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1. ENG. ALEX LEVY QA/QC SEISMIC CONSULTANT 1 CRC & TRANSMIT ERROR Introduction: The land seismic theory and operations are about producing an…
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  • 1. ENG. ALEX LEVY QA/QC SEISMIC CONSULTANT 1 CRC & TRANSMIT ERROR Introduction: The land seismic theory and operations are about producing an excitation (by a source) to the land and “listening” (Travel Time) the wave from the “objective” to the surface. How this wave is acquired and transmitted is a very complex process which involves:  Analog to Digital Conversion,  Multiplexing and Demultiplexing,  Sigma-Delta Filter Application,  Convolution process,  Communications protocols (TCP-IP; Ethernet) and so many others stages that we are going to cover during this presentation. This document is based on a Sercel System Model 428XL. In the Appendix are explained all the concepts marked as A#XX
  • 2. ENG. ALEX LEVY QA/QC SEISMIC CONSULTANT 2 THE SEISMIC SERCEL NETWORK The complete system can be described into 4 nodes: 1. Control Node: Server and Human Control Interface (e428 Client Software) 2. Recorder Node: LCI 3. Data Buffer Node: LAUX, LAUL, LRU 4. Acquisition Node: FDU, DSU3, DSU1. A. The samples of seismic data are always from the Acquisition Node to the Data Buffer Node. B. The samples are analyzed and compressed by the Data Buffer Node and finally sent directly to the Control Node through the Asynchronous protocol. C. The status and result are sent to the Recorder Node for analysis through the asynchronous protocol too. Communication Protocol:  Communication between LAU and FDU:  On Line only @ 8 or 16 [Mbps].  Use SYNCHRONOUS COMMUNICATION.  LAU Master to FDU: Commands  FDU to LAU Slave: Seismic Data and Status.  Communication between LAU or LCI and LAU  On Line or Transverse @ 8 or 16 [Mbps].  Use ASYNCHRONOUS COMMUNICATION.  Full duplex. (A#1).  Automatic routing.  Broadcast possibilities.  Multi-Transverse possibilities  CRC Error Checking.
  • 3. ENG. ALEX LEVY QA/QC SEISMIC CONSULTANT 3 The protocol used on Line and secondary Transverse @ 8 or 16 [Mbps] is a TCP IP protocol. The TCP IP protocol description: The TCP/IP Protocol can be split into 4 layers as an standard TCP IP Protocol, so: Layer 1: PHYSICALL Layer:  Interface between Hardware and Software Communication between 2 adjacent LAU’s.  Encode cells.  Packet CRC checking. Layer 2: Data Link Layer:  Point to Point communication (P2P).  Communication between 2 adjacent LAU’s.  Frame management.  Frame CRC Checking. Layer 3: NETWORK Layer:  Routing management.  Communication between 2 Lau’s or LCI or Server.  Packets management. Layer 4: TRANSPORT Layer:  Communication between 2 Lau’s or LCI or Server  Message management by MULTIPLEXING / DEMULTIPLEXING (A#2)
  • 4. ENG. ALEX LEVY QA/QC SEISMIC CONSULTANT 4 ENCAPSULATION OF THE DATA: 16 [Mbps] Frame The 16[Mbps] frame is used in the Line The protocol used on the Transverse @ 100 [Mbps] is a STANDARD EHTERNET PROTOCOL. Module’s Description and Basic Connections: Done in the LCI: - Interfacing with the links. - Generating the Firing Order and sensing the Time Break. - Seismic line management and control. - Auxiliary links control. - Collecting system status data to be returned to the HCI (Human Control Interface). Done in the 428XL Server (PRM and HCI applications) - Collecting the data from the links (Done by the Server). - Noise editing (Zeroing/Clipping/Diversity Stack). - Correlation and Stacking.
  • 5. ENG. ALEX LEVY QA/QC SEISMIC CONSULTANT 5 Pipeline Architecture: The block handles the data as a pipeline.  They are able to process the data from different acquisitions at the same time.  This Allow the 428XL to offer a zero dead time. I.E.: no delay between consecutive acquisitions. As a result, the vibrator drivers and dynamite shooters do not have to wait for the system to be ready! LCI Architecture It contains three boards: a. LPBX: Blaster Board Interface. b. LPWX: Power Management Board. c. LPXL: Line and Transverse Management. a. LPBX: Blaster Board Interface: - This board manages the Synchronization of the spread. - This clock is tuned @ 16,384[MHz] +/- 1 ppm (1 part per million). Main components used on LPBX: - TCXO: 16,384[MHz] Reference Clock for Line Synchronization. - TCXO2: 17,920[MHz] Reference Clock for DPG Synchronization. b. LPWX: Power Management Board: - Manages all the different power supply need by the other cards. - Uses MosFet Transistor Technology: The advantages of using this technology are:  Allows to manage a very high frequency for the power supply.  Reduction in the working temperature.  Reduction of components size. c. LPXL: Line and Transverse Management: - The board is able to manage the communication through the LINE and the TRANSVERSE. Important:  Remind that the LINE speed is 8[Mbps] or 16[Mbps] and TRANSVERSE speed is 100[Mbps].  The Flash Memory that the LCI has contains all the programs for the DSP, FPGA and IBM.
  • 6. ENG. ALEX LEVY QA/QC SEISMIC CONSULTANT 6 DATA Exchange between LINE – LCI – 428XL Server For the SlipSweep + Navigation the Default Mode is Continuous Asynchronous Mode. Acquisition Modes: - An Acquisition is a set of Elementary Acquisitions - The Elementary Acquisition is always started by a T0 and the length of this Elementary Acquisition is limited. - In the Asynchronous Mode (SLIPSWEEP MODE) for the FIRST T0 the LAU SYNCHRONIZE the Acquisition and for the following T there’s no re-synchronization. - In case of Error found by the LAU during the Acquisition, the only solution is to reset the LAU MEMORY is an Abort from Operator then apply LINE OFF/ON. Retrieve Mode: The Continuous Asynchronous Mode will be the default mode for the 428XL. Errors Management:  On Samples – Overscalling: An error message is able to be generated every 16 samples. What’s more, the PRM application will do a summary of the error for the complete Acquisition.  On Acquisition – AcqError due to link unplugged during Acquisition:  An Error message is able to be generated every 16 samples.  The LAU will not transmit the blocks (16 samples) of the default traces.  The Samples value missed are replaced by the value 0  On LAU Memory – Memory Overflow: All traces concerned by the trouble are non-valid.  Acquisition synchronization trouble – T0 too Early: Non applicable for SlipSweep Mode.  Transmission Error During Retrieve - Transmit Time-out: the Operator has the possibility to Retry, Cancel, or Record the Retrieve.
  • 7. ENG. ALEX LEVY QA/QC SEISMIC CONSULTANT 7 Topology for the Sercel Land Equipment The SPREAD includes: - FDU - LAUL - LAUX - Recording Truck: LCI, Server, Etc. FDU – FIELD DIGITIZER UNIT The main role of the FDU is to DIGITIZES the Seismic Data. The FDU CONVERTS the analog data coming from the geophone to a digital one and send the digital data to the LAUL after that, the data will go to the LAUX and finally to the LCI. The Analog to Digital conversion is based on a SIGMA DELTA CONVERTER which works @ 256 [KHz]. The output from the Sigma Delta Converter is a sample of 24 bits @ 4 [KHz] frequency (The 4[KHz] frequency is due to the sample rate of 0.25[ms], so f= 1/0.25[ms]= 4 [KHz]).
  • 8. ENG. ALEX LEVY QA/QC SEISMIC CONSULTANT 8 FDU MAIN COMPONENTS: a. Power Supply that generates 6.3[v] for the analog part of the board and 2.7[V] for the digital one. b. FDU-INT that reshapes the signal, and contains the 2 PLL’s (A#3) used to synchronize the FDU c. FDU-COM that manages the communication with the LAUL, processes the data: performs a first decimation (rate 64) from 256[KHz] to 4 [Khz] and manages the other functions. d. Sigma-Delta Converter that converts the analog signal from the geophone to a digital bit stream @ 256 [KHz]. e. EEPROM contains the FDU’s identity and the calibration parameters. Initialization Sequences The INITIALIZATION is done once the FDU is power-on. The process consists in 4 steps: - CLOCK SYNCHRONIZATION: The PLL’s in the FDU-INT are synchronized with the data clock @ 8.192[MHz] or 16.384[MHz]. - ALIGNMENT: The FDU-COM detects the beginning of the data frames. The FDU can now interpret the orders coming from the equipment. - ORIENTATION PHASE: As the FDU can be connected either way, so, an automatic internal orientation is done at this stage in order to select the active and passive pair. - INITIALIZATION TESTS: Field test of the string and Instrument Tests. Seismic Data Path The data is scrambles to transfer white noise (A#4) on the line, to have a better synchronization of the field units.
  • 9. ENG. ALEX LEVY QA/QC SEISMIC CONSULTANT 9 A CRC IS INSERTED TO ENABLE THE LAU TO DETECT ANY TRANSMISSION TROUBLE Geophone Data Path The analog data is converted into a 24 bits with a 0.25[ms] Sample Rate (SR) and then sent to the LAUL where it is filtered (linear or minimum phase filter), decimated, compressed and sent to the CM. FDU A phase comparison between the data and the clock (output of the VCO (1)) is done. Then the voltage generated by the phase comparator (2) is filtered and amplified before being applied to the VCO. According the input of the VCO (Tension or Voltage value), the VCO delivers a new signal with a new frequency. The comparison is done until the phase comparison is not null. In each LAUL an additional PLL performs and extra jitter filtering (A#5) in the forward path. The clock Synchronization circuitry is also implemented in the LAUL, LAUX, and LCI
  • 10. ENG. ALEX LEVY QA/QC SEISMIC CONSULTANT 10 ACQUISITION CHANNEL CIRCUITRY Definition:  ADC Analog to Digital Converter, a device that will convert an analog signal into a digital one.  DSP (A#6) Digital Signal Processing. Description: The signal acquisition circuitry is composed by the following four circuits:  Input filter (FDU): Performing initial high-cut filtering and some noise cancellation.  Modulators (FDU): Consisting of a Delta Sigma (∑-Δ) Analog to Digital converter (∑-Δ ADC).  Delay Memory (Used in LAU Slave): Consisting of a RAM used to provide temporary storage for signal processing and remove the sample skew by synchronizing the start acquisition. It is important to take into account that the RAM (Random Access Memory) is cleared (all data is deleted) once the line status is power-off.  Digital Signal Processors (FDU and used in LAU Slave): Removing all that is of no use (including the quantization noise and high frequency components.
  • 11. ENG. ALEX LEVY QA/QC SEISMIC CONSULTANT 11 Analog to Digital Converter (ADC) – How it works: Conventional ADC: Any ADC will take the value of the analogue signal at the time t and will convert it to a digital value (here 4.5v). Then it will do the same at time t+1 (here 1.25V) and so on: Aliasing Effect: In the example below we assume Fmax is the maximum frequency in the signal spectrum. So, Fs/2 must be greater that Fmax (Fs>2Fmax), this is in order to AVOID ALIASING EFFECT. In case of aliasing, the original signal cannot be recovered.
  • 12. ENG. ALEX LEVY QA/QC SEISMIC CONSULTANT 12 Quantization Error: Assuming that a properly sampled signal like the one showed in below. The conversion from an analog signal to a digital one gives a quantization error. Let’s analyze an example with 8 bits: 8 bits allow us to read 28 . Then we will have 256 different values on a 5[V] Scale. This means that the quantization error will be: 5 256 =20[mV] For example, with a 9 bits, we can read 512 different values, with a quantization error of 10 [mV]. Oversampling: In seismic data acquisition the maximum frequency of interest is 500 [Hz]. As a result, the sampling frequency should not be less than 1[KHz] (done by 2xFmax). The 428XL has a sampling frequency of 256[KHz], then, the oversampling ratio is 256. Advantages of OVERSAMPLING:  Easier ANTIALIASING.  Bit Gain All ADC generate WHITE NOISE. This noise (known as quantization noise) is evenly spread across the range from 0[Hz] to Fs/2.  By Oversampling @ 256[KHz] the quantization noise is spread from 0[Hz] to 128[KHz] with a lower level.
  • 13. ENG. ALEX LEVY QA/QC SEISMIC CONSULTANT 13 Sercel Delta Sigma Converter (∑-Δ) Input Protection: - High Frequency (HF) noise elimination. - Antialiasing filter for oversampling frequency (256[KHz] How it works: - The digital signal generated by the ∑-Δ converter is encoded as a 1-bit stream and transmitted on a serial line. - That is why, a Digital Signal Processor (DSP) is used to convert the serial signal to a parallel 24-bit signal @ 4[KHZ] (T=0.25[ms]) - By INCREASING the order of the ADC (we can achieve this connecting in cascade mode/configuration) and increasing the oversampling ratio, then, the SIGNAL-TO-NOISE RATIO and the CONVERTER’S DYNAMIC RANGE and RESOLUTION ARE INCREASED.
  • 14. ENG. ALEX LEVY QA/QC SEISMIC CONSULTANT 14 Noise Shaping: Many bits can be gained if the noise is shaped: A DIGITAL CONVERTER CONSIST OF AN ADC, DAC AND AN AMPLIFIER The aim of noise shaping is to decrease noise at low frequencies. So the transfer function applicable for the noise shaping is: Qa= A/D quantization noise Qd= D/A errors
  • 15. ENG. ALEX LEVY QA/QC SEISMIC CONSULTANT 15 So; Qa is the quantization error relative to the 1-bit ADC. This is 50% error Qd is the error relating to the 1 bit DAC. It is virtually zero, only caused by the noise of the DAC H must be designed such that its value is very high (𝟏𝟎 𝟔 ), and in the band of interest (e.g. 0 to 1000[Hz], and very low (aprox. =0) out of the band of interest. Assuming Qd is negligible: DELAY MEMORY (LAU) Description: The RAM DELAY memory or called also ROTATING BUFFER, is located in the LCI board, LAUL, and LAUX. It is the key element, permitting the following functions to be performed:  Data Storage for digital signal processing  Removal of sampling skew.  Implementation of different anti-aliasing filter characteristics.
  • 16. ENG. ALEX LEVY QA/QC SEISMIC CONSULTANT 16 Data Processing a. Sample Skew Processing: When a control word is sent to the field units, a delay of a few milliseconds arises between the moment the first unit receives the word and the moment the last one receives it. Each repeater brings about a delay of 1.5[µs] and cables give rise to a delay of 5[ƞs] per meter. That is why, the start of all FDU must be synchronized and we need to use the following method to do it. After the line is formed, each station unit starts to acquire data. This is fed to the FDU, then the LAU and displayed on the HCI (Human Computer Interface) into the screen (Seismonitor in Jline environment), but it is not recorded. The memory of the LAU Slave gets filled up and retains the latest samples milliseconds of data. During the line forming, the microcontroller (µcontroller) (of the LAU master for each segment and the LCI) determines the delay corresponding to the time associated with the selected filter, plus the delay associated with the propagation time for the messages between the acquisition module and each link. As a result, the first data processed is the data that was recorded before the order to start acquisition was received. b. DATA MEMORY DELAYS: Some delays are required in order to implement the required convolution process. They depend on the sample rate and the filter type. Filters Used The decimators are used to perform three functions: a. Achieving a high-cut filter (anti-aliasing), which is required because the signal is under-sampled. b. Decimating the data from 256[KHz] to the sample rate. c. Applying a linear-phase filter (no phase shift) or minimum-phase filter to the data.
  • 17. ENG. ALEX LEVY QA/QC SEISMIC CONSULTANT 17 LAUX Function: In our case study we are going to mention the more characteristics features of the LAUX: a. Same functionalities as LAUL b. Multipath capabilities c. Rerouting capabilities d. For more features check the User’s manual. How it Works:  The LAUX wakes up by sensing the voltage on its 4 transmission ghost pairs (hablar del ghost pairs) (minimum VDC=5[V]), then supplies both sides with the +/-24[V] on each transmission ghost pair and the transverse transmission ghost with a DC voltage to wake up the next LAUX and so on.  The LAUX power on/off is software controlled and therefore any part of the spread can be powered OFF if wanted on the HCI (Human Computer Interface).  The LAUX contains two independent boosters to supply its two sides.  The LAUX will work correctly if the voltage is above 10,5[V]. This field equipment interfaces the lines to the CM (Control Module) and also collects, decimates, filters, and compresses the data before sending it to the Server (Seismic Data) or to the LCI (Status, results). It also synchronized all the samples with the time break. The sample rate can be 0.25[ms], 0.5[ms], 1[ms], 2[ms], 4[ms]. LAUX Architecture: It has  A transverse and a line interface (PLL, signal shaping)  An IBM403 (µcontroller)  Peripherals  DSP (to decimates, filters and compressing the data)  It is made of 1 sandwich box which contains two boards:  LPWX (Line Power Crossing board) board containing the boosters and Power supply of the unit, the equalizers and the PLL’s.  LPXL (Line Processor Crossing and Line board) board containing the digital hardware. LPWX Board: Provides  The charger for the Line Tester; 9.5[V]  The waking bloc that produces a 3.3[V] to manage the power-up circuits.  The power control block that manages everything and check the leakage.  The battery control block that protects the boards against and over-voltage.  ADC that measures the voltages. LPXL Board: This board runs the program stored in the flash memory. Also managed the FDU and process the data. LPXL Board Architecture: Its main architecture contains the following components: a. Digital Signal Processing (DSP): Decimates, filters and compress the data b. Flash memory: Used to store the program. c. DRAM: One is used by the DSP to process the data, the other one by the IBM. d. IBM 405: Microprocessor @ 33[MHz]. e. DPR: Dual Port Ram, interfaces between the different buses. f. FPGA: Interfaces with the LIPX board. g. UART: Communication device to Xdev plug (TMS, LT).
  • 18. ENG. ALEX LEVY QA/QC SEISMIC CONSULTANT 18 h. EhtPhy: Component for Ethernet management. LAUL Function:  High Storage Memory capacity for non-Real Time operation.  Receives/Transmit FDU statuses and results.  Power supply the line @ 50[V]. How it works:  It wakes up by sensing voltage on the transmission ghost lines, then supplies with voltage the opposite transmission ghost line  It interprets the orders coming from the LCI or 428XL Server, controls a line of FDU (up to 120 channels depending on interface) and collects the data from them.  On the data it performs:  Decimating,  Filtering,  Compressing  After all these it sent back information to the LCI (status) or 428XL Server (Seismic Data), through the LAUX.  Synchronizes all the samples with the Time Break. LAUL Architecture: It is made of only 1 board:  LPL16 (Line Processor Land board and 16 stands for 16[Mbps] – It means 16 Mega-bits-per- second). Cables The Transmission is made using two pairs between all the elements: LCI, LAUX, LAUL, and FDU. Cables Constitution: The transverse and the line cable are exactly the same. It is important to know about the type and main characteristic of the Sercel Cables: o Standard: ST+6.5mm(0.25in) o Waterproof: and traction resistant: WPSR 9.5mm (0.37), with a Kevlar braid. o Waterproof: and traction resistant long range: WPSR-LR 9.5mm, with a Kevlar braid Must be used in the secondary transverse only with LAUX408. o EXT: 0.27mm (0.31in), extension cable that can be used in the transverse only with LAUX 408.
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