Harmonic Analysis: Six-Segment Downhole Acoustic Waveguide - Measurement While Drilling Signal Analysis, Optimization and Design - page 50

Negative pulsers can be idealized as “holes” in drill collars that slowly
open and shut. When the orifice opens, drilling fluid in the collar is diverted
into the annulus and pressure decreases - this pressure returns to ambient
conditions upon closure. For this reason, the signal generator is known as a
“negative pulser.” Negative versus ambient pressures are used to communicate
“0's” and “1's” to the surface. Such pulsers, in addition to requiring high levels
of mechanical power to operate, erode valve components and may damage the
formation and lead to well control problems. For this reason, they present
liabilities in deepwater applications and are not recommended.
Sirens are high-data-rate devices. While both positive and negative pulsers
are lower in data rate, typically offering 1 bit/sec or less, sirens potentially offer
10 bits/sec or more. The mud siren is known as a “continuous wave” pulser
with a rotor component that rotates relative to an immobile stator. It resembles a
single-stage turbine except that the turbine's thin blades are replaced by thick
block-like “lobes” which almost completely block the flow when in a closed
position. In industry publications, a positive over-pressure is shown traveling
uphole much as it would with positive pulsers - however, the ability of the siren
to increase and decrease its rotary speed allows it to send data at much higher
speeds than with positive and negative pulsers. The siren makes use of acoustic
transmissions, whereas slower positive and negative pulsers often do not.
2.2.2 Acoustics at higher data rates .
Now we re-examine positive pulser operation at higher axial reciprocation
speeds. As the valve rapidly closes, it creates an over-pressure (by “banging”
into the mud) that travels upstream as before; but at the same time, it creates an
under-pressure (“pulling away” from the flowing mud) that travels downstream.
When the valve opens, an under-pressure travels uphole while an over-pressure
travels downhole. In other words, the pulser creates both positive and negative
signals that travel away from the valve at both sides, noting that, at either side of
the valve, signals may be positive, negative or zero. While this description is
correct at high reciprocation speeds, we will continue, for historical purposes, to
refer to high speed poppet valve pulsers as positive pulsers.
We might note that, because the dynamic pressure field (relative to
hydrostatic conditions) corresponding to a positive pulser is antisymmetric with
respect to the source position, a nonzero pressure difference (or “delta-p,”
usually denoted by “'p”) generally exists across the valve. We will refer to
such antisymmetric pressure fields as “dipole” fields produced by dipole tools.
If the valve were positioned in an infinite uniform pipe, “½ 'p” will propagate
in one direction while the other “½ 'p” will travel in the opposite direction with
an opposite sign by virtual of symmetry. If “transmission efficiency,” a new
term introduced here, were defined as the pressure transmitted in a single
direction relative to the source 'p, the transmission efficiency for an infinite