# Migration and depth conversion

The migrated and depth-converted stacks of the FIRE project are available as a set of SEG-Y files. The file sizes range from 45 MB (FIRE 2-3) to 655 MB (FIRE 4 & 4A). The number of CMPs varies from 1,537 to 22,699.

Typically, each stack represents a different subdivision of a line – FIRE 2-1 is separate from FIRE 2-2 and so on. The only exception is FIRE 4 & 4A. They form a continuous line but were surveyed in different directions, the former from south to north (Ranua to Sirkka) and the latter from north to south (Näätämö to Sirkka). In OpenFIRE, FIRE 4 and 4A  have been joined to avoid possible confusion.

The pre-migration steps are described separately under NMO and DMO stacks. All stacks preserve their original sample rate at 2 milliseconds (15,000 or 8,001 samples per trace).

The migration was applied using the conventional Stolt method. The following RMS velocity table is an average obtained from previous refraction seismic studies in Southern Finland and was applied to the profiles 1, 2, 2A, 3, and 3A:

 time [s] rms velocity [m/s] 0.000 5830 0.700 5943 1.500 6008 2.528 6075 3.445 6127 4.693 6188 6.393 6262 8.709 6358 11.865 6502 16.164 6756 22.021 7166 27.028 7455 30.000 7600

The following rms velocities from the POLAR refraction profile were used for FIRE 4, 4A and 4B:

 time [s] rms velocity [m/s] 0.000 5843 0.400 5985 1.100 6097 1.900 6165 2.500 6197 3.600 6215 4.300 6237 6.000 6299 8.000 6366 10.000 6446 12.000 6545 14.200 6708 22.021 7236 27.028 7455 30.000 7600

In addition, for the PDF books, a Stolt stretch factor of 0.95 has been applied in the migration stage.

Since the migration step adds energy to the top part of stacks where reflectors are abundant and fades out the deep crust, a post-migration scaling step was performed to restore the original amplitude decay. The program used for this (su_balance_fire) is included in the OpenFIRE tools Github repository.

Depth conversion was applied using the following table of interval velocities, obtained from the table above using the Dix equation:

 time [s] velocity [m/s] 0.000 5870 0.700 6022 1.500 6125 2.528 6233 3.445 6312 4.693 6406 6.393 6528 8.709 6721 11.865 7090 16.164 7770 22.021 8521 26.971 8667 30.000 8700

Similarly for FIRE 4, 4A and 4B, the velocities are:

 time [s] velocity [m/s] 0.000 5911 0.400 6108 1.100 6227 2.200 6306 2.900 6225 3.600 6321 4.300 6413 6.000 6495 8.000 6613 10.000 6944 12.000 7096 14.200 8246 26.971 8667 30.000 8700

The NMO sections were resampled to 12.5 meters (6,401 depth samples per trace) and the DMO sections to 6.25 meters (6,401 samples per trace). The depth extent is 80 kilometers for the NMO sections and 40 kilometers for the DMO sections. The depth sampling interval is not assigned in any header since it is not mentioned in the SEG-Y rev 1 standard.

Finnish Reflection Experiment is a 2-D crooked-line seismic survey covering around 2,100 km of survey lines. The recording geometry is split-spread (asymmetric at the end of lines) with 362 active channels separated by a nominal group interval of 50 meters. This results in a nominal fold of ~90 for the entire survey.

The CMP interval in the stacked data is 25 meters.

FIRE 1, 2 and 2A form a continuous line. The CMPs of FIRE 1 and FIRE 2 match as indicated in the table below.

 CMP (line 1-3) CMP (line 2-1) 19380 182 20009 361 20187 539

The CMP 1381 in line 2A matches with the CMP 8655 in line 2-3.

#### Gaps

• There is a gap of 392 CMPs between lines 1-1 and 1-2 due to the crossing of Lake Rehja-Nuasjärvi near the city of Kajaani.
• There is a gap of 166 CMPs between lines 2-1 and 2-2 due to the town of Orivesi.
• There is a gap of 260 CMPs between lines 2-2 and 2-3 due to the city of Hämeenlinna.
• There is a gap of 196 CMPs between lines 3-1 and 3-2 due to the city of Joensuu.

#### Crossings

• FIRE 3A crosses FIRE 1 near the CMP 12200 of line 3A and the CMP 16823 of line 1-3.
• FIRE 3A departs from FIRE 3 near the CMP 1858 of line 3-1. There is a 300-meter gap between the two.
• The CMP 9919 in FIRE 4 corresponds to the CMP 212 in FIRE 4B. This is also where the lines depart from one another.

The naming of files follows the general pattern

 Fire_(line)_(subdivision)_[nmo/dmo]_mig[_depth].sgy

The 3200-byte textual header contains general information of the data and is organized as follows:

C1  FIRE Finnish Reflection Experiment 2001-2003                        * C2  Client : FIRE consortium: University of Helsinki, University of Oulu* C3                            and Geological Survey of Finland          * C4  Contractor: Spetsgeofyzika, Russia                                  * C5  LINE [NUMBER] [REGION] ; acquisition: [TIME INTERVAL]               * C6                                                                      * C7  MIGRATED [& DEPTH CONVERTED] [NMO/DMO] STACK.                       * C8                                                                      * C9  Band-pass filtering: 6, 12-80, 125 Hz + notch filter 50 Hz          * C10 Geometrical spreading correction: velocity-dependent sphdiv         * C11 Deconvolution: operator length 160 ms, white noise 0.1 per cent,    * C12                evaluation windows remain constant                   * C13 Whole-trace equalisation at the University of Helsinki.             * C14 Variable-velocity Stolt migration at the University of Helsinki.    * C15 [Time-to-depth conversion at the University of Helsinki.]           * 

C16 C17 CDP range: [RANGE]                                                  * C18 Number of samples: [NO OF SAMPLES]  Sampling interval: [INTERVAL]   * C19 Horizontal extent:  [KILOMETERS]                                    * C20   Vertical extent:  [KILOMETERS OR SECONDS]                         * C21                                                                     * C22 NOTE! There is a gap of 196 CDPs between FIRE 3-1 and 3-2 due to    * C23       the city of Joensuu.                                          * C24                                                                     * 

... [OTHER MISCELLANEOUS NOTES, CUT TO SAVE SPACE]

C40 END TEXTUAL HEADER                                                  *

The 400-byte binary header has been set, at least for the following values:

• bytes 5–8 (line number)
• bytes 17–18 (sample interval in microseconds, typically 2000 us)
• bytes 25–26 (data format, set to 1, ie. 4-byte IBM floats)
• bytes 27–28 (fold, here 1 because stacked data)
• bytes 29–30 (sorting code, set to 3, ie. single-fold continuous profiles)

In the trace headers, the following values should at least be set:

• bytes 1–4 (trace sequence number within line)
• bytes 5–8 (trace sequence number within the SEG-Y file)
• bytes 21–24 (CMP number)
• bytes 73–76 = 81–84 (CMP coordinates, northing)
• bytes 77–80 = 85–88 (CMP coordinates, easting)
• bytes 115–116 (number of samples, 15000 or 8001)
• bytes 117–118 (sample interval in microseconds, typically 2000 us)

The trace sequence number within line (bytes 1–4) continues to increase across FIRE 1 and 2. Where the stacked sections overlap, the numbers are shared. Similarly has been done for lines 4 & 4B and 3-1 & 3-2.

The header variable at bytes 1–4 should be primarily used for cutting and joining lines. As an example, if the user wants to combine FIRE 4 & 4B into a single profile, they both should be split at trace sequence number 9912 (corresponding to CMP 9919 and 212, respectively).

The coordinate system of the data is the Finland Uniform Coordinate System (EPSG:2393). As a convention, the "X" (bytes 73–76 or 81–84) in trace headers is the northing and "Y" (bytes 77–80 or 85–88) is the easting.