STANDARD HANDBOOK OF PETROLEUM & NATURAL GAS ...

MWD and LWD 937
The early system was transmitting 1.5 bits/s (4 sine waves to identify one bit).
Later systems went to 3 bits/s. Now with a 24-Hz carrier frequency, 6 bits/s
can be transmitted.
Then, with data compression techniques (sending only changes for most of
the words in the frame and rotating the data), an effective transmission rate of
10 bits/s can be achieved.
The continuous wave technique has a definite advantage over the other
techniques: a very narrow band of frequencies is needed to transmit the
information. The pulse techniques, on the contrary, use a large band of frequencies,
and the various noises, pump noises in particular, are more difficult
to eliminate.
In principle, several channels of information could be transmitted simultaneously
with the continuous wave technique. In particular, a downward channel
to control the tool modes and an upward channel to bring up the information.
Fluidic Pulser System. A new type of pulser is being developed at Louisiana
State University. It is based on a patent by A. B. Holmes [lo?’]. The throttling
of the mud is obtained by creating a turbulent flow in a chamber as shown in
Figure 4-249.
A vortex is generated by momentarily introducing a dissymetry in the chamber.
The resulting change in pressure loss can be switched on and off very rapidly.
The switching time is approximately 1 ms and the amplitude of the pressure
loss change can be as high as 145 psi (10 bars). The prototype tool can operate
up to 20 Hz. Using a continuous wave with two cycles per bit could lead to a
rate of 10 bits/s. With a data compression technique, 15 effective bits per second
could be transmitted, corresponding to 1.5 data per second.
Voltage signal pulses
from instruments
Figure 4-249. Fluidic mud pulser principlet. (Courtesy Louisiana State University
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