seismic reflection is the principal way in which we can image the outer part of the solid earth it can create spectacular images like this this image comes from the browse basin which is just off the northwest coast of australia and it shows how sedimentary layers have built out from the continent into the surrounding ocean the image is built by sending sound waves into the earth and then collecting them again in receivers and we use these to build an image you may be more familiar with this approach through the sonar mapping of the seabed essentially a form of echo location where sound waves are sent into the water to reveal the seabed seismic takes it further to reveal what lies below the seabed and create images like this well this is a two-dimensional slice down into the earth but you can actually create three-dimensional images using the seismic reflection method so here's a seismic image of part of the seabed offshore nigeria in the eastern atlantic showing a submarine channel and by working in three dimensions you can create a cube of seismic imagery the sides of which you can see here and then you can slice this horizontally in so-called time slices to reveal ancient channel structures preserved in the rock record way below the seabed spectacular 3d structures geologists use these to get a better understanding of the three-dimensional structure of the subsurface in this case here for planning wells drilled into the floor of the northern gulf of mexico offshore the southern part of the united states but for now let's just consider two dimensions how are images like this created first we need a seismic source a source of acoustic energy that could be transmitted into the earth and this will send out a wavefront down into the subsurface radiating away from this source the trajectory taken by this wavefront defines a series of ray paths that are radial from the seismic source let's just consider one of these which migrates down through this subsurface through this yellow layer and it hits an interface with an underlying layer shown here in green this boundary represents a contrast in the acoustic impedance just like electrical impedance acoustic impedance represents the resistance to the transmission of the seismic energy through the medium and it depends on the velocity at which that seismic energy can be transmitted and the density of the material a contrast in these properties will deflect the ray path part of it will be deflected back to the surface as a reflection and other components of the seismic energy will continue further into the subsurface refracted at this interface to have two components a reflected component and a refractive component the refracted component continues on its way to another interface where it again can be reflected back towards the surface and again a residual part of that seismic energy can continue further still into the subsurface so these reflected rays must be collected back at the earth's surface by receivers so that's the basis of the method we have a seismic source which sends energy into the subsurface some of that energy is reflected back and we collect that reflected energy in a series of receivers and by repeating this type of experiment again and again we can build a profile such as this one here from southeast spain so here's the basis of this that we have a seismic source with a controlled input which is a waveform which goes into the earth and then reappears at the earth's surface highly modified the earth then acts as a filter which modifies the input to form an output wave structure that we record in the receivers so our mission is to try and understand what this filter consists of so of course there are a number of naturally occurring seismic sources such as earthquakes and volcanic eruptions but these are rather uncontrolled the seismic reflection method uses a controlled source in water this could be provided by air guns and on land it may be provided by explosives or more commonly by vibrasize which are shown here in the photograph as an array of bouncy trucks that send controlled vibrations down into the earth so those are the principal seismic sources the receivers consist of hydrophones if they're on water or geophones on land and our mission then is to understand what goes on in the earth in terms of differences in its ability to transmit seismic energy the velocity structure in other words and its layering so let's consider how the velocity at which scientific or acoustic energy can be transmitted varies from different types of material acoustic energy sound in other words is transmitted in air and that velocity varies depending on the air temperature at 20 degrees centigrade and at sea level air transmits sound at around 343 meters per second water transmits seismic energy at around 1.4 to 1.5 kilometers a second this variation may relate to the temperature as with air but also its salinity and therefore density sandstones also show variation in this case between three and four kilometers a second these variations largely reflect how lithified the sandstone is therefore in terms of its porosity granite in contrast has a seismic velocity of around six kilometers a second it's denser than sandstone and has no porosity so let's see how this might play if we put some seismic energy through these materials so here it comes we're going to put a seismic wave through here and here it goes the granite's arrived the sandstone's arrived here comes the water about to get there and the air it's still coming through the seismic energy is still taking a long time to get through the air so these animations show how much slower it is to get sound through air compared to the equivalent energy source through granite or sandstone or even water so materials have different seismic velocities so let's imagine we're looking at granite and sandstone underwater seen in a profile and we have a survey vessel that will cruise across the water's surface with the seismic source on the boat and there's a receiver strung out the back floating on the water surface and that will send seismic energy down into the subsurface some of that energy will reflect back off the interface between the sandstone and granite and be collected at the receiver in detecting this reflection the survey vessel has detected the top of the granite and as the survey vessel moves along it can map out the top of the granite creating a profile so that's the basis of how the seismic method works here is a single reflective horizon now of course a seismic survey vessel will not map reflectors one reflector at a time it'll collect a swathe of data representative of depth so it'll build up a profile something like this the top reflector here represents the sea bed so these data were collected offshore italy in the mediterranean well this is quite a small piece of the data set the real virtue of the seismic method particularly in the offshore is the ability to collect very great profile lengths so that's the amount that we can see here is part of a bigger profile where we can trace the continuity of reflective horizons across large distances but actually this profile was collected across an even greater part of the mediterranean so covering all this area here so the seismic method is enormously powerful for relatively quickly mapping out the structure of the outer part of the earth over significant distances you might have noticed as we've been looking at these various images that the colour shown on the images is variable well actually that has very little geophysical significance in creating the image you can select different color bars in the software so all these are effectively scientific images and it's about aesthetics not the data themselves so the size of a reflection image is a gift of sound to create an image a gift of sound and vision and these images are enormously powerful they've revolutionized the way in which we can understand the stratigraphy of sedimentary basins their structure and tectonics and because of this we can use them to identify faults and other hazards such as submarine landslides in the seabed and below the earth's surface they're fundamental for discovering and developing earth resources and for engineering seismic reflection methods have revolutionized the way in which we view the earth the images that we've used here come from the virtual seismic atlas and there's a lot more besides that you can view in this open resource