SaneSeisA Ian Vincent Oct 2015 Updated 18-Oct2015 A proposed seismic acquisition file format

Transcription

1 SaneSeisA Ian Vincent Oct 2015 Updated 18-Oct2015 A proposed seismic acquisition file format The SEG file formats currently used for seismic acquisition (SEGD, SEGY SEG2) are based on decades old concepts that do not apply to modern seismic operations Presented here is a format proposal for shot domain data sets that attempts to resolve the issues with the SEG file formats. It attempts to eliminate all ambiguities. The object is to be able to quickly and easily write and read seismic data files. Informational parts of the header are text that can be read by anyone with a text editor. It is an acquisition level format only not a processing format. A processing format may be defined later depending on reception of this format description. Little endian encoding (Intel) will be used Floating point values and trace data values will be encoded as 32 bit floating point values as per IEEE754 Text entries are ASCII All headers are mandatory and all entries must be filled. File length will be limited to a maximum of 60 Seconds regardless of sample rate. All channels in the file use the same sample rate and number of samples The first sample is T0 (IE for a record length of 1000mS at 1mS SR, there are 1001 discrete samples, this is the way Sercel currently work) Coordinates are UTM and coordinate values in metres A source table contains source and start time information Up to 255 sources are permitted in a file (ID 1 255) A virtually unlimited number of source starts may appear in any file A source may start more than once within a file each start requires an entry in the source table Continuous recording is supported. A manufacturer and user header are included The file format uses Flat headers except for the source and receiver tables which may be variable length. This means that parts of headers do not need to be decoded to interpret later parts which in turn must be decoded to arrive at the number of channels in a file as is the case with SEGD. There are no channel sets as in SEGD. The source table may include null entries IE unused start entries. This allows for a fixed table size to be defined for projects if desired. E.g. if 22 vibrators are assigned to a crew, but only 16 used at any one time in continuous production, the source table may be fixed at 22 sources if desired. The receiver table includes array sensitivity, meaning true amplitude can be recovered The manufacturer header can contain any manufacturer data (text and/or binary data), but for the file to conform to the format, the manufacturer must publish a full description of all entries in the header. The user header is intended for such things as weather station information or other miscellaneous data that is project specific. It is text only and free form. Trace headers are flat and contain all positioning, trace assignment and QC information Currently it is at a formative stage. Full implementation will likely require additions and changes, however the basic concept will not change no optional sections, all entries defined, all headers mandatory and flat, fixed size (except for the Source and Receiver Tables).

2 SaneSeis File Header File Header Description Item Format nbytes Example Description Format Code Text 9 SaneSeisA Mandatory (A indicates Acquisition, P will be Processing) Format Version Integer 4 1 Increments as required, first version is 1 Client company Text 32 PDO Contractor Text 32 BGP Crew Text A Country Text 32 Oman Block name Text 32 Yibal 3D Datum Text 32 PSD93 0 if unknown Recording System Text 32 Sercel 428 Source Type Text 32 Vibrator Mixed source, use 'Vibrator, Airgun' etc. Record type Byte 1 0 0=normal, other =test see table A Correlated Byte 1 0 0=No, 1=Yes Low UTM Zone Byte 1 40 If cross UTM boundary, then use lowest File Number Integer Time of first sample Long Integer 8 GPS time accurate in Microseconds, -1 if not used Sample Rate Byte 1 4 1,2 or 4 ms Number of Integer channels Record length Integer Record length in ms Acquisition Mode Byte 1 1 0= Normal recording, 1=continuous SlipSweep Byte 1 1 1=yes, 0= No Recorder Low Line Float Recorder location in spread IE where the recorder connects in Recorder Low Float Point Number of sources Integer 4 8 Number of source starts in this file, 0 if microseismic Number of Receiver Types Integer 4 2 Number of different receiver types or arrays = Number of Receiver Table entries

3 Table A Record Type 0 Normal record. 1 Field (Sensor) noise. 2 Field (Sensor) tilt. 3 Field (Sensor) crosstalk. 4 Instrument noise. 5 Instrument distortion. 6 Instrument gain/phase 7 Instrument crosstalk 8 Instrument common mode 9 Synthetic. 10 Field (Sensor) pulse. 11 Instrument pulse. 12 Field (Sensor) distortion. 13 Instrument gravity. 14 Field (Sensor) leakage 15 Field (Sensor) resistance These codes are Sercel Codes, more can be added as required Source Table Description Item Format nbytes Example Description Source 1 ID Byte if not used Source 1 Type Text 1 V V=Vibrator, E=Explosive, A=AirGun, W=Water Gun, D=Weight Drop, X=Not used Source 1 Model Text 8 AHVIV380 Freeform short identifier. Useful if sources have different outputs e.g. 380 and 326 vibs Source 1 Line Float Source 1 Point Float Index 1 Integer 4 1 Increments as required, starts at 1 Source 1 Easting Float Source 1 Northing Float Source 1 Elevation Float Source 1 Position Mode Source 1 Start Time Source 1 Start Source 1 End Source 1 Sweep Length Source 1 Sweep Name Source 1 Number of Vibrators Source 1 Charge Size Text 1 A A=Actual (from Vib Nav), S=Surveyed, P=Preplan, E=Error or missing Long 8 GPS time accurate in Microseconds, -1 if not Integer used Float 4 1 Float 4 10 Integer Sweep Length in ms Text 16 Linear Freeform Sweep descriptor Integer 4 1 Float 4 0 Charge size in Kg

4 Source 1 Hole Float 4 0 Average hole depth in metres Depth Source 1 number Integer 4 0 of holes Source 1 Airgun or Text 1 X A or W or x for unused WaterGun Source 1 Airgun Float 4 0 Cubic inches array Volume Source 1 number Integer 4 0 of guns Source 1 Error Byte 1 0 0=No, 1=Yes for any problem affecting integrity of the shot 97 Bytes for each source entry (repeats as required based on Number of Sources defined in file header) Item Format nbytes Example Description Source n ID Byte if not used Source n Type Text 1 V V=Vibrator, E=Explosive, A=AirGun, W=Water Gun, D=Weight Drop, X=not used Source n Model Text 8 PLS326 Freeform short identifier. Useful if sources have different outputs e.g. 380 and 326 vibs Source n Line Float Source n Point Float Index n Integer 4 1 Increments as required, starts at 1 Source n Easting Float Source n Northing Float Source n Elevation Float Source n Position Mode Text 1 A A=Actual (from Vib Nav), S=Surveyed, P=Preplan, E=Error or missing Source n Start Time Long Integer 8 GPS time accurate in Microseconds, -1 if not used Source n Start Float 4 5 Source n End Float 4 76 Source n Sweep Integer Sweep Length in ms Length Source n Sweep Text 16 PDO7 Freeform Sweep descriptor Name Source n Number Integer 4 1 of Vibrators Source n Charge Float 4 0 Charge size in Kg Size Source n Hole Float 4 0 Average hole depth in metres Depth Source n number Integer 4 0

5 of holes Source n Airgun or Text 1 X A or W, X for unused WaterGun Source n Airgun Float 4 0 Cubic inches array Volume Source n number Integer 4 0 of guns Source n Error Byte 1 0 0=No, 1=Yes for any problem affecting integrity of the shot Receiver Table Description Item Format nbytes Example Description Type ID Integer 4 1 Unique ID of the receiver type = table entry number Sensor Byte 1 2 As Per SEGD Sensor code (See table) Type Model Text 16 Sercel SG5 Short description Integer 4 1 Number of elements in the array Number of elements Array Float 4 0 Crossline size in metres (0 for single element) width Array Float 4 0 Inline size in metres (0 for single element) Length Array Sensitivity Resistance Limit Low Resistance Limit High Tilt Limit Leakage Limit Capacitance High Limit Capacitance Low Limit Float 4 80 Total Sensitivity of the array - V/m/S for geophones, mv/m/s^2 for Accelerometer, V/Bar for Hydrophones Float Ohms Float Ohms Float 4 5 % Float 4 3 Megohms Float 4 0 Nano Farads Float 4 0 Nano Farads

6 Sensor SEGD code (from Sercel s Documentation) 0 : not defined. 1 : Hydrophone. 2 : Geophone, Vertical. 3 : Geophone, Horizontal, In-line. 4 : Geophone, Horizontal, Crossline. 5 : Geophone, Horizontal, other. 6 : Accelerometer, Vertical. 7 : Accelerometer, Horizontal, In-line. 8 : Accelerometer, Horizontal, Crossline. 9 : Accelerometer, Horizontal, other. Data Section Each trace contains a flat header followed by data encoded as 32 bit floating point values. Trace numbers start at 1 and increment until the last trace is written. The trace contents are defined within the header, so there is no need for channel sets. It also allows for mixing of Aux and Seis data within a record, but I would discourage that unless there is a good reason. There is provision within the content description for storing source data such as vibrator Ref, GF, Mass and BP signal data. In these cases, it is likely that the signals will be shorter than the main record, so they will be padded with zeros at the end. The Trace header format borrows from the Sercel 428 format. This is because it is well described, quite complete and Sercel is currently the dominant equipment manufacturer, and likely to remain so for some time. Trace Header Description Item Format n Bytes Example Comments Trace Number Integer Trace within file Trace Contents Integer 4 0 0=Seis,1=TB,2=Pilot,3=Autocorr +,4=AutoCorr -, 5=Radio Ref, 6=Ext. ref Osc(100Hz), 110=Source 1 Ref, 111=Source 1 GF, 112=Source 1 BP Acc, 113=Source 1 Mass Acc, 114=Source 1 Overload, repeats starting at 120 for source 2 etc. Line Number Float 4 Line Point Float 4 Point Easting Float 4 Point Northing Float 4

7 Point Elevation Float 4 Point Code Text 2 G1 As per SPS Point Depth Float 4 Water Depth Float 4 0 if not used Receiver Type Integer 1 2 As defined in receiver table Digitiser Type Integer 4 15 Manufacturer s Code Code Digitiser Serial Long Number Integer Channel within Integer 4 1 Digitiser Channel Gain Integer 4 12 Gain in db Controller Type Integer 4 6 Manufacturer code Controller Serial Long Refers to device that controls this channel Number Integer Resistance Float Resistance Byte =Low,0=None, 1=High Error Tilt Float Tilt Error Byte =Low,0=None, 1=High Leakage Float Value in MegOhms Leakage Error Byte =Low,0=None Capacitance Float 4 0 nano Farads Capacitance Byte =Low,0=None, 1=High Error Channel RMS Float 4 Amplitude in mv IE conversion factor applied amplitude Channel Byte 1 0 0=No, 1=Yes Overscale Channel Error Byte 1 0 0=No, 1=Yes - from recording system, Acq error or other error which affects integrity Offset removal Byte 1 1 0=No, 1=Yes Applied Sample to mv Conversion factor Channel Alias Filter Type Channel Alias Filter Channel Low Cut Filter Float Unambiguous conversion factor for this channel Byte 1 1 1= Min Phase, 2= Linear Phase Float dB point in Hz Float 4 0-6dB point in Hz Interpolations Integer 4 0 Number of interpolated samples in this trace (Sercel)

8 Comments SaneSeisA easily accommodates conventional normal recording where there is only one source. In this case, only one entry is required in the source table. It also accommodates the recently proposed methods of blended acquisition where many sources are operating either simultaneously or at random times and locations. In this case, a source table entry is required for each source start encountered in the time span of the file. Header size comparison between SaneSeisA and SEGD V2 Consider a conventional Vibroseis record containing traces (including Aux) from a Sercel 428. This means 1 source table entry for SaneSeisA and to be realistic 4 receiver tables. SaneSeisA SEGD V2 File Header 2700 Bytes 2656 Bytes Trace headers 2,380,000 (119 x 20000) 4,880,000 (244 x 20000) Although the file header is slightly bigger, there is a useful saving by using SaneSeisA, and the benefit increases with channel count. This will be offset slightly with many entries in the source and receiver tables, but it is unlikely that we will see more than 100 of each, and then only with a huge channel count which favours SaneSeisA. In any case, it shows that there is considerable room to expand the headers before exceeding the currently used format size. And just as an exercise focusing on very large crews: Channels 50 sources 10 receiver types SaneSeisA SEGD V2 File Header 7819 Bytes 2656 Bytes Trace headers 23,800,000 (119 x 20000) 48,800,000 (244 x ) Each file would save 25MB bytes. Not a big thing in today s world until we consider a project with these parameters would likely have around 1,000,000 (or more) source points, which makes the saving over the project around 25TB, which is probably worthwhile.