Shale shakers Suction line

Negative Mud Pulse Telemetry
In all systems fluid must be circulating through the drillstring. In the negative mud
pulse system a valve inside the MWD tool opens and allows a small volume of
mud to escape from the drill string into the annulus. The opening and closing of
this valve creates a small drop in standpipe pressure (50 - 100 psi), which can be
detected by a transducer on surface.
(ii) Positive Mud Pulse
In the positive mud pulse system a valve inside the MWD tool partially closes,
creating a temporary increase in standpipe pressure.
(iii) Frequency Modulation (Mud siren)
In the frequency modulation system a standing wave is set up in the mud column by
a rotating slotted disc. The phase of this continuous wave can be reversed. The data
is transmitted as a series of phase shifts.
Many tools also include the ability to record downhole data for later retrieval at
surface. Although this undermines the principle of access to ‘real time’ data it
allows the operator to gather large volumes of data (typical petrophysical data) and
therefore eliminate expensive electric wireline logging operations.