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This application is a divisional of application Ser. No. 10/755,930 filed Jan. 13, 2004, which is a divisional of application Ser. No. 10/078,121, filed Feb. 19, 2002, now U.S. Pat. No. 6,675,914. 

   CROSS-REFERENCE TO RELATED APPLICATIONS 
   Not applicable. 
   STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
   Not applicable. 
   FIELD OF THE INVENTION 
   The invention relates to a method and apparatus for the removal of undesirable materials on the wall of an earth formation so as to allow the measurement of formation characteristics such as pressure. More particularly, the invention relates to a device that creates a wave discharge by pulsing a volume of fluid so as to produce a resonant oscillation in the fluid. The wave discharge is directed in the form of a concentrated beam against at least partially non-permeable membranes formed on the earth wall of a borehole in order to remove these materials from the wall of the borehole. Still more particularly, the described device creates oscillations that produce the wave discharge by using a Helmholtz resonance frequency in pulsing a fluid volume. The wave discharge will disintegrate mudcake formed on the earth formation borehole wall to allow the unobstructed measurement of formation pressure within the formation. 
   BACKGROUND OF THE INVENTION 
   The efficient recovery of subterranean hydrocarbons such as oil and gas is assisted by obtaining reliable data about the physical conditions in a formation of interest. For example, a target formation typically includes hydrocarbon fluids that are under high pressure. Accurately measuring the formation pressure where such pressurized materials reside promotes safe and cost-effective operations in nearly all phases of hydrocarbon recovery. However, techniques for measuring formation pressure must overcome a number of technical challenges. One obstacle to pressure measurement is the mudcake that drilling mud tends to deposit on the wall of the wellbore. 
   A wellbore is typically filled with a drilling fluid such as water or a water-based or oil-based drilling fluid. The density of the drilling fluid is usually increased by adding certain types of solids that are suspended in solution. Drilling fluids containing solids are often referred to as drilling muds. The drilling fluids cool and lubricate the drill bit and carry the cuttings uphole to the surface. The solids in drilling fluids also increase the hydrostatic pressure of the wellbore fluids. By selecting drilling fluids weighted to a particular density, the column of drilling fluids creates a pressure downhole, which is greater than the pressure of the fluids in the formation. When the drilling fluid pressure is greater than the formation fluid pressure, the well is said to be in an over balanced condition. Conversely, if the formation pressure is greater than the fluid column, then the well is said to be in an under balanced condition. Control of formation fluids flowing into the well under high pressure minimizes the risk of a well blowout. 
   While an over balanced condition prevents well blowouts, it also has disadvantages, such as increased drilling costs due to slower penetration into the formation. Drilling fluid pressure in excess of formation pressure slows the penetration of the drill bit into the formation. In certain well environments it is preferred to maintain a neutral or slightly under balanced condition so as to achieve drilling speeds faster than those achieved while drilling in an over balanced condition. Drilling Practices Manual, Preston Moore, P. 18–22 Pennwell Publishing, 1974. Consequently, it is desirable to maintain a neutral balance or a slightly under balanced condition to maximize drilling penetration into the formation. 
   Drilling fluids create a mudcake as they flow into a formation by depositing solids on the inner wall of the wellbore. The mudcake on the wall of the wellbore tends to act like a filter and tends to isolate the high-pressure fluids of the wellbore from the relatively lower pressures of the formation. The mudcake helps prevent excessive loss of drilling fluid into the formation. The static pressure in the wellbore and the surrounding formation is typically referred to as hydrostatic pressure. Pressure in the formation beyond the mudcake gradually tapers off with increasing radial distance outward from the wellbore. 
   The measurement of formation pressures during drilling operations assists in locating strata most likely to produce hydrocarbons efficiently. Typically after the borehole is drilled, the well is logged by lowering a package of sensors downhole that gather data about the formation. Pressure data is useful in judging when a formation contains hydrocarbons and when such a formation may economically produce hydrocarbons. Often a wellbore may pass through more than one hydrocarbon-bearing formation, and formation pressure data assists the drilling engineer in determining whether to halt or continue drilling. 
   Further, the ability to monitor formation pressure during drilling is important to the desired practice of continuously adjusting the drilling mud density. This facilitates drilling through the maximum amount of formation in the shortest amount of time 
   To maintain the proper condition during drilling, whether neutral, over balanced or under balanced, it is necessary to measure the pressure of the formation fluids at the vicinity of the drill bit. However, the dynamic environment near the drill bit makes measurement of the formation fluids particularly difficult during logging while drilling (LWD) operations. In addition, the mudcake that forms on the wall of the borehole presents a further difficulty in determining formation fluid pressure at the bit during drilling. This mudcake forms a relatively non-permeable barrier between the instrument on the one side and the formation fluids on the other. The mudcake barrier hinders accurate measurement of the pressure of the formation fluids. 
   Prior art sensors are generally not capable of measuring formation fluid pressure during drilling. Consequently, rig personnel must closely monitor the drilling fluids flowing from the borehole for signs of increased formation fluid pressure. This often entails temporarily halting the drilling operation to allow pressure measurement of the formation. Once the drilling fluids show evidence of formation fluids flowing up the borehole, drilling is stopped and corrective measures are taken. However, this approach has particular drawbacks; and, it would be desirable to determine formation fluid pressure at the bit during drilling. 
   One such prior art instrument is a reservoir description tool (RDT) such as that disclosed in U.S. Pat. No. 5,644,076 (the &#39;076 patent) entitled “Wireline Formation Tester Supercharge Correction Method”, incorporated herein by reference in its entirety. The RDT of the &#39;076 patent includes a pressure sensing element mounted within a chamber of a housing having a piston to create a vacuum within the housing chamber. Hydraulic pads force the housing against the borehole wall; and, as the piston retracts to create a pressure reduction, a drawdown pressure removes the mudcake lining from the borehole wall. Fluids in the formation then enter the housing chamber allowing the pressure-sensing element to take a pressure reading. This tool allows only stationary measurements because drawdown pressure requires a tight seal between the housing and the borehole wall. This is undesirable because, aside from being time consuming, stationary measurements provide only discrete data points, not a continuous log. The drawback to discrete data points is that the fluid pressure between the discrete data points may vary dramatically and unpredictably. 
   Another borehole tool for removing the mudcake to measure the pressure of the formation fluids is disclosed in U.S. Pat. No. 5,969,241 (the &#39;241 borehole tool) incorporated herein by reference. The &#39;241 borehole tool measures pressure from within the borehole. A portion of the borehole wall is isolated from the surrounding borehole fluids by placing the chamber of the &#39;241 borehole tool against the borehole wall. The chamber comprises a recess in an exterior surface of the &#39;241 borehole tool. This patent describes an acoustic horn as the mechanism by which to excite fluids in a chamber. The mudcake present on the isolated portion of the borehole wall is disintegrated by an ultrasonic transducer, actuated by a piezoelectric stack, housed within the chamber. A pressure gauge then measures the pressure of the chamber to indicate the pressure of the earth formation. 
   Such a prior art tool also has deficiencies. For example, this borehole tool is inefficient because its vibrational energy does not transfer directly to the fluid. The vibrating horn is limited in the efficiency by which it transfers electrical energy to acoustical wave energy. Excitation of the piezoelectric stack creates a longitudinal wave resonance within the ultrasonic transducer. As the ultrasonic transducer resonates longitudinally, the vibrational energy is transferred to the fluid. However, the mechanical coupling of the ultrasonic transducer to the fluid is poor, thus much of the vibrational energy imparted by the piezoelectric stack remains in the ultrasonic transducer. This inefficient energy transfer is expected to reduce the vibrational energy available to break down the mudcake. Further, such tools are not compact and arel not easily installed in the drill string, which must pass through the confined area of the borehole. 
   Notwithstanding the foregoing described prior art, there remains a need for a device that possesses the features of efficiently transferring vibrational energy to create a focused wave discharge that may be used to remove mudcake from a borehole wall. Further, it is desired that such a device may be utilized so as to minimize any interruption to the drilling process. It is also desired that such a tool be capable of use on different down hole assemblies such as wire line operations and near the drill bit in drilling operations. Additionally, the tool should be able to take pressure measurements on a continuous or near-continuous basis as the drill string descends the well bore. 
   SUMMARY OF THE INVENTION 
   The present invention overcomes the aforementioned deficiencies of the prior art by providing a device that generates rhythmic pressure pulsations within a fluid-filled chamber, thereby producing a pressure wave discharge, which exits through an orifice of the chamber in a focused beam. The pulsations produced by the device include Helmholtz resonant frequencies for the geometry of the chamber; Helmholtz resonant frequencies efficiently transfer energy from pulse elements of the device to the fluids in the chamber. The device directs pressure waves in the fluids in the chamber through an orifice that focuses the waves against the borehole wall in the form of a concentrated beam. The wave discharge removes mudcake from the borehole wall, thereby opening a passage from the interior of the formation to the device chamber. In this manner pressure transducers associated with the device may accurately measure pressure from the formation. The device of the present invention operates with a speed that allows it to be used on a continuous to near-continuous basis. If disposed on a drill string, the drilling operation need not be slowed or halted in order for the present acoustic jet to function. Further the device may be used on both wireline operations and drilling operations. 
   The pressure reading tool of the present invention overcomes the deficiencies of the prior art by applying a fundamentally different approach to the removal of mudcake from borehole walls. For example, the &#39;241 borehole tool induces vibrational frequencies in an acoustic horn to transfer the vibratory energy to the fluid. The tool of the present invention induces a resonance in the fluid itself. Thus, the poor energy transfer between the acoustic horn and fluid is eliminated. Further, the tool of the present invention concentrates and focuses the wave energy so as to minimize the loss of energy while simultaneously maximizing the energy brought to bear against the borehole wall. 
   One embodiment of the present invention includes pressure reading tool having a housing with an interior chamber and an orifice extending from the chamber to the exterior of the housing. A pulse member with a magnetostrictive ring and excitation source is disposed within the housing chamber to produce a highly agitated fluid discharge through the orifice. The magnetostrictive ring, chamber volume, and orifice may be designed to cooperate to induce Helmholtz resonance frequencies in the fluid in the chamber to thereby enhance the agitation of the fluid discharge. A sheathing may be used to encapsulate the pulse member to protect it from contact with the fluid. A dampening element may also be interposed between the pulse member and housing to isolate vibration. 
   In operation, the tool is disposed in the wall of the drill stem having a drill bit for penetrating the formation and forming a borehole. An impermeable membrane in the form of mudcake forms on the borehole wall due to the drilling fluids. A portion of the borehole wall is isolated by placing the tool against the borehole wall. The pulse member is actuated to modulate the chamber volume to produce agitated fluids within the chamber. The fluids are agitated at a high frequency within the chamber. The tool directs a stream of pressure waves through the orifice and against the impermeable membrane to remove the impermeable membrane. A pressure transducer communicates with the chamber to read the pressure of the formation fluids. These pressure readings are communicated with the surface to direct the drilling of the bit through the formation. The readings may be continuous while drilling. 
   Thus, the present invention comprises a combination of features and advantages that enable it to overcome various problems of prior art pressure measuring devices. The various characteristics described above, as well as other features, objects, and advantages, will be readily apparent to those skilled in the art upon reading the following detailed description of the preferred embodiments of the invention, and by referring to the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a detailed description of a preferred embodiment of the present invention, reference will now be made to the accompanying drawings, which form a part of the specification, and wherein: 
       FIG. 1  is a cross-sectional close-up view of a drill string and well bore; 
       FIG. 2  is a cross-sectional view of a preferred embodiment of the present invention; 
       FIG. 3  is a cross-sectional close-up view of the preferred embodiment of  FIG. 2 ; and 
       FIG. 4  is a cross-sectional view of three pressure reading tools positioned in three stabilizer blades of a down hole assembly. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   It should be appreciated that the invention may be embodied in many different forms. There are shown in the drawings, and herein will be described in detail, specific embodiments of the present invention. However, the present disclosure is an exemplification of the principles of the invention. It is not intended to limit the invention to the particular illustrated embodiments, which can be modified in the practice of the invention. For example the present invention may be used while logging on a wireline cable or in logging while drilling. The present invention is particularly advantageous in logging while drilling as further described below. The term “logging” is used herein in its broadest sense to include recording any type of data representing characteristics of the formation as a function of depth, including particularly the measurement of formation fluid pressure. 
   Referring initially to  FIG. 1 , there is shown the use of an embodiment of the present invention for logging while drilling. The pressure reading tool  60  is shown disposed in a bottom hole assembly  10  for drilling a borehole  12 . The borehole  12  extends from the surface down through a plurality of different earth formations such as exemplary formation  14 . Formation  14  may include various formation fluids  16  such as water, gas and hydrocarbons. These formation fluids  16  are under pressure. The logging while drilling embodiment of the bottom hole assembly  10  includes various members including a drill collar or drill stem  18  with a drill bit  20  connected thereto. It can be seen that the drill bit  20  is penetrating the formation  14  at the bottom  22  of the borehole  12 . 
   Drilling fluids  24  are pumped down through the drill string on which the bottom hole assembly  10  is disposed, to the bottom  22  of the borehole  12  and then return up the annulus  26 , formed by the drill string and wall  28  of borehole  12 , to the surface. The drilling fluids  24  lubricate and cool the bit  20  and remove the cuttings to the surface. As the column of drilling fluids circulates through borehole  12 , some of the drilling fluid solids  24  accumulate on wall  28  of borehole  12  forming a mudcake  30 . Mudcake  30  forms a relatively impermeable membrane between the drilling fluids and earth formation  14 . A pressure drop typically occurs across mudcake  30 . 
   The present pressure reading tool  60  is schematically shown disposed in aperture  32  of one of the drilling string members, such as drill stem  18 . Alternatively, the tool may be disposed on various pieces of downhole machinery. For example, in the embodiment shown in  FIG. 4  pressure reading tools are placed in stabilizer blades  40 . Alternatively, the pressure reading tools may be placed on the drill stem  18  or on the drill collar. Alternatively, pressure reading tools may be positioned on a dedicated piece of machinery that is itself attached to the drill string. Similarly, the tool may be employed on a wireline. 
   While  FIG. 1  portrays a single pressure reading tool disposed within the drilling apparatus, it should also be understood that more than one such tool may be included in any particular down hole assembly. For example, in one embodiment three pressure reading tools are disposed within the same drill collar or drill stem. As shown in  FIG. 4 , a particular drill collar has three stabilizer blades  40 . There is a tool of the present invention disposed in each of the three stabilizer blades. In that embodiment, each of the three tools is at the same horizontal position on the drill stem; however, each tool is separated radially. In this manner, the three tools record formation pressure from sections of the formation at differing azimuthal positions. In an alternative embodiment a drill collar or drill stem may be arrayed with multiple tools at differing horizontal positions. There is an advantage associated with the use of multiple pressure reading tools. As the number of such tools increases, so does the chance of successfully obtaining an accurate formation pressure reading at a particular location. Conditions inherent in drilling, such as the vibrations and mechanical shocks found in the drilling environment, raise the possibility that mechanical equipment such as the pressure reading tool may be rendered inoperable. Likewise, a poor seal between the borehole wall  28  and orifice  76  of the tool may affect the pressure reading taken by the tool. In both these instances, the placement of multiple tools on the drill string increases the chance of a successful reading. 
   In both  FIG. 1  and  FIG. 4  tool  60  is directed radially outward toward mudcake  30 . In this manner, tool  60  produces and directs a wave discharge for removing the mudcake to allow the measurement of formation fluid pressure while drilling as hereinafter described in detail. 
   Referring now to  FIG. 2 , there is shown a preferred embodiment of the tool  60 , which includes a pressure reader  64 , a housing  66 , and pulse device  70  for producing an agitated fluid discharge using Helmholtz resonance frequencies, thus enabling pressure readings of the earth formation. Housing  66  is generally defined by a cylindrical wall  82 , an outer cap  74 , and an inner cap  75 . The generally hollow interior of housing  66  forms a chamber  84 . Chamber  84  itself is generally cylindrical in shape, as it is defined by cylindrical wall  82 , outer cap  74 , and inner cap  75 . Outer cap  74  includes an orifice  76 . Outer cap  74  may be at least partially hardened against frictional wear caused by movement across borehole wall  28 . Hardening of outer cap  74  may be through a surface treatment or a “wear plate” mounted on outer cap  28 . Inner cap  75  is adjacent the inside diameter of drill stem  18  and includes a conduit  78  at its center, which is substantially opposite orifice  76  in outer cap  74 . Inner cap  75  also includes one or more feed-through holes  80 ,  81  for receiving electrical conduits  83 ,  85 . Outer cap  74  or inner cap  75  may be removable to allow access to chamber  84 . 
   The tool has been described as having a chamber with a generally cylindrical interior geometry. While such a shape is believed to be advantageous for the transfer of energy from an electrical form to an acoustic form, the chamber may nevertheless assume other configurations. Any chamber geometry is possible, including, but not limited to, conical, spherical, cubic, rectangular, tetragonal, pyramid-shaped, elliptical, ovoid, parabolic, and polygonal. 
   Conduit  78  is also preferably substantially opposite orifice  76 . While this is believed advantageous, alternative placements of conduit  78  are also possible. For example, conduit  78  could be placed in cylindrical wall  82 . Also, conduit  78  could be placed in an off-center position on inner cap  75 . These examples are for illustrative purposes only and are not meant to be limiting. 
   According to the embodiment as shown in  FIG. 2 , outer cap  74  is curved so as to follow the shape of borehole wall  28 . Outer cap  74  would be disposed adjacent the borehole wall. In this embodiment, outer cap  74  may be hardened to withstand the contact with borehole wall  28 . In alternative embodiment, however, outer cap  74  is positioned some distance from borehole wall  28  so as to avoid direct contact with borehole wall  28 . As shown in  FIG. 4 , the pressure reading tool is positioned in a stabilizer blade of the downhole assembly. In this configuration, stabilizer blade  40  contacts borehole wall. Outer cap  74  is slightly recessed so that it does not directly contact borehole wall. In the configuration of  FIG. 4 , outer cap  74  need not assume a curved shape; nor does it need to be hardened. 
   Preferably, housing  66  is sufficiently compact to fit into a drill collar, drill stem  18 , stabilizer blade  40 , or wireline device. The pressure reading tool may be preassembled and installed as a unit in a machined or precut aperture  32  of a selected drill piece. Some known attachment means may be used in order to affix the pressure reading tool to the drill piece. Known attachment methods include, but are not limited to, a pressure fitting, pins, threading, bolting or gluing. Preferably, a threaded lock ring  42 , shown in  FIG. 4 , secures the pressure reading tool to the drill piece. The body of housing  66  may also seal aperture  32  so as to prevent the interior of the drill string passing fluids to or from the exterior of the drill string. This is preferably accomplished by o-ring seals  44 . Material selection for housing  66  is largely driven by downhole environment conditions. Generally, a corrosion resistant steel will provide the necessary ruggedness for borehole applications. Acceptable materials include steels such as 17-4PH or MP-35N. 
   Referring now to  FIGS. 2 and 3 , cylindrical wall  82  of chamber  84  is preferably at least partially lined with dampening element  86 . Preferably, dampening element  86  is made of a relatively soft material such as lead. Because tool  60  may be used along with an array of wireline instruments, it is preferred that the operation of tool  60  be dampened to prevent the transmission of vibrations along the drill string. This serves to minimize interference with other drill string instruments. Thus, cylindrical wall  82  of chamber  84  is lined on its interior preferably with a layer of lead to absorb much of the vibrations. In lieu of a lining, dampening element  86  may be a lead ring formed to seat at least partially along interior cylindrical wall  82 . It is emphasized that these are only two non-limiting examples of elements suitable for dampening. It is also emphasized that the dampening element is a convenient feature and may not be essential to the satisfactory operation of tool  60 . Alternatively any members that constitute housing  66  such as cylindrical wall  82 , outer cap  74 , and inner cap  75  may be selected of a material and dimension sufficient to perform any needed dampening function. 
   Pulse device  70  is disposed within chamber  84  and comprises a member or members that can physically oscillate in response to a signal. In the preferred embodiment of  FIG. 2 , pulse device is a generally annular or ring-shaped member disposed within chamber  84 . Pulse device  70  seats substantially contiguously along the interior surface of cylindrical wall  82 , or, if present, along the interior surface of dampening element  86 . Preferably, pulse device  70  extends along the length of cylindrical wall  82  such that the ends of pulse device  70  rest against the interior surfaces of outer cap  74  and inner cap  75 . 
   In the preferred embodiment, pulse device  70  seats substantially contiguously along the interior surface of cylindrical wall  82 . In this manner, the physical oscillations of pulse device  70  efficiently transfer energy to fluid in chamber  84  at all positions along the interior surface of pulse device  70 . However, it is possible to configure pulse device  70  in an alternative manner. For example, rather than being configured as a single, ring-shaped body, pulse device  70  could comprise any number of discrete units, of any geometry. These separate units could be placed at different locations within chamber  84 . A plurality of individual pulse device units could approximate the form and function of a ring-shaped pulse device when such individual units are placed in proximity to one another along the interior surface of cylindrical wall  82 . Alternatively, discrete pulse device units could be placed on the interior surfaces of outer cap  74  and inner cap  75 . Additionally, pulse device units could even be placed at some interior position of chamber  84 . If housing  66  is selected such that it defines chamber  84  to have a non-cylindrical geometry, then pulse device  70  may also have an alternative configuration and placement in the chamber. It would also be possible, and would be within the scope of this invention, to construct housing  66  with recesses or voids so as to have a honeycombed configuration. In such a configuration, pulse device units could be disposed within the recesses of housing  66 . 
   Pulse device  70  may itself be composed of separate elements. In the ring-shaped, preferred embodiment, shown in  FIG. 3 , pulse device  70  has pulse elements  88  at its core. Excitation source  90  wraps around pulse elements  88 , and sheathing  72  wraps around excitation source  90 . Sheathing  72  thus forms the external surfaces of the preferred pulse device  70 . 
   Sheathing  72  is preferably made of an elastomeric material to insulate the pulse device  70  from harmful contact with borehole fluids and particulates. Accordingly, the material for sheathing  72  should be selected to provide a impermeable barrier between the borehole environment and pulse device  70 . Another consideration in material selection is the need to efficiently couple the energy of pulse device  70  to the fluid in chamber  84 . Thus, sheathing  72  should be a resilient medium that provides efficient transfer of pulsing motion from pulse device  70  to the fluid. Generally, the modulus of elasticity of the material for sheathing  72  should be closer to that of rubber than that of steel. Materials with relatively high material stiffness will tend to limit the motion of pulse device  70 . Rubber meets the requirements of elasticity and impermeability. Other materials such as Teflon may also be designed to have the requisite material properties. Further, sheathing  72  also provides a resilient support for pulse device  70  in housing chamber  84 . Preferably, the thickness of sheathing  72  should secure pulse device  70  within housing  66  without unduly impeding the oscillating motion of pulse device  70 . 
   Still referring to  FIG. 3 , pulse device  70  includes a plurality of pulse elements  88  wrapped within excitation source  90 . Pulse elements  88  physically distort in response to an excitation signal. As pulse elements  88  physically distort, the volume of chamber  84  rhythmically increases and decreases, thereby producing a pulsation of the fluid within chamber  84 . Preferably, pulse elements  88  are a ring of magnetostrictive elements capable of radial oscillatory expansion and contraction when activated. Excitation source  90  can include windings that are capable of transferring magnetic flux signals. Magnetic flux is the excitation signal that causes magnetostrictive elements to physically distort. The windings of excitation source  90  are wrapped around the magnetostrictive elements and exit housing  66  via housing feed-through holes  80 ,  81 . Outside the housing, the wires may connect with an external signal source. While feed-through holes  80 ,  81  allow the winding wires of excitation source  90  to exit, it is otherwise sealed to segregate fluid within chamber  84 . Pressure boots may provide one mechanism by which to make the electrical connection from wiring to the pressure reading tool. 
   Alternatively, pulse elements  88  may be a plurality of piezoelectric elements. As with the magnetostrictive ring, the piezoelectric elements are formed into an annular or ring shape. A preferred piezoelectric material is PZT-5A Piezoelectric Material, available from EDO Corporation, Salt Lake City, Utah, 84115. Whether piezoelectric elements or magnetostrictive elements are used depends on the demands of a particular application. For example, it is generally understood that piezoelectric elements are more brittle than magnetostrictive elements and may be more easily damaged. However, a particular situation may require the higher frequency oscillations that are more efficiently provided by piezoelectric elements. In any event, magnetostrictive and piezoelectric elements are given as illustrative examples of a material that can produce harmonic pulsation of the fluids in chamber  84 . Pulse elements  88  are not intended to be limited to these two materials. 
   Orifice  76  will focus the pressure wave discharge into a concentrated beam. However, one skilled in the art will understand that the profile of orifice  76  can be easily modified for alternate fluid discharges. Thus, nearly any profile may be utilized for chamber  84  and orifice  76 . If a Helmholtz chamber is desired, the resulting volume and geometry must satisfy the Helmholtz resonance frequency requirements. In certain downhole applications, it is foreseeable that it may not be possible to design housing  66  to create Helmholtz resonance frequencies. In such cases, it will be apparent to one skilled in the art to adjust the geometry of housing  66  and orifice  76  to produce an agitated fluid discharge. 
   A Screen  68  is preferably positioned within chamber  84  on outer cap  74  proximate to orifice  76 . Screen  68  can prevent borehole particulates from entering chamber  84 . When the fluid in chamber  84  is vibrated, fluid in the immediate vicinity of orifice  76  develops the highest fluid velocity. It is preferable not to restrict such fluid movement. However, if screen  68  is placed too far from orifice  76 , it may allow borehole particulates to enter chamber  84  and damage pulse device  70 . Preferably, screen  68  is placed to allow the highest velocity fluid movement through orifice  76 . Further, screen  68  includes a plurality of openings designed to minimize impedance to fluid movement. Preferably, screen  68  is formed of stainless steel and secured to outer cap  74 . While particulates capable of damaging tool  60  are often present in a borehole environment, it is emphasized that satisfactory operation of tool  60  is not dependant on the presence of screen  68 . 
   A pressure reader  64  is mounted to housing  66 . Conduit  78  provides fluid communication between pressure reader  64  and chamber  84 . Pressure reader  64  preferably includes a threaded portion that may engage mating threads within conduit  78 . Alternatively, pressure reader  64  may be secured to housing  66  by some alternative means. Because conduit  78  provides access to chamber  84 , the fluids in chamber  84  pass through conduit  78  and contact a surface of pressure reader  64  such that the pressure of the fluids can be measured. It is preferable to locate pressure reader  64  as closely as possible to chamber  84 . A remotely mounted pressure reader  64  requires a longer conduit  78 , which may be more susceptible to plugging by borehole particulates. Commercially available pressure transducers can be utilized as the pressure reader  64  in the present invention. One such pressure transducer is a strain gage based pressure transducer manufactured by Paine, Inc. Quartz gage pressure transducers are more accurate and may be used. Such devices are usually more bulky and thus of limited suitability to borehole applications. 
   While it is not essential to the invention, in the preferred tool  60 , the geometry of housing  66 , chamber  84 , orifice  76 , and pulse device  70  are selected to produce Helmholtz resonance frequencies in the fluid expected to be encountered in the drilling environment. Helmholtz resonance is a well-known scientific principle. The shape and design of Helmholtz cavities or Helmholtz resonators is also known in the industry. One kind of Hehnholtz resonator is an enclosed cavity of fluid with an open port. If the volume of fluid in the cavity is compressed, the fluid attempts to spring back to its original volume. Physical oscillations in the fluid within a ported cavity tend to resonate at specific frequencies. 
   The natural resonant frequency for a spherical Helmholtz resonator ported with a cylindrical neck in an atmospheric environment may be represented by the following equation: where 
   
     
       
         
           
             f 
             r 
           
           = 
           
             
               c 
               
                 2 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 π 
               
             
             ⁢ 
             
               
                 A 
                 
                   L 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   V 
                 
               
             
           
         
       
     
       
       
         
           c=speed of sound in the fluid 
           V=cavity volume 
           A=cross sectional area of the neck, and 
           L=length of the neck
 
This equation necessarily changes as the fluid is changed from air to another medium. Likewise, as other factors such as the geometry of the chamber and neck become more complicated, the classical equation breaks down. Hence the selection of an optimal frequency in the pressure reading tool must also be guided by trial-and-error methods. Given the changing environment in an active wellbore arising from factors such as changing pressures and the changing densities of fluids present in the wellbore, it is sometimes necessary to design a resonating chamber that can function across a variety of frequencies.
 
         
       
     
  
   A preferred design of the present invention was tested in laboratory conditions. The fluid was a drilling mud with density of approximately 1500 kg/M 3 . The speed of sound in this material was estimated at 1500 m/s. At approximately 42 kHz the preferred embodiment of the present invention displayed a relatively low impedance while retaining good sound pressure levels. At this frequency the design was found to generate a cylindrical standing wave in laboratory testing. 
   One preferred embodiment of pressure reading tool  60  previously described has the following dimensions. The diameter of the chamber  84  in the fully assembled tool, i.e., the chamber diameter as defined when pulse device  70  is in place, is approximately 1.10 in. The diameter of chamber  84  with pulse device  70  removed is approximately 1.75 in. No dampening element  86  was present. The annular pulse device  70  thus has a ring thickness of approximately of 0.325 in. The depth of chamber  84  is approximately 1.00 inch. Outer cap  74  has a thickness of approximately 0.250. Inner cap  75  has a thickness of approximately 0.50 in. The cylindrical interior wall is approximately 0.25 in. thick. Orifice  76 , centered in outer cap  74 , has an opening diameter, measured at the exterior wall of outer cap  74 , of approximately 0.50 in.; and orifice  76  widens toward the interior of chamber  84  at an angle of approximately 28°. 
   In this preferred embodiment, pulse device  70 , with an annular ring thickness of approximately 0.325 in., was further designed as follows. Sheathing  72  was as long as the interior length of chamber  84 , approximately 1.00 in., and assumed the ring thickness of the pulse device  70 , approximately 0.325 in. An annular-shaped magnetostrictive assembly, composed of a magnetostrictive ring with windings, was approximately 0.75 in. long and approximately 0.10 in. in thickness. The magnetostrictive assembly formed the interior of pulse device  70 . The magnetostrictive assembly had an interior diameter of approximately 1.30 in. and an exterior diameter of approximately 1.50 in. Given the differences in diameters, the magnetostrictive assembly was thus placed in sheathing  72  in a slightly off center position. The distance from the interior surface of sheathing  72  to the interior surface of the magnetostrictive assembly was approximately 0.20 in. However, the distance from the exterior surface of sheathing  72  to the exterior surface of the magnetostrictive assembly was approximately 0.25 in. In the assembled pulse device the magnetostrictive assembly was placed equidistant from the interior surfaces of outer cap  74  and inner cap  75 , approximately 0.125 in. from each. 
   In operation, rig personnel will install preferred tool  60  into a drilling structure such as a drill stem  18 , on a stabilizer blade  40 , or drill collar. The appropriate electrical connections are made to link pulse device  70  with a signal source. Pressure reader  64  may also be linked with an appropriate display device or recording device, usually located at a control point on the surface. Such a link is preferably done through an electronic data connection. 
   To take pressure readings during LWD, the assembled tool is lowered into borehole  12 . When the drill string approaches a formation region of interest, several steps will take place. Of initial importance is the seal between orifice  76  of tool  60  and borehole wall  28 . The measuring of formation pressure with the pressure reading tool is best accomplished when the tool is placed firmly against the formation wall. In one embodiment, the face, or outer cap  74 , of tool  60  is curved so as to make full contact against the curved face of the borehole wall  28 . Outer cap  74  seals against borehole wall  28  and traps fluids, such as drilling fluids within chamber  84 . Alternatively, where outer cap  74  is recessed relative to stabilizer blade  40 , it is stabilizer blade  40  or alternate drill string structure that forms a seal with borehole wall  28 . A tight seal is provided between preferred tool  60  and borehole wall  28  to ensure that pressure reader  64  receives the pressure of formation  14 , and not the fluids in borehole  12 . Placement of multiple tools on a drill string, each tool placed at a differing radial position, increases the probability that the orifice of at least one such tool will be in sufficiently sealed contact with the borehole wall to assure an accurate pressure reading. 
   The procedure for obtaining a pressure reading continues with electrical signals of a chosen frequency or frequencies delivered to tool  60 . These signals activate pulse device  70  at a corresponding mechanical frequency. Activation of pulse device  70  causes it to oscillate, thereby imparting a rhythmic expansion and contraction of the volume of chamber  84 . The rhythmic expansion and contraction of the volume in chamber  84  imparts pressure waves in the fluid. This wave energy flows through the only point of discharge, orifice  76 . Orifice  76  focuses the wave discharge into a concentrated beam. Because the pulsation frequency causes the fluid to resonate at a Helmholtz frequency, pulse device  70  efficiently transfers energy to the fluid discharge. 
   The near instantaneous result is a flow of wave energy expelled from the tool. Orifice  76  directs the wave discharge toward borehole wall  28  layered with mudcake  30 . The fluid pulsations strike mudcake  30 , flush away the mudcake  30 , and thereby restore permeability to borehole wall  28 . 
   At this point electrical signals to the tool can stop, and the fluid oscillation thereby ceases. The necessary period is allowed for the hydrocarbons in formation  14  to pressurize tool chamber  84 . The time period needed to pressurize chamber  84  will vary depending on factors such as the permeability of the formation and the pressure in the formation. The fluids in formation  14  seep through borehole wall  28  and into chamber  84  through orifice  76 . With hydraulic communication established via conduit  78 , chamber  84  and orifice  76 , pressure reader  64  can measure formation fluid pressure. As is known in the art, it is possible to estimate formation pressure without the need for the pressure to equalize between that of the formation and that of the chamber. Pressure reader  64  transmits the pressure data to the surface. 
   The tool allows for continuous or near-continuous readings of formation pressure. In the logging while drilling embodiment, the movement of the drill string downward as drilling progresses also moves the tool vertically downward. However, the tool receives pressure readings from a given point on the borehole wall prior to the time that the tool descends past this point of the borehole wall. The tool clears mudcake from the borehole wall and records the formation pressure associated with the cleared area of borehole wall, prior to the orifice moving past that cleared point. Once the orifice does descend past a point on the borehole wall that has been cleared and measured for pressure, the process can begin anew. At a new, lower point on the borehole wall, the tool clears mudcake and again records formation pressure. The points of pressure measurement can be closely spaced so as to allow recording of pressure data in a continuous or near-continuous fashion. In this manner the tool will take formation pressure readings at a series of points, in an ongoing fashion, while the drill string makes its normal descent in the formation. There is no need to halt drilling in order to make these pressure readings. 
   Preferred tool  60  provides a direct reading of formation fluid pressure that can be used to adjust the borehole pressure. That is, rig personnel can select a borehole pressure that prevents formation fluid from invading the borehole  12  without creating an excessive borehole pressure that slows drilling speed. Referring back to  FIG. 1 , during LWD, preferred tool  60  can be linked with a downhole telemetry system  100  to transmit formation pressure data uphole. For example, downhole telemetry system  100  could include control circuitry  102  to energize preferred tool  60  and a drive circuitry/transmitter  104  to receive pressure data from preferred tool  60  to transmit the pressure data to the surface. Drive circuitry/transmitter  104  may utilize a mud siren to transmit data in the form of pressure pulses in the drilling mud flowing uphole. Monitors  106  on the surface receive and process the pressure data transmitted by downhole telemetry system  100 . Such a system could be configured to provide continuous transmission of pressure data. Alternatively, the drive circuitry could be designed to transmit pressure data only after a threshold pressure is sensed by pressure transducer. In any event, data transmission systems for LWD in the prior art are well known, and one of ordinary skill in the art will understand how to relay pressure readings obtained from preferred tool  60  to monitoring systems on the surface. Further, one of ordinary skill in the art will know how to modify drilling mud to create a specific borehole pressure. 
   A similar approach is followed for deploying preferred tool  60  during wireline logging operations. For wireline logging, a preferred tool  60  is usually one of several tools in a package lowered downhole. Thus, preferred tool  60  may transmit pressure data via the wireline cable to the surface. A continuous log requires that preferred tool  60  be dragged along borehole wall  28 . While it is believed that tool  60  will remove mudcake nearly instantaneously, a similarly instantaneous pressure reading may not be possible. A lag time may be involved with wireline logging. Lag time calculations are discussed in the &#39;076 patent referenced above and incorporated by reference in its entirety. Thus, pressure reader  64  provides pressure data that allows an accurate reading of formation fluid pressure even though the fluid pressure in chamber  84  and formation  14  have not equalized. 
   While preferred embodiments of this invention have been shown and described, modifications thereof can be made by one skilled in the art without departing from the scope or teaching of this invention. The embodiments described herein are exemplary only and are not limiting. Accordingly, the scope of protection is not limited to the embodiments described herein, but is only limited by the claims that follow, the scope of which shall include all equivalents of the subject matter of the claims. In the claims, the recitation of steps in a sequential order is not intended to require that the steps be performed in that order, unless explicitly so stated.

Summary:
The pressure reading tool includes a housing with an interior chamber and an orifice extending from the chamber to the exterior of the housing. A pulse member with a magnetostrictive ring and an excitation source are disposed within the chamber to produce a highly agitated fluid discharge through the orifice. The magnetostrictive ring, chamber volume, and orifice cooperate to induce Helmholtz resonance frequencies in the fluid in the chamber to thereby enhance the agitation of the fluid discharge. A sheathing encapsulates the pulse member to protect it from contact with the fluid. A dampening element is also interposed between the pulse member and housing to isolate vibration.