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BACKGROUND OF THE INVENTION 
     The present invention relates to the field of small diameter holes drilled in hard geologic formations such as rock. More particularly, the invention relates to a portable apparatus for efficiently creating holes such as seismic shotholes in rock. 
     Rock holes are drilled for excavation blasting, mining operations, and many other purposes. For example, explorative searches for hydrocarbons, minerals, and other products require the physical penetration of geologic formations. Seismic operations typically detonate explosive charges to generate shock wave source signals for penetrating subsurface geologic formations. The shock waves are reflected from subsurface geologic structures and interfaces and the reflected energy is detected with sensors such as geophones at the surface. Transducers reduce the reflected energy into signals which are recorded for processing. 
     In many land-based geophysical seismic operations, vibrator trucks contact the soil and discharge energy into subsurface geologic formations. However, survey regions frequently comprise mountainous, tropical, or other regions inaccessible to seismic trucks. Because of accessibility constraints and the large source energy provided by explosive materials, explosive charges detonated in shot-holes provide a preferred source of seismic source energy. Shot holes up to four wide and between ten and thirty meters deep are drilled in surface geologic formations. Explosive charges are placed in the bottom of the shot-hole and are detonated to generate shock waves transmitted into the subsurface geologic formations. 
     Seismic shot-holes require different parameters than excavation blast holes because the objective of shot-holes is not to displace or fracture rock, but to efficiently transfer elastic shock wave energy downwardly into subsurface geologic formations. Accordingly, shot-hole equipment and drilling techniques are relatively specialized. As representative examples, U.S. Pat. No. 3,939,771 to McReynolds (1976) disclosed a seismic explosive charge loader and anchor. U.S. Pat. No. 4,278,025 to McReynolds (1981) disclosed a seismic explosive charge loader having a spring anchor for retaining the charge in the borehole. U.S. Pat. No. 4,546,703 to Thompson (1985) disclosed a device for placing an explosive charge into a borehole. U.S. Pat. No. 4,660,634 to Johnson, Jr. (1987) disclosed an automatic drill pipe breakout especially suited for geophysical seismic drilling. U.S. Pat. No. 5,281,775 to Gremillion (1994) disclosed a vibration hole forming device for shot-hole drilling from a lightweight drill. 
     The diameter of conventional explosive charges is smaller than the shot-hole diameter to facilitate placement of the explosives into the lower shot-hole end. The resulting annulus between the explosive charge and the shot-hole wall does not efficiently couple the shock wave energy to the subsurface geologic formations. Moreover, a large portion of the shock wave energy is discharged upwardly through the shot-hole because of the relatively low resistance provided by the open hole. To limit this energy loss, plugs are placed in the shot-hole as shown in U.S. Pat. No. 4,066,125 to Bassani (1978). U.S. Pat. No. 4,736,796 to Arnall et al. (1988) disclosed other techniques for sealing shot-holes with cement, gravel, and bentonite. 
     Hard rock drills use compressed air to drive a hammer in mining and tunneling operations. A rotary percussion hammer drives a narrow, hexagonal shaped bit into the rock to pulverize the rock and to create the rock hole. Such drills are not useful at distances from the rock surface because such drills jam within the rock hole and become stuck. This sticking is caused by variations in the hole annular area due to the hexogonal bit shape, by the tendency for rock particles to lodge against the exterior bit edges, and by insufficient airflow velocity through the hexagonal bit. The failure to remove rock particles from the hole generated increases the probability of bit sticking within the hole and the loss in efficiency caused by such factors. 
     Regional seismic operations require multiple shothole locations for a seismic survey, and large surveys can require thousands of shotholes. The average cost for each shothole multiplied by the number of shotholes significantly determines the economic efficiency of the survey and the data sets obtainable from a survey design. A need exists for improved techniques for efficiently creating holes such as seismic shotholes in areas inaccessible by heavy equipment. 
     SUMMARY OF THE INVENTION 
     The present invention provides an apparatus engagable with a portable rock drill and compressed air supply for forming a hole in rock. The apparatus comprises a rock bit having a selected radial dimension for breaking the rock into rock cuttings to form the rock hole, a connector attached to the bit and to the rock drill wherein the connector includes a substantially tubular shaft having an exterior radial dimension less than the selected radial dimension of the rock bit, and an aperture through the connector for receiving compressed air from the rock drill and for conveying the compressed air to the rock bit for transporting rock cuttings from the rock hole. 
     In different embodiments of the invention, the connector can include a tool adapter having a port for receiving a compressed air supply, a drill pipe body attached to the tool adaptor, or a tool crossover for attaching a drill pipe body to a tool adaptor, or a drill pipe end for attachment to the rock bit. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates a conventional hexagonal rock bit and rotary percussion drill. 
     FIG. 2 illustrates a connector attached to a drill body and drill bit. 
     FIG. 3 illustrates a tool adapter. 
     FIG. 4 illustrates a sectional view showing the relative diameter of tool adapter and the aperture. 
     FIG. 5 illustrates bit crossover attachable to a tool adapter and a drill pipe body. 
     FIG. 6 illustrates a drill pipe body attached to a bit crossover and drill pipe end. 
     FIG. 7 illustrates a crossectional view of a drill pipe body. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The invention provides a unique portable system for forming holes in hard geologic formations such as rock. As used herein, the term “rock” means any geologic formations having tough or hard particles difficult to penetrate with a drill, and includes aggregates, agglomerates, hard rock, clays, gravel deposits, and similar formations. 
     FIG. 1 illustrates a conventional rotary percussion drill such as rock drill  10  having drill body  12 , handle  14 , air hose swivel housing  16 , bit chuck  18 , and drill bit  20 . Compressed air enters air hose  16  to rotate or reciprocate bit  20 , and is partially routed through hose  22  to enter aperture  24  through bit  20 . Such air travels through aperture  24  and is discharged through port  26  to clean bit  20  and to transport rock cuttings from the hole formed in the rock by bit  20 . As previously discussed, conventional drill bits such as bit  20  are hexagonal and have a relatively small aperture  24  therethrough for discharging compressed air through port  26 . 
     FIG. 2 illustrates one embodiment of the invention wherein connector  30  is attached to drill body  12 -and to drill bit  20 . As illustrated in FIG. 2, connector  30  includes tool adapter  32 , bit crossover  34 , drill pipe body  36 , and drill pipe end  38  connected with threaded connections or threadforms  40 ,  42 ,  44 ,  46  and  48 . Tool adapter  32  is connected to drill body  12  with threadform  40 , bit crossover  34  is connected to tool adapter  32  with threadform  42 , drill pipe body  36  is connected to bit crossover  34  with threadform  44 , drill pipe end  38  is connected to drill pipe body  36  with threadform  46 , and bit  20  is connected with a threadform  48  to drill pipe end  38 . 
     FIG. 3 illustrates one embodiment of tool adapter  32  engagable with drill body  12 . Tool adapter  32  includes swivel connection  50  for connection with air hose  16  and aperture  52  for transporting compressed air therethrough. FIG. 4 illustrates a sectional view wherein the diameter of tool adapter  32  is shown and the size of aperture  52  is illustrated. FIG. 5 illustrates bit crossover  34  attachable to tool adapter  32  with threadform  42  and attachable to drill pipe body  36  with threadform  44 . Aperture  54  through bit crossover  34  is aligned with aperture  52  for transporting compressed air therethrough. 
     FIG. 6 illustrates drill pipe body  36  having threadform  44  for engagement with bit crossover  34  and having threadform  46  for engagement with drill pipe end  38 . A crossectional view of drill pipe body  36  is illustrated in FIG. 7, wherein the size of aperture  56  through drill pipe body  36  and the structure of exterior surface  58  is) shown. Drill pipe body  36  preferably comprises substantially the entire length of connector  30  and provides several important functions. Drill pipe body  36  must be sufficiently strong to transmit significant impact forces from drill body  12  to bit  20 . Additionally, drill pipe body  36  is preferably cylindrical to eliminate edges susceptible to entrapment of rock cuttings. By providing a smooth profile on the exterior surface  58 , the likelihood of rock cuttings binding between exterior surface  58  and the interior surface of the hole drilled in the rock is reduced because there are no edges or discontinuities to interrupt the fluid flow. This configuration facilitates a relatively smooth laminar flow of compressed air around exterior surface  58 , which increases the probability of laminar flow for the rock cuttings entrained within such compressed air. 
     By providing a cylindrical aperture  56  through drill pipe body  36 , the relative size of aperture  56  can be maximized relative to the radial diameter of exterior surface  58 . This configuration uniquely provides an efficient relationship which maximizes the amount of compressed airflow possible through connector  30 . By providing optimal compressed air flow, rock cuttings are efficiently removed from the rock hole and the pos sibility of binding between the rock hole side wall and bit  20  is significantly reduced. 
     Drill pipe end  38  is attached to drill pipe body  36  with threadform  46  and to bit  20  with threadform  48 , and aperture  60  extends the compressed air path to bit  20  and port  26 . Although drill pipe end  38  is illustrated as having two male threadform ends, such connections can be male, female, snap-locked, or engaged as other mechanical connector types. The configuration of the invention permits alternative materials such as aluminum to be used in drill pipe body  36 , thereby facilitating manufacture and increasing the relative diameter of aperture  56  relative to the diameter of exterior surface  58 . 
     The invention significantly improves the performance of rock hole formation by portable drills manually operable by a single person. The invention is more efficient than hexagonal drills conventionally used in hard rock drilling. Drill bit  20  is attached to a cylindrical drill pipe end  38  and drill pipe body  36  which is slightly smaller in radial dimension than the gauge of bit  20 . This configuration provides a relatively small annulus between exterior surface  58  and the rock wall of the hole, and maximizes the internal size of aperture  56  and the quantity of compressed air transportable therethrough at a given pressure. By making such annulus smaller and more uniform in dimension, and by increasing the volume of compressed air transported to the bottom of the rock hole formed by bit  20 , the annular velocity of rock cuttings is increased. This increased rock cutting velocity reduces the possibility that rock cuttings will be drawn by gravity to the bottom of the rock hole, when such cuttings would cause “binding” or “sticking” between exterior surface  58  and the rock wall. By reducing the possibility of rock cutting build-up around exterior surface  58 , overall drilling efficiency is increased. 
     Although the invention has been described in terms of certain preferred embodiments, it will become apparent to those of ordinary skill in the art that modifications and improvements can be made to the inventive concepts herein without departing from the scope of the invention. The embodiments shown herein are merely illustrative of the inventive concepts and should not be interpreted as limiting the scope of the invention.

Summary:
A portable apparatus for creating holes in rock. A rock drill connected to a compressed air supply is engaged with a connector attached between the rock drill and a rock bit. The connector has a substantially tubular shaft for transporting compressed air from the air supply to the rock bit. The tubular shaft uniquely maximizes the air available for removing rock cuttings and minimizes the possibility of rock bit sticking in the rock hole. The invention is particularly suited to seismic shothole drilling requiring portable equipment operable by a single person.