WEBVTT 00:00.000 --> 00:03.700 For the exploration of raw materials, seismic measurements produce 00:03.700 --> 00:04.500 huge amounts of data. 00:05.340 --> 00:07.960 Such large-scale measurements are also called acquisitions. 00:09.240 --> 00:13.440 In a typical marine acquisition, areas of several thousand square 00:13.440 --> 00:15.740 kilometres are examined, which can take several weeks. 00:17.580 --> 00:23.080 Airguns, which are underwater sound sources, send 100,000 acoustic 00:23.080 --> 00:24.020 signals during such large investigations. 00:24.560 --> 00:28.400 An airgun shot is often registered by several thousand hydrophones. 00:28.400 --> 00:33.880 They record the data in abstract intervals of two milliseconds, for 00:33.880 --> 00:35.820 detailed measurements even below that. 00:36.280 --> 00:39.760 This high value is necessary to map the subsurface with a high 00:39.760 --> 00:40.380 resolution. 00:41.680 --> 00:46.700 With a record length of 10 seconds, this results in several hundred 00:46.700 --> 00:48.560 megabytes of seismic measurement data per shot, for example. 00:49.220 --> 00:52.020 A whole measurement campaign can therefore easily generate data 00:52.020 --> 00:54.620 amounts of many terabytes in magnitude. 00:54.620 --> 00:59.380 In order to obtain a single image of the subsurface from these large 00:59.380 --> 01:02.360 amounts of data, the measurement signals must be prepared, combined 01:02.360 --> 01:04.680 and the relevant signals amplified. 01:05.500 --> 01:07.720 This is the aim of seismic processing. 01:16.850 --> 01:18.290 Hello and welcome. 01:19.190 --> 01:23.090 In this video, I will give you an introduction to data preparation for 01:23.090 --> 01:25.050 seismic reflection measurements, i.e. 01:25.050 --> 01:25.530 processing. 01:26.250 --> 01:30.490 I will show you which steps are necessary to reduce the large data 01:30.490 --> 01:34.030 amounts of seismic measurements to a single interpretable image of the 01:34.030 --> 01:34.490 subsurface. 01:35.950 --> 01:39.390 The first step is the so-called pre-processing, i.e. 01:39.390 --> 01:41.410 the first steps in data preparation. 01:42.370 --> 01:47.210 In this, it is mainly about reading the data and combining the 01:47.210 --> 01:51.890 information for measuring geometry, checking and sorting the data, as 01:51.890 --> 01:55.430 well as correcting geometric recording characteristics. 01:56.430 --> 01:59.930 Pre-processing removes the effects of all known subsurface 01:59.930 --> 02:01.390 characteristics from the data. 02:01.990 --> 02:05.730 In the following steps, the unknown geological information can be 02:05.730 --> 02:06.290 analyzed. 02:07.610 --> 02:10.470 Processing is part of seismic data acquisition. 02:10.910 --> 02:12.070 You can see its course here. 02:12.810 --> 02:14.730 A source generates seismic waves. 02:15.050 --> 02:17.010 These spread through the subsurface. 02:18.110 --> 02:23.130 In seismic reflection, as the name suggests, seismic waves are 02:23.130 --> 02:24.690 reflected on rock layers. 02:25.450 --> 02:30.470 Other types of waves, such as refracted waves or surface waves, are 02:30.470 --> 02:31.070 neglected. 02:32.250 --> 02:35.250 On the earth's surface or on the sea, the waves are measured by 02:35.250 --> 02:39.050 geophones or hydrophones and then processed, i.e. 02:39.090 --> 02:39.590 processed. 02:40.830 --> 02:43.330 Then they are transformed into an image of the subsurface. 02:44.650 --> 02:48.530 In the last step, the interpretation for the raw material search is 02:48.530 --> 02:48.530 carried out. 02:50.050 --> 02:55.210 The main goals of data processing are, first, the amplification of the 02:55.210 --> 02:57.850 signal by removing unwanted parts. 02:58.390 --> 03:02.730 Second, the optimization of the resolution in time and space, i.e. 03:02.790 --> 03:05.530 both in the seismogram and in the depth profile. 03:06.270 --> 03:10.450 And third, the placement of the seismic signals in their specific 03:10.450 --> 03:11.970 position in the earth's subsurface. 03:13.290 --> 03:16.430 Processing is made up of many different steps. 03:16.430 --> 03:21.750 The first part is the so-called preprocessing, in which the data is 03:21.750 --> 03:26.590 prepared for further processing and the unwanted, known data 03:26.590 --> 03:27.750 properties are removed. 03:28.150 --> 03:31.430 Here are some important steps in preprocessing. 03:33.150 --> 03:37.130 For many terabytes of data, it is important to check the data quality 03:37.130 --> 03:40.110 and also to verify the sorting of the data based on measurement 03:40.110 --> 03:40.670 protocols. 03:41.270 --> 03:47.610 First, the data is read in, provided with geometry information and 03:47.610 --> 03:49.150 checked for completeness and errors. 03:49.990 --> 03:51.930 This process is called editing. 03:53.670 --> 03:57.710 This is the checking of the data and the cleaning of seismogram traces 03:57.710 --> 03:58.690 of poor quality. 04:00.870 --> 04:05.850 One unwanted property of the recorded seismograms is the removal of 04:05.850 --> 04:08.170 the amplitude with increasing distance. 04:08.170 --> 04:13.370 In the simple case of a homogeneous half-space, seismic space waves 04:13.370 --> 04:14.510 spread out radially symmetrically. 04:15.450 --> 04:19.630 Their energy is distributed on the surface of a half-ball A of the 04:19.630 --> 04:22.750 size 2 times pi times radius r squared. 04:23.450 --> 04:26.730 The energy therefore decreases proportionally to the square of the 04:26.730 --> 04:27.150 distance. 04:28.190 --> 04:31.370 The amplitude correction recreates the signal strength lost due to the 04:31.370 --> 04:32.590 geometric loss of expansion. 04:35.410 --> 04:39.070 Later incoming weak signals are thus amplified. 04:39.530 --> 04:41.790 This is called gain recovery in English. 04:42.770 --> 04:46.910 Since the size of the amplitude loss can be estimated, the weakening 04:46.910 --> 04:47.410 can be corrected. 04:48.530 --> 04:51.890 However, it should be noted that in such a simple correction, 04:52.210 --> 04:54.030 disturbing noise signals can also be amplified. 04:54.830 --> 04:58.770 These can be removed by more complex corrections or in later steps of 04:58.770 --> 05:00.850 processing, such as stacking. 05:04.070 --> 05:08.010 Local effects also influence seismic data and must be removed. 05:08.610 --> 05:12.390 These are, for example, static corrections for the topography and the 05:12.390 --> 05:15.130 top rock layers, as I will show you now. 05:15.610 --> 05:17.570 They are particularly important in land seismics. 05:18.690 --> 05:22.750 On the sea, however, the times and variable wave velocities must be 05:22.750 --> 05:23.250 considered in seawater. 05:24.510 --> 05:28.130 Here you can see an example of a vertical cut through the earth. 05:29.630 --> 05:30.670 Geophones are placed on the surface. 05:30.670 --> 05:33.810 Below them is a weathering layer. 05:34.570 --> 05:39.790 Such layers usually have a very low spread rate for seismic waves. 05:40.110 --> 05:44.170 They are not of interest for exploration, as the desired raw material 05:44.170 --> 05:45.210 is stored in deeper layers. 05:46.390 --> 05:49.710 For the running times of seismic waves, this means later arrivals for 05:49.710 --> 05:54.130 higher -lying stations and earlier arrivals for lower-lying ones. 05:54.950 --> 06:01.150 On the other hand, geophones are systematically later arrival times 06:01.150 --> 06:03.850 registered as such, which lie on a thinner layer. 06:04.890 --> 06:09.190 The spread rate of the weathering layer and the rocks below it is 06:09.190 --> 06:10.070 taken as a constant. 06:10.930 --> 06:14.130 It is determined together with the corresponding magnitude by 06:14.130 --> 06:15.410 additional examinations. 06:16.350 --> 06:23.250 Then the running time effects of the topography and the weathering 06:23.250 --> 06:26.810 layer are calculated at a reference level, the so-called date, for 06:26.810 --> 06:29.430 example, this line here. 06:31.850 --> 06:35.530 In addition to these static corrections, there are also dynamic, i.e. 06:35.550 --> 06:37.030 time-dependent corrections. 06:37.430 --> 06:41.290 They depend on the depth of the reflectors where the seismic waves are 06:41.290 --> 06:41.790 reflected. 06:43.030 --> 06:46.190 For example, the normal move-out correction counts for this. 06:46.750 --> 06:51.390 For curved reflectors, it corrects the running time of reflected waves 06:51.390 --> 06:53.650 to that of a vertical beam. 06:54.570 --> 06:57.310 This correction is part of the speed analysis. 06:58.630 --> 07:01.910 Now you have learned the most important corrections of preprocessing. 07:03.010 --> 07:06.830 This has removed the effects of all known subsurface properties from 07:06.830 --> 07:07.190 the data. 07:08.370 --> 07:13.790 In the further steps of processing, the unknown rock information can 07:13.790 --> 07:14.830 then be worked out. 07:15.310 --> 07:18.370 I will now present these steps to you in a short overview. 07:19.550 --> 07:21.330 First, the deconvolution. 07:21.330 --> 07:25.470 It sharpens the signal and increases the vertical resolution in the 07:25.470 --> 07:25.970 subsoil. 07:26.750 --> 07:32.170 The speed analysis determines a medium propagation speed of the 07:32.170 --> 07:34.290 seismic waves in the survey area from the seismic data. 07:34.850 --> 07:37.290 Then comes the stacking of the data. 07:37.870 --> 07:41.790 The individual preprocessed seismograms are added to this, while 07:41.790 --> 07:43.450 coherent signals are amplified. 07:44.010 --> 07:48.210 The stacking not only reduces the amount of data, but also improves 07:48.210 --> 07:49.990 the signal-to-noise ratio. 07:50.810 --> 07:55.310 A seismic section is then the arrangement of many such stacked 07:55.310 --> 07:58.070 seismograms next to each other into a single image. 07:59.190 --> 08:03.570 However, the reflectors are still in the time range of the seismogram 08:03.570 --> 08:07.990 and are not assigned to their actual depth or their lateral position. 08:09.390 --> 08:14.410 These three steps, deconvolution, speed analysis and stacking, are 08:14.410 --> 08:20.090 often used repeatedly in practice, as their results influence each 08:20.090 --> 08:20.090 other. 08:21.090 --> 08:25.610 For example, improvements in the speed model lead to an improvement in 08:25.610 --> 08:26.330 the stacking seismograms. 08:27.330 --> 08:30.910 In general, the individual steps of the processing are flexible. 08:31.390 --> 08:33.810 Depending on the application, their order can vary. 08:35.110 --> 08:37.350 The last step represents migration. 08:38.190 --> 08:42.690 It forms the stacked waveforms of the seismic section on their actual 08:42.690 --> 08:46.990 reflectors in the depth and thus enables an interpretation of the 08:46.990 --> 08:48.130 geological structures. 08:48.530 --> 08:51.190 These then form the basis for finding raw materials. 08:54.450 --> 08:57.750 In this video, I have shown you the most important steps of 08:57.750 --> 09:02.550 preprocessing and given you a brief overview of the further processing 09:02.550 --> 09:03.910 in the reflection seismic. 09:04.770 --> 09:08.710 You have learned that large amounts of data have to be checked and 09:08.710 --> 09:09.230 cleaned. 09:09.770 --> 09:11.130 This is called editing. 09:12.090 --> 09:16.630 Amplitude correction reverses the effects of geometric loss of 09:16.630 --> 09:19.170 expansion by reinforcing the signal in terms of distance. 09:21.070 --> 09:26.110 Static corrections remove well-known influences of terrain topography 09:26.110 --> 09:30.110 and weathering from the data of land seismology and, for example, 09:30.190 --> 09:32.470 geological effects in marine seismology. 09:33.490 --> 09:36.690 Dynamic corrections remove time-dependent effects. 09:38.110 --> 09:42.570 At the end of preprocessing, seismic data remain, from which all known 09:42.570 --> 09:44.170 effects of the subsoil have been removed. 09:45.090 --> 09:48.790 In the following steps of processing, the location of the selected 09:48.790 --> 09:52.430 reflectors can be determined and a geological interpretation of the 09:52.430 --> 09:53.650 subsoil becomes possible.