You are an expert at summarizing long articles. Proceed to summarize the following text:

You are an expert at summarizing long articles. Proceed to summarize the following text: 
This application is the US national phase of international application PCT/AU2005/001812 filed 2 Dec. 2005 which designated the U.S. and claims benefit of Australian Application No. 2004906893 filed 2 Dec. 2004, the entire contents of both applications are hereby incorporated by reference. 
   FIELD OF THE INVENTION 
   The present invention relates to core barrel capacity gauge. 
   BACKGROUND OF THE INVENTION 
   When it is required to obtain a cross sectional sample of a particular geological formation, it is known to use a core barrel assembly in place of a standard drill bit. 
   The core barrel assembly utilizes a specialized core bit attached to a number of outer barrels that are interconnected to make up the desired length. The core bit drills downwardly and has a central opening such that the core bit cuts around a column of the formation that is to be the sample. An inner barrel is provided within the outer barrel for receiving the core sample. The inner barrel is provided with an adaptor at the lower end that allows the core to pass into the inner barrel but not to fall back out. 
   The process of obtaining a core sample generally commences by connecting the core barrel assembly to the standard drill pipe string and lowering it to the bottom of the hole. Fluid is pumped through the drill string into the core barrel assembly where it passes through the inner barrel and the cavity between the inner and outer barrels to flush them of debris. A diverter ball is dropped through the drill string before commencement of sampling to seal the opening to the inner barrel so that fluid pumped down the drill string is passed only through the cavity between inner and outer barrels and coring commences. During coring, the core bit is designed to drill around a vertical column of the sample such that the inner barrel passes downwardly around the sample. A known problem that can occur in such a situation is that if the core column is not sufficiently stable, it can collapse downwardly within the inner barrel. The collapsed core column can create additional friction on the inner surface of the inner barrel resulting in jamming of the core. 
   Observations of the drilling fluid pressure, the torque and the rate of penetration can provide some indication of whether this core collapse has occurred, however it is not possible to rule out the possibility that changes in these values are the result of some other event (such as a change in the formation). The driller is therefore forced to make a decision that could result in continuing drilling when the core is jammed or stopping drilling when the core is not jammed, both situations resulting in an expensive loss of time and effort. 
   The present invention attempts to overcome at least in part the aforementioned problem of detecting collapse of a core sample within a core barrel assembly. 
   SUMMARY OF THE INVENTION 
   In accordance with one aspect of the present invention there is provided a core barrel capacity gauge for use on a core barrel assembly having a barrel for receiving a core sample, wherein the core barrel capacity gauge includes a core sample marker located within the barrel such that the core sample marker rests against the top of the drilled core sample and a marker location sensor, the marker location sensor being arranged to detect the location of the core sample marker within the barrel. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will now be described, by way of example, with reference to the accompanying drawings, in which: 
       FIG. 1  is a side cross sectional view of a core barrel assembly of known configuration; 
       FIG. 2  is a side cross sectional view of the core barrel assembly of  FIG. 1  during the process of obtaining a core sample; 
       FIG. 3  is a side cross sectional view of the core barrel assembly of  FIG. 1  during the process of obtaining a core sample where the core sample has collapsed; and 
       FIG. 4  is a side cross sectional view of a core barrel assembly having a core barrel capacity gauge in accordance with the present invention. 
   

   DESCRIPTION OF THE INVENTION 
   Referring to the  FIGS. 1 to 3 , there is shown a core assembly  10  on which the core barrel capacity gauge of the present invention may be used. The core barrel assembly  10  includes a core bit  12  attached to the lower end of one or more outer barrels  14 . The outer barrels  14  are connected to a top adaptor  24  that includes a swivel assembly  18  onto which is attached an inner barrel  16  for receiving the core sample. Stabilisers  20  are provided between adjacent outer barrels  14 . The inner barrel is provided with an adaptor  22  at the lower end that allows the core to pass into the inner barrel but not to fall back out. 
     FIG. 1  shows the core barrel assembly  10  before the commencement of the coring process. Drilling fluid is passed downwardly through the top adaptor  24  and passes via the swivel assembly  18  into the inner barrel  16  and the cavity between the inner barrel  16  and the outer barrel  14 . Before the commencement of the coring process, a diverter ball  26  is dropped down into the swivel assembly to prevent drilling fluid passing into the inner barrel  16 . The core sample  28  is then received within the inner barrel  16  as shown in  FIG. 2  during a normal core sampling operation.  FIG. 3  shows an example of the coring process in which the core sample  28  has collapsed. As can be seen, the collapsed core sample  28  fills the clearance left between the core sample  28  and the inner barrel  16  thereby creating friction. 
   Referring to  FIG. 4  there is shown a core sample capacity gauge  30  provided on a core barrel assembly  10  of the type shown in  FIGS. 1 to 3 . The core barrel capacity gauge  30  comprises a core sample marker  32  and a marker location sensor  34 . The marker location sensor  34  is arranged to detect the location of the core sample marker  32  within the inner barrel  16 . 
   In the embodiment shown, the core sample marker  32  comprises a housing having a magnetic field detection means and a signal generator. The magnetic field detection means comprises suitable electronics to determine the presence of a magnetic field of predetermined strength. The inner barrel  16  is provided with a plurality of position markers  36  at regular intervals along the length, each comprising a magnet  38 . 
   The magnetic field detection means is arranged to detect the magnetic field generated by the magnets  38  as the core sample marker  32  passes the magnets  38 . Upon detection of the magnet field of one of the magnets  38  by the magnetic field detection means, the signal generator produces a signal in the form of a percussion wave which is transmitted up the inner barrel  16  in the drilling fluid. 
   The marker location sensor  34  is provided within the inner barrel  16  adjacent the swivel assembly  18 . The marker location sensor  34  detects the percussion wave generated by the core sample marker  32  and transmits, by a suitable means, a signal to a signal receiver (not shown) at the surface. The signal transmitted to the surface by the marker location sensor  34  may also be in the form of a percussion wave signal transmitted through the drilling fluid. The signal receiver at the surface includes a suitable means to indicate to the driller the location of the core sample marker  32  within the inner barrel  16  based on the signals received from the marker location sensor. 
   As the driller is then able to determine the position of the core sample marker  32  (and therefore the top of the core sample) with respect to the inner barrel  16 , it is possible to determine any collapse of the core sample  28 . That is, if the distance the distance the inner barrel  16  has passed the core sample marker  32  is significantly less than the distance drilled down, then the driller will know that some collapse of the core sample  28  has occurred. 
   The core barrel capacity gauge  30  may also be provided with a pressure sensor (in sensor assembly  31 ) and a temperature sensor (in sensor assembly  31 ) to provide information to the operator regarding the pressure of the drilling fluid and temperature within the core barrel assembly. Further a rotational sensor (in sensor assembly  31 ) may be provided to indicate to the operator whether the inner barrel  16  is rotating with outer barrel  14 . The temperature, pressure and rotational information may be used by the operator to further assess the progress of the coring operation. 
   Modifications and variations as would be apparent to a skilled addressee are deemed to be within the scope of the present invention

Summary:
A core barrel capacity gauge for use on a core barrel assembly having a barrel for receiving a core sample. The core barrel capacity gauge includes a core sample marker located within the barrel such that the core sample marker rests against the top of the drilled core sample and a marker location sensor. The marker location sensor is arranged to detect the location of the core sample marker within the barrel.