WELLOG DAQ SYSTEM APPLICATIONS
© 2006 2008 WELLOG
All Rights Reserved
ELECTRIC LOG (ELOG):
The typical Electric Logging tool, produces three or four signals depending on the service.
Acquisition is performed on eight analog signals as follows:
Four analog signals are applied to the inputs of four programmable gain amplifiers, channels 1 thru 3 (or 4) as listed
Three signals are recorded in addition to the above signals. They are Vout, Iout, signal ground. These voltages are recorded on channels 5 thru 7.
Borehole temperature is logged on channel 0.
The depth display is set to zero at the tool zero point as the tool is positioned to go into the hole. As the logging tool is lowered into the hole en-route to the bottom, programmable gains are adjusted to give full a/d span on all signal inputs. Bottom of the hole depth (TD) is noted, and logging progresses as the tool is raised to the surface. All parameters are logged and recorded with the depth at which the measurement was made.
Surface calculation of resistivities is performed using the logged data.
Single point resistance (SPR) is derived from logged Vout and Iout.
SPR = Vout/Iout
16 inch resistivity = (v16/Iout) * K1
32 inch resistivity = (v32/Iout) * K2
64 inch resistivity = (v64/Iout) * K3
Where K1, K2, and K3 are the respective geometric constants.
Resistivities can be corrected for borehole conditions, formation effects and temperature.
A log is created from the data obtained and the resulting calculations and corrections.
The Electric log transmitter produces a bipolar formation current which switches continuously positive, off, negative, off. The acquisition system is programmed to sample the voltage at the appropriate time for V16, V32, V64, temp, Iout, Vout, SP, and signal ground.
After sampling, the data is transmitted to the surface using a bipolar pulse position method. Data is transmitted in packets at a selected interval. Logging at a rate of 60 feet per minute is considered nominal in a non deviated borehole. At the rate of 60 feet per minute, data packets are sent at a rate determined by the sample interval. Each data packet contains eight parameters using two 8 bit bytes per parameter. Data transmission includes a start bit and stop bit. A data packet therefore contains 10 * 16 = 160 bits per packet. The resulting data transmission rate at 10 packets per second is 1600 bits per second at a logging speed of 60 feet per minute and sample interval of 10 samples per foot.
Primary wireline Telemetry Method (PWM):
Pulse Width Modulation methods vary pulse width according to logic level.
Positive and negative pulses are alternated and the pulse width varies according to the logic level as follows:
Using 8N1 protocol = 1 start bit + 8 data bits + no parity +1 stop bit
DATA: 0 1 0 1 1 1 0 1 1 1 Decoded: |0-|1|0-|1|1|1|0-|1|1|1
Pulse +: |-- |-- |- |-- |- 0 1 0 1 1 1 0 1 1 1
Pulse -: |- |- |- |- |-
Bipolar pulses are 50 microseconds. Bit spacing for a 0 is 200 microseconds. Bit spacing for a 1 is 100 microseconds.
Alternate pulse wireline Telemetry: (10K Bits Per Second)
Bits are defined as follows: Logic 0 = no pulses for 100 micro-seconds. A logic 1 is defined as two 50 microsecond pulses.
A start bit is always a logic 1 and a stop bit is always a 1 so data is framed by four 50 microsecond pulses.
Error checking can be performed using a method called parity checking. The method counts the number of logic 1s in a data byte to
Determine if an error has occurred. When odd parity is used, the parity bit is set or cleared to make the number of 1s and odd number. If even parity is selected, then the parity bit is set or cleared to make the number of 1s an even number. At the receiving end, the parity is checked according to the selected even or odd parity generated at the transmitting end. If the parity does not agree, then it is assumed that an error has occurred. When parity is selected, an additional parity bit is added to the transmission. The parity bit precedes the stop bit in the transmission sequence.
CRC error checking uses a method of cyclical redundancy checking. Upon transmission, all of the bits in a data packet are subjected to a CRC algorithm and a check sum is generated. The checksum is added to the end of the packet as a separate number. At the receiving end, the data packet is subjected to the same CRC algorithm and the resulting checksum is compared to the checksum received with the packet. If the checksums are not in agreement, then it is assumed that an error has occurred.
For answers to your Data acquisition and telemetry questions, ask WELLOG.