Apparatus and method for downhole measurement

1. An apparatus for downhole measurement during operation of a drilling system, the apparatus comprising:

a first sensor configured to detect a property of a subsurface formation;

a radiation source configured to, when triggered, emit radiation to excite a part of the formation;

a processing unit configured to trigger the radiation source if a change of the property of the formation is detected;

a second sensor positioned at a known distance from the radiation source and operably connected to the processing unit, the second sensor is configured to detect said excited part of the formation and is more distant from a drill bit of the drilling system than the radiation source is;

the processing unit is further configured to, after the excited part of the formation is detected, calculate a rate of penetration of the drill bit based on the following: a time duration between the radiation source is triggered and the excited part of formation is detected, and the distance between the radiation source and the second sensor.

2. The apparatus of claim 1 , wherein the radiation source is more distant from the drill bit than the first sensor is.

3. The apparatus of claim 1 , wherein the processing unit is configured to calculate a cumulative sum (CUSUM) of the property of the formation, and trigger the radiation source if the cumulative sum indicates a change of the property.

4. The apparatus of claim 3 , wherein values of the property detected by the first sensor is smoothed and then used to calculate the cumulative sum.

5. The apparatus of claim 3 , wherein the cumulative sum is calculated based on the following formulas:

for i= 1,CUSUM( i )=property( i )− mu,

for i> 1,CUSUM( i )=CUSUM( i− 1)+(property( i )− mu ),

wherein i is a positive integer and is the serial number of values of the property detected by the first sensor, property (i) is the i-th value of the property detected by the first sensor, mu is an average of all available values of the property that have been detected.

6. The apparauts of claim 5 , wherein the processing unit is further configured to determine that the property of the formation changes at the moment in time when property (i) is detected if:

CUSUM( i+ 2)>CUSUM( i+ 1) and CUSUM( i+ 1)≤CUSUM( i )

or if

CUSUM( i+ 2)≤CUSUM( i+ 1) and CUSUM( i+ 1)>CUSUM( i ).

7. The apparatus of claim 1 , wherein the property of the formation detected by the first sensor includes at least one of the following:

radiation,

density,

sound velocity.

8. A drilling system comprising an apparatus for downhole measurement during operation of a drilling system, the apparatus comprising:

a first sensor configured to detect a property of a subsurface formation;

a radiation source configured to, when triggered, emit radiations to excite a part of the formation;

a processing unit configured to trigger the radiation source if a change of the property of the formation is detected;

a second sensor positioned at a known distance from the radiation source and operably connected to the processing unit, the second sensor is configured to detect said excited part of the formation and is more distant from a drill bit of the drilling system than the radiation source is

the processing unit is further configured to, after the excited part of the formation is detected, calculate a rate of penetration of the drill bit based on the following: a time duration between the radiation source is triggered and the excited part of formation is detected, and the distance between the radiation source and the second sensor.

9. A method for downhole measurement during operation of a drilling system, comprising:

detecting a property of a subsurface formation;

determining if a change of the property of the formation is detected;

if a change of the property of the formation is detected, triggering a radiation source to emit radiation to a part of the formation to excite said part of the formation;

detecting said excited part of formation with a sensor which is positioned at a known distance L from the radiation source and is more distant from a drill bit of the drilling system than the radiation source is;

calculating a rate of penetration of the drill bit, after the excited part of formation is detected, based on the following: a time duration between the radiation source is triggered and the excited part of formation is detected, and the distance between the radiation source and the sensor.

10. The method of claim 9 , wherein the radiation source is more distant from the drill bit than the first sensor is.

11. The method of claim 9 , wherein determining if a change of the property of the formation is detected comprising:

calculating a cumulative sum (CUMSUM) of the property of the formation, and

determining if the cumulative sum indicates a change of the property of the formation.

12. The method of claim 11 , wherein values of the property detected by the first sensor is smoothed and then used to calculate the cumulative sum.

13. The method of claim 11 , wherein the cumulative sum is calculated based on the follow formulas:

for i= 1,CUSUM( i )=property( i )− mu,

for i> 1,CUSUM( i )=CUSUM( i− 1)+(property( i )− mu ),

wherein i is a positive integer and is the serial number of values of the property detected by the first sensor, property (i) is the i-th value of the property detected by the first sensor, mu is an average of all available values of the property that have been detected.

14. The method of claim 13 , wherein determining if the cumulative sum indicates a change of the property comprising:

determining that the property of the formation changes at the moment in time when property (i) is detected if:

CUSUM( i+ 2)>CUSUM( i+ 1) and CUSUM( i+ 1)≤CUSUM( i )

or if

CUSUM( i+ 2)≤CUSUM( i+ 1) and CUSUM( i+ 1)>CUSUM( i ).

15. The method of claim 9 , wherein the property of the formation includes at least one of the following:

radiation,

density,

sound velocity.