Reference signal encoding for seismic while drilling measurement

A method for making seismic while drilling (SWD) measurements by determining and analyzing the reference signal downhole near the drill bit and sending a limited quantity of information to the surface by measurement-while-drilling (MWD) telemetry. In one embodiment, a library of anticipated drill bit wavelets is stored in memory downhole and in memory at the surface. The best matching wavelet is identified by the processor downhole and then a code identifying the wavelet and a scaling factor is sent to the surface. At the surface, the best matching wavelet is retrieved based on the code received and then a reconstructed signal is created using the retrieved wavelet and the scaling factor. In another embodiment, key characteristics of the signal such as central frequency, frequency band, etc., are calculated downhole and transmitted to the surface. These key characteristics are then used to reconstruct the reference signal which is then used for correlation of surface detected signals.

BACKGROUND AND SUMMARY OF THE INVENTION 
The present invention relates to an improved method of determining, while 
drilling in the earth with a drill bit, the positions of geologic 
formations in the earth. More particularly, it relates to a method for 
improving the quality of a reference signal. 
Conventional Reflection Seismology 
Conventionally, reflection seismology utilized surface sources and 
receivers to detect reflections from subsurface impedance contrasts. The 
obtained image often suffered in spatial accuracy, resolution and 
coherence due to the long travel paths between source, reflector, and 
receiver. To overcome this difficulty, a technique commonly known as 
Vertical Seismic Profiling (VSP) was developed to image the subsurface in 
the vicinity of a borehole. With VSP, a source is suspended at a discrete 
borehole depth with a wireline and is activated to generate seismic 
signals. Field sensors are positioned at the surface to detect these 
seismic signals. Data is recorded and the process is repeated for several 
borehole depths. With the source positioned downhole, data can be acquired 
simultaneously at many surface locations with little more expense than for 
a single location. To reduce rig downtime, the drill bit was used as the 
source. However, because the drill bit signal is an uncontrolled 
pseudo-random process, a reference signal needs to be detected. Thus, in 
currently available methods of performing VSP, the signal generated by the 
drill bit travels up the drill string to the reference sensor and also 
propagates upward to the field sensors. By correlating the signal detected 
by the reference sensor with the signal detected by the field sensors, the 
travel time of the energy traveling from the drill bit to the field 
sensors may be determined. However, noise which is present in the 
reference signal will degrade the quality of the correlated signal. 
Seismic While Drilling 
One approach used in commercially available Seismic-While-Drilling (SWD) 
systems (e.g., TOMEX) to eliminate noise from the reference signal is 
through the use of an accelerometer 401 as the reference sensor. An 
example of a system utilizing an accelerometer is shown in FIG. 4. The 
signal generated by the drill bit 325 is used as a signal for seismic 
measurements and surface signals are recorded by an array of geophones 320 
distributed around the drilling rig 301. The "reference signal", which is 
used for cross-correlation and following source signal detection, is 
recorded by an accelerometer 401 at the top of the drillstring 310. The 
accelerometer 401 has two sensors, one of which is sensitive to the noise 
but substantially insensitive to the acoustic signal transmitted up the 
drill string 310 from the source. The frequency band within which there is 
high coherence between the energy in the noise signal and the reference 
signal is determined. The noise signal is then amplified by a factor equal 
to the average ratio of the energy amplitude of the pilot signal to the 
noise signal within this frequency band, and this weighted noise signal is 
subtracted from the reference signal to reduce the noise in the reference 
signal. However, in many practical cases, the quality of the signal 
recorded at the top of the drillstring using an accelerometer is still 
very poor and still does not contain adequate information about the bit 
signature, especially for deviated extended reach wells. Often, even 
utilizing long term stacking (a method where, because the drill bit signal 
is not clear, the signal is averaged over a period of several minutes to 
extract a typical drill bit signature) does not improve the 
signal-to-noise ratio. Therefore, the results of SWD become 
unsatisfactory. Thus, there is a need to have a method for more accurately 
determining the reference signal. 
Measurement-While-Drilling Telemetry 
Measurement-While-Drilling (MWD) telemetry is a well known method for 
transmitting information between the bottom of the borehole and the 
surface. In MWD telemetry, information is transmitted by creating pressure 
pulses in the drilling fluid. This is performed by interrupting the flow 
of drilling fluid inside the pipe. At first glance, it might seem that the 
obvious solution to the problems of obtaining an improved reference signal 
lay in simply detecting the drill bit signal downhole near the drill bit 
and then transmitting this signal to the surface via MWD telemetry. 
However, MWD telemetry is limited to only a few bits per second. This is 
far below the rate of transmission needed to transmit to the surface the 
vast quantity of information needed in any representation of the waveform. 
Therefore, other approaches must be taken. 
Improvement of Quality and Reliability of Seismic While Drilling (SWD) 
Measurements Due to Improvement of Quality of the Reference Signal 
The present application discloses a method for making seismic while 
drilling (SWD) measurements by determining and analyzing the reference 
signal downhole near the drill bit and sending information about the 
signal to the surface using a limited number of transmission bits. In one 
embodiment, a library of anticipated drill bit wavelets is stored in 
memory downhole and in memory at the surface. This library of anticipated 
drill bit wavelets is based on long term experience (several years) in 
collecting drill bit signals downhole and, in fact, could also be 
considered a data base of these collected drill bit signals. The best 
matching wavelet is identified by the processor downhole and then a code 
identifying the wavelet and a scaling factor are sent to the surface. At 
the surface, the best matching wavelet is retrieved based on the code 
received and then a reconstructed signal is created using the retrieved 
wavelet and the scaling factor. In another embodiment, key characteristics 
of the signal such as central frequency, frequency band, etc., are 
calculated downhole and transmitted to the surface. These key 
characteristics are then used to reconstruct the reference signal which is 
then used for correlation of surface detected signals. 
The disclosed innovations, in various embodiments, provide one or more of 
at least the following advantages: 
allows reference signal information to be sent to the surface using a 
limited number of bits since measurement-while-drilling (MWD) telemetry 
from downhole to the surface is limited to a few bits per second, 
provides a better reference signal for cross-correlation purposes than is 
provided by present methods, and 
provides a real time determination of the reference signal much faster (in 
around 1 minute) than the current method of utilizing long-term stacking 
which may take as long as 20 minutes.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The numerous innovative teachings of the present application will be 
described with particular reference to the presently preferred embodiment 
(by way of example, and not of limitation). 
Definitions: 
Following are short definitions of the usual meanings of some of the 
technical terms which are used in the present application. (However, those 
of ordinary skill will recognize whether the context requires a different 
meaning.) Additional definitions can be found in the standard technical 
dictionaries and journals. 
Borehole: a deep narrow circular hole, especially one made in the earth to 
find water, oil, etc. 
Formation (or rock formation): earth strata of a particular type of rock or 
a combination of different rocks surrounding a borehole. 
Measurement-While-Drilling: a procedure whereby measurements of downhole 
and surface parameters such as temperature, weight on bit, surface RPM, 
received energy, torque, etc. are taken as a drill is being operated in a 
bore hole. 
Measurement-While-Drilling Telemetry: a method of transmitting information 
between the bottom of a borehole and the surface. Measurements are taken 
downhole and these measurements are encoded into 1's and 0's and 
transmitted to the surface as a signal. At the surface, these signals are 
decoded to retrieve information. One method for transmitting this 
information is by creating pressure pulses in the drilling fluid 
corresponding to the 1's and 0's. These pulses propagate through the 
borehole and are received by sensors at the surface. Other methods of 
transmission such as using electromagnetic or acoustic waves are also 
possible. Furthermore, the process may be reversed and information 
generated at the surface may be transmitted downhole as well. 
Anticipated Bit Wavelet Library Method 
In FIG. 1 there is shown a flow chart of the presently preferred embodiment 
for obtaining a reference signal for use in Seismic While Drilling (SWD) 
measurements. A library of anticipated drill bit wavelets and a code 
corresponding to each wavelet is constructed (step 100). In one example, 
this library of anticipated drill bit wavelets is based on long term 
experience (several years) of collecting drill bit signals downhole and 
could be considered a data base of these collected drill bit signals. In 
this example, the library could contain 256 different wavelets. Of course 
the library could contain more or less wavelets depending on the 
requirements of the system user. This library is then stored in memory 
both down the borehole and at the surface (step 105). During drilling, the 
drill bit generates seismic vibrations (step 110). These seismic 
vibrations may be used to gain information about geologic formations in 
the vicinity of the drill bit while drilling. 
Because the drill bit signal is an uncontrolled pseudo-random process, the 
signal generated by it needs to be determined to provide a reference 
signal to cross-correlate the collected data with so that the collected 
data can be effectively represented as if the source spectrum were 
controlled. Therefore, a reference signal (the signal generated by the 
drill bit) is detected down hole near the drill bit (step 120). The 
reference signal needs to be detected down hole rather than at the surface 
because the signal degrades as it travels up the drill string to the 
surface. By the time the reference signal reaches the surface, it does not 
contain adequate information about the drill bit signature (especially for 
deviated extended reach wells). This reference signal is then analyzed 
downhole (step 125). A best matching wavelet from the library along with a 
scaling factor are determined downhole (step 130). The scaling factor 
defines compression or stretching of the wavelet in time. A code 
corresponding to this best matching wavelet along with the scaling factor 
are then transmitted to the surface, typically using 
Measurement-While-Drilling (MWD) telemetry (step 140). By reducing the 
information transmitted to a code and a scaling factor, only two 8-bit 
words need to be sent to the surface. This is particularly important since 
MWD telemetry from downhole to the earth's surface is limited to a few 
bits per second. After the code and scaling factor are received at the 
surface, a processor reconstructs the reference signal based on the 
wavelet which corresponds to the code received and on the scaling factor 
(step 145). 
The signals generated by the drill bit propagate, not only to the nearby 
downhole detector, but also away from the well bore and through the earth. 
With multiple reflecting layers (boundaries between different geologic 
formations), the signals generated by the drill bit propagate to the 
surface via various paths, i.e., by various direct and reflecting paths. 
These direct and reflected signals are then received by various detectors 
placed at known locations on the earth's surface (step 150). The sounds 
received by the surface detectors contain not only the signals associated 
with drilling, but also components which may be unrelated to drilling. The 
latter may include cultural noise such as from vehicles, people, animals, 
weather (wind and rain), etc. Therefore, the direct and reflected signals 
need to be corrected to account for these extraneous sources of sound. 
Thus, the direct and reflected signals are cross-correlated with the 
reconstructed reference signal to distinguish the drill bit generated 
signals from interference (step 155). These corrected direct and reflected 
signals can then be used to determine information about the subterranean 
formations around and near the borehole (step 160). Such information 
includes information that bears on the likelihood that hydrocarbons will 
be present in that location; information which is of particular importance 
in the petroleum exploration industry. Examples of methods for extracting 
such information from these signals may be found in U.S. Pat. No. 
5,050,130 issued to Rector et al., U.S. Pat. No. 4,849,945 issued to 
Widrow, and U.S. Pat. No. 5,191,557 issued to Rector et al. all of which 
are hereby incorporated by reference. Those skilled in the art will be 
familiar with such methods. 
Drill Rig 
In FIG. 3 is shown a drill rig 301 and equipment suitable for performing 
the presently disclosed methods for obtaining a reference signal. Rig 301 
supports a string of sectionalized drill pipe 310 in the well or borehole 
314 which is usually filled with a fluid 316 such as drilling mud. A drill 
bit 325 is secured to the lower end of drill string 310. Also secured to 
the lower end of drill string 310 and just above the drill bit 325 are 
downhole electronics 328. The downhole electronics 328 include a 
processor, a receiver, and a transmitter and may include a recording 
device as well. The downhole electronics 328 need to be near the drill bit 
325. In a typical example, the downhole electronics 328 would be between 
10 and 20 feet from the drill bit 325. However, the downhole electronics 
328 could be nearer or further from the drill bit 325, but, obviously, the 
nearer the downhole electronics 328 are to the drill bit 325, the better 
the quality of the reference signal obtained. The downhole electronics 328 
can be configured to detect any acoustic mode desired. As an example, in 
the TOMEX system, both acoustic shear and compressional waves are 
detected. 
In operation, the drill string 310 is rotated at a rate that depends upon 
the type of formation 322 being drilled. As the drill string 310 turns, 
the respective teeth of the drill bit 325 impact the bottom-hole formation 
to crush or disintegrate the rock at the bottom of the hole 314 to 
penetrate the formation 322 and also to produce a pseudorandom sequence of 
pulses. Some of the seismic energy that is generated by the drill bit 
radiates directly into the surrounding formation as compressional and 
shear wavefields. Some of the energy is detected by receivers located in 
the electronics section 328 downhole as a reference signal. A down hole 
processor in the electronics section 328 processes the signal to determine 
information about the reference signal such as key characteristics of the 
reference signal or the identity of the best matching wavelet from a 
library of anticipated wavelets stored in downhole memory. The down hole 
processor then transmits this information about the reference signal to a 
first processor 360 located at the surface 334. One method for 
transmitting this information to the surface processor 360 is by 
measurement-while-drilling (MWD) telemetry. 
Receiver arrays 320 are located near the surface of the earth 334 in a 
substantially horizontal plane from the wellhead 315. Although shown in 
FIG. 3 with only one receiver array, multiple receiver arrays can and 
would normally be used in practice. An example of receivers 320 
appropriate for this purpose are geophones. The receivers 320 detect 
seismic wavefields propagating through the subsurface earth formation 322. 
Some of these seismic wavefields travel along a direct path 350 from the 
drill bit to the receivers 320. Other seismic wavefields are detected by 
the receivers 320 which have traveled along a downward path 351 to a 
reflector 340 and then are reflected upward and travel along a reflected 
path 352 to the receivers 320. Optionally, if real time processing is not 
to be performed, the direct and reflected signals may be recorded by a 
recording device 368 which samples and records the signals for later 
processing. 
The first processor 360 reconstructs the reference signal from key 
characteristics or codes transmitted to the surface from the downhole 
electronics 328. The reconstructed reference signal is sent to a cross 
correlator 362. The direct and reflected signals are also transmitted to 
the cross correlator 362 either directly from the receivers 320 or, 
optionally, from the recorder 368 depending on whether analysis will be 
performed in real time or at a later time. The cross correlator 362 then 
determines which part of the signal detected by the receivers 320 was 
generated by the drill bit and which part is extraneous noise and then 
eliminates the extraneous noise from the direct and reflected signals. The 
direct and reflected signals with extraneous noise eliminated is then sent 
to a second processor 364 where information about the subterranean 
formations is extracted and sent to a display 366. 
Alternate Embodiment: Key Characteristics 
In an alternative embodiment, rather than transmitting a code corresponding 
to a wavelet in a library, key characteristics of the reference signal are 
transmitted to the surface. A flow chart for this key characteristic 
method is shown in FIG. 2. Again, a drill bit generates a signal (step 
210). This drill bit signal is detected down hole near the drill bit to 
provide a reference signal (step 220). The reference signal is analyzed 
down hole to obtain key characteristics about the reference signal (step 
230). The key characteristics of the reference signal are transmitted to 
the surface (step 240). Once the key characteristics are received by 
electronics at the surface, the reference signal is reconstructed based on 
these key characteristics (step 250). These key characteristics may 
include such information as the central frequency, the frequency band, 
and/or statistical parameters as well as any other information which may 
be determined to be relevant and necessary to reconstruct the reference 
signal to the appropriate level of accuracy at the surface as determined 
by the system user. 
As previously mentioned in the discussion of the preferred embodiment, the 
signals generated by the drill bit propagate away from the well bore and 
through the earth and reach the earth's surface through various direct and 
reflected paths. These direct and reflected signals are then received by 
various detectors placed at known locations on the earth's surface (step 
260). The direct and reflected signals are cross-correlated with the 
reconstructed reference signal to distinguish the drill bit generated 
signals from interference (step 270). These corrected direct and reflected 
signals can then be used to determine information about the subterranean 
formations around and near the borehole (step 280). 
According to a disclosed class of innovative embodiments, there is 
provided: a data collection method, comprising the steps of: (a.) in a 
downhole processor, analyzing the acoustic signal generated by an 
operating drill bit to derive data therefrom; (b.) in at least one second 
processor, reconstructing a copy of said acoustic signal from said data, 
and using said copy as a reference signal, in combination with acoustic 
information detected at positions separate from said drill bit, to provide 
seismic characterization. 
According to another disclosed class of innovative embodiments, there is 
provided: a seismic-while-drilling method, comprising the steps of: (a.) 
in a downhole processor, analyzing the acoustic signal generated by an 
operating drill bit to derive data therefrom; (b.) transmitting said data 
to at least one processor located at or near the earth's surface; (c.) 
reconstructing a copy of said acoustic signal from said data; and (d.) 
using said copy as a reference signal to interpret acoustic information 
detected at positions separate from said drill bit. 
According to another disclosed class of innovative embodiments, there is 
provided: a method of gathering geophysical information; comprising: 
drilling a well bore with a drill bit which produces seismic energy; in a 
down hole processor, analyzing a reference signal generated by said drill 
bit to determine key characteristics of said reference signal; 
transmitting said key characteristics to the surface; obtaining a 
reconstructed reference signal at the surface based on said key 
characteristics; detecting a seismic signal at the surface; correlating 
said seismic signal with said reconstructed reference signal to obtain 
information about subterranean geologic formations. 
According to another disclosed class of innovative embodiments, there is 
provided: a method of gathering geophysical information; comprising: 
creating a library of anticipated bit wavelets and storing said library in 
downhole memory and in surface memory; generating a reference signal down 
a borehole; analyzing said reference signal downhole to identify a code 
corresponding to a best matching wavelet from said library and a scale 
factor; transmitting said code and said scale factor to the surface; 
obtaining a reconstructed reference signal at the surface from said code 
and said scale factor; using said reconstructed reference signal to 
correlate said detected reference signal to determine geophysical 
information. 
According to another disclosed class of innovative embodiments, there is 
provided: a system for seismic exploration; comprising: a downhole 
processor that analyzes the acoustic signal generated by an operating 
drill bit to derive data therefrom; a second processor operatively 
connected to receive said data; and at least one acoustic detector remote 
from said downhole processor; wherein said second processor is remote from 
said downhole processor and uses said data to interpret acoustic 
information received by at least one of said acoustic detector. 
Modifications and Variations 
As will be recognized by those skilled in the art, the innovative concepts 
described in the present application can be modified and varied over a 
tremendous range of applications, and accordingly the scope of patented 
subject matter is not limited by any of the specific exemplary teachings 
given. 
Although described with reference to a library of wavelets, other libraries 
of signal information could be maintained which could be used to transfer 
reference signal information to the surface using a minimum amount of 
transferred data. 
Furthermore, although a few examples of key characteristics have been 
given, it will be obvious to those skilled in the art that other 
characteristics of a signal may be computed and transmitted to the surface 
which would enable the surface processor to reconstruct the reference 
signal. 
Additionally, although described with reference to direct and reflected 
signals, it should be noted that useful information can be gathered from 
the direct signals alone using this technique. 
Also, although described primarily with reference to onshore seismic 
exploration, the innovative teachings of this application can be applied 
to marine seismic exploration as well. 
Other modifications will be obvious to those skilled in the art. Such 
modifications include but are not limited to adding an intervening 
transmitter between the transmitter at the bottom of the bore hole and the 
surface to boost the signal and adding recording devices down hole or at 
the surface to store information for later processing.