STANDARD HANDBOOK OF PETROLEUM & NATURAL GAS ...

942 Drilling and Well Completions
Figure 4-253. Principle of the electromagnetic MWD transmission. (Courtesy
Geoservices [log].)
t
A
h
0
*
I I *
Frequency
I I I
0
5 10 15
5 10 15
(a)
(b)
Frequency
Figure 4-254. Attenuation of electromagnetic signals for 1 and 0 a m
average earth resistivity: (a) attenuation as a function of frequency; (b)
maximum depth reached versus frequency. (Courtesy Geoservices [log].)
With the downhole power available and the signal detection threshold at
surface, Figure 4-254b gives the maximum depth that can be reached by the
technique as a function of frequency. Assuming that phase-shift keying is used
with two cycles per bit, in a 10 m area (such as the Rocky mountains) a
depth of 2 km (6,000 ft) could be reached while transmitting 7 bits/s.
Coding and Decoding. Ten-bit binary codes are used to transmit the information
in most techniques. In one technique, the maximum reading to be
transmitted is divided ten times. In a word, each bit has the value corresponding
to its rank.