axis <NUM> and parallel to the XY plane, which can be broken up into its component parts, Sy and Sx. The receiver <NUM> receives reflected signals in three axes, such that received signals can be broken up into their component parts, Rx, Ry, and Rz. The received signals may further be broken up based on the component of the source <NUM> signal to which they correspond. That is, the Rx component may be broken up into SxRx and SyRx, the Ry component may be broken up into SxRy and SyRy, and the Rz component may be broken up into SxRz, and SyRz. Each of these may correspond to the compressive, P, and shear, S1, S2 components of the reflected signal. For example, for the XZ plane, the P image corresponds to SxRx, the S1 image corresponds to SxRz, and the S2 image corresponds to SyRy. For the YZ plane, the P image corresponds to SyRy, the S1 image corresponds to SyRz, and the S2 image corresponds to SxRx.

Strike is defined as the as the angle of the azimuth of a plane of the detected feature with the borehole. Dip is the angle the detected feature makes with the borehole. Based solely on the 2D images described above for the XZ plane and the YZ plane, the strike and dip values for a detected feature may not be determined. However, by taking into account the SxRy and SyRx values, strike and dip may be estimated.

<FIG> is a flow chart of a process <NUM> for taking measurements and generating 3D images of the volume around the bore, in an embodiment not forming part of the present invention. In block <NUM> a signal is emitted from the source of the measurement tool. As previously discussed, the source may emit a signal in a plane parallel to the XY plane and orthogonal to the bore axis into the volume of material surrounding the bore (i.e., cross dipole). In other embodiments, the source may be any device that generates compression and shear waves via an electrical or mechanical excitation process. The source may be any number of devices capable of emitting a signal into the volume of material surrounding the bore. The source may be capable of operating as a monopole, a dipole, or both. The signal penetrates deep (e.g., <NUM>, <NUM>, or <NUM> meters or more) into the volume of material surrounding the bore such that fractures of a wide range of sizes may be detected. For example, the signal may be emitted at an appropriate frequency that the signal penetrates deep into the volume of material surrounding the bore.

In block <NUM>, the receiver receives signals reflected from features within the volume of material disposed about the bore. The receiver contains one or more sensors capable of receiving the P, S1, and S2 components of the reflected signal. For example, the receiver may have first and second elements oriented in a plane parallel to the XY plane and orthogonal to the bore axis. A third element may be oriented coaxial to or parallel to the bore axis. The P component of the reflected signal travels along the axis of propagation, for isotropic media, the S1 component is polarized in the plane of propagation, and the S2 component is polarized perpendicular to the plane of propagation. As such, the first and second elements oriented in the plane orthogonal to the bore axis receive the P component and the S2 component of the reflected signal. The third element, oriented parallel to the bore axis, receives the S1 component. For anisotropic media, appropriate components of the P, S1, and S2 modes are fully recorded by the three components of the receiver. In some embodiments, the receiver may be a three-component (3C) sensor (e.g., a three-axis magneto resistive sensor, a piezo electric sensor, magnetorestrictive, capacitive sensor, MEMS sensors, etc.). In other embodiments, the receiver may include one or more geophones or accelerometers. In general, the receiver may be any device capable of sensing a vector quantity, such as force, velocity, acceleration, displacement, etc. Further, the receiver may include multiple sensors, either in a single package or in separate packages. In some embodiments, block <NUM> may include some signal conditioning, such as filtering, fast Fourier transforms (FFT), etc..

In block <NUM>, 3D one or more images <NUM> are generated using the collected data and output. As discussed with regard to <FIG>, the source may emit a signal in a plane orthogonal to the bore axis and parallel to the XY plane, which can be broken up into its component parts, Sy and Sx. The receiver receives reflected signals in three axes, such that received signals can be broken up into their component parts, Rx, Ry, and Rz. The received signals may further be broken up based on the component of the source signal to which they correspond. That is, the Rx component may be broken up into SxRx and SyRx, the Ry component may be broken up into SxRy and SyRy, and the Rz component may be broken up into SxRz, and SyRz. Each of these may correspond to the compressive, P, and shear, S1, S2 components of the reflected signal. By stitching the various components (SxRx, SyRx, SxRy, SyRy, SxRz, and SyRz) together and analyzing the collected data, images may be created of the various features disposed within the volume of material surrounding the bore and extending outward <NUM> meters or more.

In block <NUM>, strike <NUM> and dip <NUM> may be determined and output. As previously discussed with regard to <FIG>, strike is defined as the angle of the azimuth of a plane of the detected feature with the borehole, and dip is the angle the detected feature makes with the borehole. Once the images <NUM> of the volume of surrounding the bore have been generated, strike <NUM> and dip <NUM> values may be determined directly from the one or more images <NUM> and output.

Though 3D images allow for more thorough, more complete formation evaluation, and more accurate strike and dip values, 3D imaging may use more processing power and take more time than 2D imaging. Accordingly, the method of the present invention prefers 2D imaging, or performs 2D imaging as a preliminary step before 3D imaging.

<FIG> is a flow chart of a process <NUM> for taking measurements and generating 2D images of the volume around the bore. In block <NUM> a signal is emitted from the source of the measurement tool. As previously discussed, the source may emit a signal in a plane parallel to the XY plane and orthogonal to the bore axis into the volume of material surrounding the bore. However, the source may be any number of other devices capable of emitting a signal into the volume of material surrounding the bore. The source may be capable of operating as a monopole, a dipole, <NUM> pole, <NUM> pole, <NUM> pole, etc. The signal penetrates deep (e.g., <NUM>, <NUM>, or <NUM> meters or more) into the volume of material surrounding the bore such that fractures of a wide range of sizes may be detected. For example, the signal may be emitted at an appropriate frequency to resolve fracture targets and are of adequate strength to interrogate the deep formation.

In block <NUM>, the receiver receives signals reflected from features within the volume of material disposed about the bore. The receiver contains one or more sensors capable of receiving the P, S1, and S2 components of the reflected signal. For example, the receiver may have first and second elements oriented in a plane parallel to the XY plane and orthogonal to the bore axis. A third element may be oriented coaxial to or parallel to the bore axis. The P component of the reflected signal travels along the axis of propagation, in isotropic media the S1 component is polarized in the plane of propagation, and the S2 component is polarized perpendicular to the plane of propagation. As such, the first and second elements oriented in the plane orthogonal to the bore axis receive the P component and the S2 component of the reflected signal. The third element, oriented parallel to the bore axis, receives the S1 component. For anisotropic media, appropriate components of the P, S1, and S2 modes are fully recorded on the <NUM> components of the receiver. According to the invention, the receiver is a three-component (3C) sensor (e.g., a three-axis magneto resistive sensor, a piezo electric sensor, magnetorestrictive, capacitive sensor, MEMS sensors, etc.). The receiver may include one or more geophones or accelerometers. Further, the receiver may include multiple sensors, either in a single package or in separate packages. In some embodiments, block <NUM> may include some signal conditioning, such as filtering, fast Fourier transforms (FFT), etc..

In block <NUM>, the collected data is segregated into data for upward moving signals and downward moving signals. The data for upward moving signals and downward moving signals are used separately to generate images and then combined to give an integrated picture.

In block <NUM>, 2D images are generated for the XZ plane <NUM> and the YZ plane <NUM> and output. The SxRx, SxRz, and SyRy data are used to generate the image for the XZ plane <NUM>. In the XZ plane image, the P component corresponds to SxRx, the S1 component corresponds to SxRz, and the S2 component corresponds to SyRy. The SyRy, SyRz, and SxRx data are used to generate the image for the YZ plane <NUM>. The P component corresponds to SyRy, the S1 component corresponds to SyRz, and the S2 component corresponds to SxRx.

In block <NUM>, strike <NUM> and dip <NUM> may be determined. Based solely on the 2D images described above for the XZ plane and the YZ plane, the strike <NUM> and dip <NUM> values for a detected feature may not be determined. However, by taking into account the SxRy and SyRx values, strike <NUM> and dip <NUM> may be estimated and output.

The disclosed techniques utilize at least one source and at least one three-component receiver for formation evaluation of a volume of material disposed around a bore and extending outward <NUM> meters or more. By sensing the compressive component, P, and both shear components, S1 and S2, 2D and/or 3D imaging of the volume may be generated, allowing for estimation of birefringence of the volume, and detection of microfractures several orders of magnitude below the scale of resolution. The source may operate in a monopole mode or multi-mode (i.e., dipole, quadpole, hexpole, octopole, etc.). Further, the disclosed measurement tool and corresponding techniques may be used in cased bores and/or open bores. Further, the disclosed measurement tool may be used during logging while tripping (LWT), logging while drilling (LWD), measurement while drilling (MWD), or wireline operations.

Claim 1:
A system comprising:
a downhole measurement tool (<NUM>) configured to be run through a bore (<NUM>), the downhole measurement tool (<NUM>) comprising:
a vibrational source (<NUM>) configured to emit a source signal (<NUM>) into a volume of material (<NUM>) surrounding the bore (<NUM>), wherein the source signal (<NUM>) is configured to propagate through the volume of material (<NUM>) surrounding the bore (<NUM>) and reflect off of features (<NUM>) disposed within the volume of material (<NUM>) surrounding the bore (<NUM>), wherein the source signal comprises both compressional and shear waves; and
a three-component receiver (<NUM>), comprising:
a first element (<NUM>) oriented in a first plane, orthogonal to an axis (<NUM>) of the bore (<NUM>), wherein the first element (<NUM>) is configured to receive a first set of reflections of the source signal (<NUM>);
a second element (<NUM>) oriented in the first plane, orthogonal to the first element (<NUM>), wherein the second element (<NUM>) is configured to receive a second set of reflections of the source signal (<NUM>); and
a third element (<NUM>) oriented parallel to the axis (<NUM>), wherein the third element (<NUM>) is configured to receive a third set of reflections of the source signal (<NUM>);
wherein the downhole measurement tool is configured to acquire the received first, second, and third sets of reflections of the source signal;
and wherein the system comprises a computing device configured to analyze the received first, second, and third sets of reflections of the source signal and to generate the one or more images of the volume of material surrounding the bore based on the first, second, and third sets of reflections of the source signal,
wherein the computing device is configured to:
generate (<NUM>) two or more 2D images (<NUM>, <NUM>) of the volume of material (<NUM>) surrounding the bore (<NUM>) based on the first, second, and third sets of reflections of the source signal (<NUM>); and
characterized in that the computing device is further configured to estimate (<NUM>) strike (<NUM>) and dip (<NUM>) based directly on the two or more 2D images (<NUM>, <NUM>) prior to or instead of 3D imaging.