Acoustic data link and formation property sensor for downhole MWD system

A method for transmitting signals in a well that contains a drill string, comprising the steps of:

• (a) providing separate acoustic means for transmitting separate acoustic signals through the drill string and the mud in the annulus;
• (b) providing separate acoustic means for receiving said separate acoustic signals transmitted through the drill string and the mud in the annulus;
• (c) transmitting said separate acoustic signals through the drill string and the mud in the annulus;
• (d) receiving said separate acoustic signals from the drill string and the mud in the annulus;
• (e) correlating said separately received signals according to the times required for each signal to complete its transmission path;
• and
• (f) interpreting said correlated signals.

A method for transmitting acoustic data signals a short distance in a well containing an acoustic noise generator, said method comprising the steps of:

• (a) transmitting frequency shift keyed shear waves through said acoustic noise generator;
• (b) transmitting frequency shift keyed compression waves through mud in the well annulus;
• (c) receiving and processing said shear waves to generate a first received signal;
• (d) receiving and processing compression waves to generate a second received signal;
• and
• (e) correlating said first and second received signals and generating a received data stream.

A method for collecting information relating to a formation from a device housed in a bottomhole assembly, comprising the steps of:

• (a) transmitting acoustic signals into the formation;
• (b) receiving reflected and refracted acoustic signals;
• (c) generating an electrical signal representative of the received signals;
• (d) correlating the received reflected and refracted acoustic signals with a reference signal;
• (e) determining the time lag for the received acoustic signals;
• (f) identifying formation anomalies based upon the time lag determination.

A method for collecting information relating to a formation from a device housed in a bottomhole assembly, comprising the steps of:

• (a) transmitting acoustic signals into the formation;
• (b) receiving reflected and refracted acoustic signals;
• (c) generating an electrical signal representative of the received signals;
• (d) correlating the received acoustic signals with a reference signal;
• (e) determining the time lag for the received acoustic signals;
• (f) identifying calculating a formation speed of sound based upon the time lag determination.

A method for transmitting data around a downhole assembly, comprising the steps of:

• (a) transmitting data having a carrier frequency and a modulation frequency from a point on a first end of the downhole assembly;
• (b) receiving said data at a point on an opposite end of the downhole assembly;
• (c) correlating said received data with first and second reference signals having different frequencies to generate a modulated received signal, one of said reference signals having a frequency corresponding to said carrier frequency;
• (d) correlating the modulated received signal with a third reference signal to generate a plurality of bits of information.

A method for determining formation acoustic attenuation, comprising the steps of:

• (a) transmitting a signal A1 having a first carrier frequency f.sub.1 ;
• (b) transmitting a signal A2 having a second carrier frequency f.sub.2 said second carrier frequency being different from said first carrier frequency;
• (c) receiving said signals A1 and A2 at first and second spaced-apart receivers;
• (d) calculating the formation attenuation from the signal decay per unit distance of propagation between the receivers;
• (e) correlating said received signals A1 and A2 with first and second reference signals B1 and B2 having frequencies corresponding to said first and second frequencies respectively f.sub.1 and f.sub.2, so as to generate first and second sets of correlated data D.sub.1 and D.sub.2 ;
• (f) taking the ratio R1 of D.sub.1 and D.sub.2 ;
• (g) comparing R1 with threshold levels to determine which frequency is being received or whether noise prevented frequency detection;
• and
• (h) using the information from steps (d) and (g) to determine the frequency dependence of the formation attenuation if a frequency is received or querying the transmitter to repeat transmissions if only noise is detected.

A method for obtaining information about a formation, comprising the steps of:

• (a) transmitting a signal A1 having a first carrier frequency f.sub.1 ;
• (b) transmitting a signal A2 having a second carrier frequency f.sub.2 said second carrier frequency being different from said first carrier frequency;
• (c) receiving said signals A1 and A2 at first and second spaced-apart receivers;
• (d) calculating the formation speed of sound from the arrival times of the signal at the first and second receivers;
• (e) correlating said received signals A1 and A2 with first and second reference signals B1 and B2 having frequencies corresponding to said first and second frequencies respectively f.sub.1 and f.sub.2, so as to generate first and second sets of correlated data D.sub.1 and D.sub.2 ;
• (f) taking the ratio R1 of D.sub.1 and D.sub.2 ;
• (g) comparing R1 with threshold levels to determine which frequency is being received or whether noise prevented frequency detection;
• and
• (h) using the information from steps (d) and (g) to determine the frequency dependence of the formation speed of sound if a frequency is received or querying the transmitter to repeat transmissions if only noise is detected.

A method for obtaining information from time windowed data, comprising the steps of:

• (a) transmitting a signal A1 having a first carrier frequency f.sub.1 ;
• (b) transmitting a signal A2 having a second carrier frequency f.sub.2 said second carrier frequency being different from said first carrier frequency;
• (c) receiving said signals A1 and A2;
• (d) correlating said received signals A1 and A2 with first and second reference signals B1 and B2 having frequencies corresponding to said first and second frequencies respectively f.sub.1 and f.sub.2, so as to generate first and second sets of correlated data D.sub.1 and D.sub.2 ;
• (f) taking the ratio R1 of D.sub.1 and D.sub.2 ;
• (g) comparing R1 with threshold levels to determine which frequency is being received or whether noise prevented frequency detection;
• and
• (h) using the information from steps (d) and (g) to determine the frequency dependence of a measured property if a frequency is received or querying the transmitter to repeat transmissions if only noise is detected.

A method for obtaining information about cement behind casing, comprising the steps of:

• (a) transmitting from a transducer an acoustic pulse of sufficient bandwidth to include the thickness resonance frequencies of all applicable casing wall thicknesses;
• (b) receiving echoes from the casing with a receiving transducer;
• (c) processing the received signals by correlating with selected narrow band reference frequencies to determine casing wall thickness in the time window immediately after the first reflection from the inner casing wall;
• and
• (e) processing later time windows at the wall thickness resonance frequency to detect signals from reflective boundaries behind casing.