The seismic reflection image — Transcript

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1. 0:02seismic reflection
2. 0:03is the principal way in which we can
3. 0:05image the outer part of the solid earth
4. 0:08it can create spectacular images like
5. 0:10this
6. 0:12this image comes from the browse basin
7. 0:14which is just off the northwest coast of
8. 0:16australia
9. 0:17and it shows how sedimentary layers have
10. 0:20built out
11. 0:20from the continent into the surrounding
12. 0:22ocean
13. 0:24the image is built by sending sound
14. 0:27waves
15. 0:27into the earth and then collecting them
16. 0:30again in receivers
17. 0:32and we use these to build an image
18. 0:35you may be more familiar with this
19. 0:37approach through the
20. 0:39sonar mapping of the seabed essentially
21. 0:42a form of echo location where sound
22. 0:44waves are sent
23. 0:45into the water to reveal the seabed
24. 0:49seismic takes it further to reveal what
25. 0:52lies below the seabed
26. 0:55and create images like this
27. 0:59well this is a two-dimensional slice
28. 1:00down into the earth but you can actually
29. 1:02create three-dimensional images using
30. 1:04the
31. 1:04seismic reflection method so here's a
32. 1:07seismic image of part of the seabed
33. 1:09offshore nigeria
34. 1:10in the eastern atlantic showing a
35. 1:13submarine channel
36. 1:15and by working in three dimensions you
37. 1:17can create a cube
38. 1:18of seismic imagery the sides of which
39. 1:22you can see here
40. 1:23and then you can slice this horizontally
41. 1:26in so-called time slices
42. 1:27to reveal ancient channel structures
43. 1:30preserved in the rock record way below
44. 1:33the seabed
45. 1:36spectacular 3d structures geologists use
46. 1:39these to get a better understanding
47. 1:42of the three-dimensional structure of
48. 1:44the subsurface
49. 1:45in this case here for planning wells
50. 1:48drilled
51. 1:49into the floor of the northern gulf of
52. 1:51mexico
53. 1:52offshore the southern part of the united
54. 1:55states
55. 1:57but for now let's just consider two
56. 1:58dimensions
57. 2:00how are images like this created
58. 2:06first we need a seismic source a source
59. 2:09of
60. 2:09acoustic energy that could be
61. 2:11transmitted into the earth
62. 2:13and this will send out a wavefront
63. 2:16down into the subsurface radiating away
64. 2:19from this source the trajectory taken by
65. 2:23this wavefront
66. 2:24defines a series of ray paths that are
67. 2:27radial
68. 2:28from the seismic source let's just
69. 2:30consider one of these
70. 2:31which migrates down through this
71. 2:34subsurface through this
72. 2:35yellow layer and it hits an interface
73. 2:38with an underlying layer
74. 2:40shown here in green
75. 2:43this boundary represents a contrast in
76. 2:45the acoustic impedance
77. 2:48just like electrical impedance acoustic
78. 2:51impedance represents the resistance to
79. 2:54the transmission of the seismic energy
80. 2:56through the medium and it depends on the
81. 2:59velocity at which that seismic energy
82. 3:01can be transmitted
83. 3:03and the density of the material a
84. 3:05contrast in these properties will
85. 3:06deflect the ray path
86. 3:08part of it will be deflected back to the
87. 3:11surface
88. 3:11as a reflection and other components of
89. 3:14the seismic energy
90. 3:15will continue further into the
91. 3:17subsurface
92. 3:18refracted at this interface to have two
93. 3:21components a reflected component and a
94. 3:23refractive component
95. 3:26the refracted component continues on its
96. 3:28way to another interface
97. 3:30where it again can be reflected back
98. 3:33towards the surface
99. 3:34and again a residual part of that
100. 3:37seismic energy
101. 3:38can continue further still into the
102. 3:40subsurface
103. 3:43so these reflected rays must be
104. 3:46collected back at the earth's surface
105. 3:48by receivers so that's the basis of the
106. 3:51method
107. 3:51we have a seismic source which
108. 3:54sends energy into the subsurface some of
109. 3:58that energy is reflected back and we
110. 4:00collect that reflected energy
111. 4:02in a series of receivers and by
112. 4:05repeating this type of experiment again
113. 4:07and again we can build a profile
114. 4:10such as this one here from southeast
115. 4:13spain
116. 4:18so here's the basis of this that we have
117. 4:20a seismic source
118. 4:21with a controlled input which is a
119. 4:24waveform
120. 4:25which goes into the earth and then
121. 4:28reappears at the earth's surface
122. 4:30highly modified the earth then
123. 4:34acts as a filter which modifies
124. 4:38the input to form an output wave
125. 4:41structure
126. 4:42that we record in the receivers so our
127. 4:45mission
128. 4:45is to try and understand what this
129. 4:47filter consists of
130. 4:50so of course there are a number of
131. 4:52naturally occurring seismic sources
132. 4:54such as earthquakes and volcanic
133. 4:56eruptions
134. 4:57but these are rather uncontrolled
135. 5:01the seismic reflection method uses a
136. 5:03controlled source
137. 5:05in water this could be provided by air
138. 5:07guns and on land
139. 5:08it may be provided by explosives or
140. 5:11more commonly by vibrasize which are
141. 5:15shown here in the photograph as an array
142. 5:17of bouncy trucks
143. 5:19that send controlled vibrations down
144. 5:22into the earth
145. 5:23so those are the principal seismic
146. 5:24sources
147. 5:27the receivers consist of hydrophones if
148. 5:29they're on water or geophones
149. 5:31on land and our mission then is to
150. 5:34understand what goes on in the earth
151. 5:36in terms of differences in its ability
152. 5:39to transmit
153. 5:40seismic energy the velocity structure in
154. 5:42other words
155. 5:43and its layering
156. 5:48so let's consider how the velocity at
157. 5:51which
158. 5:52scientific or acoustic energy can be
159. 5:54transmitted varies
160. 5:56from different types of material
161. 5:59acoustic energy
162. 6:00sound in other words is transmitted in
163. 6:02air and that velocity
164. 6:04varies depending on the air temperature
165. 6:07at 20 degrees centigrade
166. 6:08and at sea level air transmits sound
167. 6:12at around 343 meters per second
168. 6:17water transmits seismic energy at around
169. 6:201.4 to 1.5 kilometers a second
170. 6:22this variation may relate to the
171. 6:24temperature as with air but also its
172. 6:26salinity
173. 6:28and therefore density
174. 6:31sandstones also show variation in this
175. 6:34case between three and four kilometers a
176. 6:36second
177. 6:38these variations largely reflect how
178. 6:41lithified the sandstone is therefore in
179. 6:44terms of its porosity
180. 6:48granite in contrast has a seismic
181. 6:51velocity of around
182. 6:52six kilometers a second it's denser than
183. 6:55sandstone and has no porosity
184. 6:58so let's see how this might play if we
185. 7:00put some seismic energy
186. 7:02through these materials so here it comes
187. 7:05we're going to put a seismic wave
188. 7:07through here
189. 7:08and here it goes the granite's arrived
190. 7:09the sandstone's arrived here comes the
191. 7:10water
192. 7:11about to get there and the air
193. 7:15it's still coming through the seismic
194. 7:16energy is still taking a long time to
195. 7:18get through the air
196. 7:20so these animations show how much slower
197. 7:22it is to get sound
198. 7:23through air compared to the equivalent
199. 7:25energy source through
200. 7:26granite or sandstone or even water
201. 7:30so materials have different seismic
202. 7:32velocities
203. 7:36so let's imagine we're looking at
204. 7:38granite and sandstone underwater
205. 7:40seen in a profile and we have a survey
206. 7:43vessel
207. 7:44that will cruise across the water's
208. 7:45surface with the seismic source
209. 7:47on the boat and there's a receiver
210. 7:50strung out the back
211. 7:51floating on the water surface and that
212. 7:53will send seismic energy
213. 7:55down into the subsurface some of that
214. 7:57energy will reflect back
215. 7:59off the interface between the sandstone
216. 8:01and granite and be collected
217. 8:03at the receiver
218. 8:06in detecting this reflection the survey
219. 8:09vessel has detected
220. 8:10the top of the granite and as
221. 8:13the survey vessel moves along it can map
222. 8:16out
223. 8:16the top of the granite
224. 8:19creating a profile so that's the basis
225. 8:24of how
226. 8:24the seismic method works
227. 8:28here is a single reflective horizon now
228. 8:31of course a seismic survey vessel
229. 8:33will not map reflectors one reflector at
230. 8:35a time
231. 8:36it'll collect a swathe of data
232. 8:39representative of depth so it'll build
233. 8:42up a profile
234. 8:44something like this
235. 8:48the top reflector here represents the
236. 8:51sea bed so these data were collected
237. 8:53offshore italy in the mediterranean well
238. 8:56this is quite a small piece
239. 8:57of the data set the real virtue of the
240. 9:01seismic method
241. 9:02particularly in the offshore is the
242. 9:04ability to collect
243. 9:06very great profile lengths
244. 9:09so that's the amount that we can see
245. 9:10here is part of a bigger profile
246. 9:14where we can trace the continuity of
247. 9:17reflective horizons
248. 9:18across large distances but actually this
249. 9:20profile was collected
250. 9:22across an even greater part of the
251. 9:23mediterranean
252. 9:25so covering all this area here
253. 9:31so the seismic method is enormously
254. 9:34powerful
255. 9:35for relatively quickly mapping out the
256. 9:37structure of the outer part of the earth
257. 9:39over significant distances
258. 9:43you might have noticed as we've been
259. 9:44looking at these various images
260. 9:46that the colour shown on the images is
261. 9:49variable well actually that has very
262. 9:50little
263. 9:51geophysical significance
264. 9:55in creating the image you can select
265. 9:57different
266. 9:58color bars in the software so all these
267. 10:00are effectively
268. 10:01scientific images and it's about
269. 10:03aesthetics
270. 10:04not the data themselves
271. 10:13so the size of a reflection image is a
272. 10:15gift of sound
273. 10:16to create an image a gift of sound and
274. 10:18vision
275. 10:19and these images are enormously powerful
276. 10:22they've revolutionized the way in which
277. 10:24we can understand the stratigraphy
278. 10:25of sedimentary basins their structure
279. 10:28and tectonics
280. 10:30and because of this we can use them to
281. 10:32identify faults
282. 10:33and other hazards such as submarine
283. 10:35landslides
284. 10:37in the seabed and below the earth's
285. 10:39surface
286. 10:40they're fundamental for discovering and
287. 10:44developing earth resources and for
288. 10:46engineering
289. 10:49seismic reflection methods have
290. 10:51revolutionized the way
291. 10:52in which we view the earth
292. 10:56the images that we've used here come
293. 10:58from the virtual seismic atlas
294. 11:00and there's a lot more besides that you
295. 11:02can view in this
296. 11:03open resource