Automated configuration of telemetry transmission
A method for configuring telemetry transmission includes receiving a transmitted telemetry signal at the earth's surface. The received signal is selected and processed to compute a measured power spectral density (PSD). A theoretical PSD is computed of the transmitted signal at the known carrier frequency and processed in combination with the measured PSD to compute an attenuation factor of the transmitted signal at a plurality of frequencies.
1. A method for configuring telemetry transmission of a downhole telemetry tool deployed in a subterranean wellbore, the method comprising:
(a) receiving an electromagnetic telemetry signal at a surface of the Earth during a process of drilling the subterranean wellbore, the electromagnetic telemetry signal transmitted at a known carrier frequency, ranging from 0.5 to 40 Hz, from within the subterranean wellbore using the downhole telemetry tool, the received electromagnetic telemetry signal including a telemetry signal and noise;
(b) selecting a portion of the received electromagnetic telemetry signal;
(c) computing a measured power spectral density (PSD) of the portion selected in (b);
(d) computing a theoretical power spectral density (PSD) of the transmitted electromagnetic telemetry signal at the known carrier frequency; and
(e) processing the measured PSD computed in (c) in combination with the theoretical PSD computed in (d) to compute an attenuation factor of the transmitted electromagnetic telemetry signal at a plurality of frequencies.
2. The method of claim 1 , further comprising:
(f) changing a transmission configuration of the downhole telemetry tool in response to the attenuation factor computed in (e).
3. The method of claim 2 , wherein (f) comprises changing at least one of a carrier frequency, a bit rate, and an amplitude of the transmitted electromagnetic telemetry signal.
4. The method of claim 1 , wherein (a) comprises:
(i) causing the downhole telemetry tool to transmit the electromagnetic telemetry signal at the known carrier frequency; and
(ii) receiving the transmitted telemetry signal at the earth's surface.
5. The method of claim 1 , wherein the portion of the received electromagnetic telemetry signal selected in (b) is transmitting during a static surveying operation.
6. The method of claim 1 , wherein the portion of the received electromagnetic telemetry signal selected in (b) is transmitting during a drilling operation such that the selected portion has a signal to noise ratio greater than a predetermined threshold.
7. The method of claim 1 , wherein the theoretical PSD is computed in (d) based on quadrature phase shift keying (QPSK) modulation for the transmitted electromagnetic telemetry signal at the known carrier frequency and having an identical bit rate to the electromagnetic telemetry signal received in (a).
8. The method of claim 1 , wherein the processing in (e) further comprises:
(i) applying a scaling factor to the measured PSD computed in (c) or the theoretical PSD computed in (d) such that an amplitude of the measured PSD substantially equals an amplitude of the theoretical PSD at the known carrier frequency;
(ii) computing a difference between the measured PSD and the theoretical PSD at each of the plurality of frequencies; and
(iii) processing the difference computed in (ii) to determine the attenuation factor.
9. The method of claim 8 , wherein the difference is computed in (ii) at frequencies corresponding to at least one side lobe peak above and below the known carrier frequency in the measured PSD and the theoretical PSD.
10. The method of claim 1 , wherein the processing in (e) further comprises:
(i) applying a scaling factor to the measured PSD computed in (c) or the theoretical PSD computed in (d) such that an amplitude of the measured PSD substantially equals an amplitude of the theoretical PSD at the known carrier frequency;
(ii) computing a difference between the measured PSD and the theoretical PSD at each of the plurality of frequencies;
(iii) fitting the difference to a mathematical function in a selected frequency range; and
(iv) processing the function to compute the attenuation factor at the plurality of frequencies.
11. The method of claim 10 , wherein the mathematical function is a linear function.
12. The method of claim 10 , wherein the frequency range is automatically selected in (iii) such that a linear mathematical function achieves the fitting within a predetermined threshold.
13. The method of claim 1 , wherein the processing in (e) further comprises:
(i) applying a scaling factor to the measured PSD computed in (c) or the theoretical PSD computed in (d) such that an amplitude of the measured PSD substantially equals an amplitude of the theoretical PSD at the known carrier frequency;
(ii) computing a difference between the measured PSD and the theoretical PSD at each of the plurality of frequencies;
(ii) fitting the difference to a mathematical function in a selected frequency range; and
(iv) adding the function to the theoretical PSD in the selected frequency range to compute a modeled attenuated PSD; and
(v) taking a ratio of the modeled attenuated PSD to the theoretical PSD to compute a plurality of attenuation factors at the plurality of frequencies.
14. The method of claim 13 , wherein (e) further comprises:
(vi) fitting the plurality of attenuation factors to a function;
(vii) processing the function to compute an attenuation factor at a frequency other than the plurality of frequencies.
15. The method of claim 1 , further comprising computing a ratio of power spectral density amplitudes of a first frequency to a second frequency to compute the attenuation factor.
16. The method of claim 1 , further comprising:
changing at least one of a carrier frequency, a bit rate, and an amplitude of the transmitted electromagnetic telemetry signal in response to the attenuation factor computed in (e).
17. A method for configuring an electromagnetic telemetry transmission of a downhole telemetry tool deployed in a subterranean wellbore, the method comprising:
(a) receiving a telemetry signal at a surface of the Earth during a process of drilling the subterranean wellbore, the telemetry signal transmitted at a known carrier frequency from within the subterranean wellbore using the downhole telemetry tool;
(b) computing a measured power spectral density (PSD) of the telemetry signal received in (a);
(c) computing a theoretical power spectral density (PSD) of the transmitted telemetry signal at the known carrier frequency;
(d) applying a scaling factor to the measured PSD computed in (b) or the theoretical PSD computed in (c) such that an amplitude of the measured PSD substantially equals an amplitude of the theoretical PSD at the known carrier frequency;
(e) computing a difference between the measured PSD and the theoretical PSD at each of a plurality of frequencies;
(f) fitting the difference to a mathematical function in a selected frequency range;
(g) adding the mathematical function to the theoretical PSD in the selected frequency range to compute a modeled attenuated PSD; and
(h) taking a ratio of the modeled attenuated PSD to the theoretical PSD to compute attenuation factors at the plurality of frequencies.
18. The method of claim 17 , further comprising:
(i) fitting the attenuation factors to the mathematical function; and
(j) processing the function to compute an attenuation factor at a frequency other than the plurality of frequencies.
19. The method of claim 17 , further comprising:
(i) changing at least one of a carrier frequency, a bit rate, and an amplitude of the transmitted telemetry signal in (a) in response to the attenuation factors computed in (h) (i).