Hello everybody! We are in the middle of our last bit of operations, and so far so good. We have a really special treat today on this blog with Christine Peirce taking the helm to explain the science. Take it away Chris!
Imaging the lower crust and upper mantle – a seismologist’s perspective
Throughout the cruise we have been deploying ocean-bottom seismographs (OBSs) onto the seabed, each recording its own multi-component dataset from three geophone sensors used to measure ground motion in three dimensions, and a hydrophone which measures pressure waves in the water column. These OBSs come from the University of Texas in the USA, University of Durham in the UK and Geomar in Germany.
Why do we record airgun seismic signals using seabed instruments?
There are two approaches to seismically imaging beneath the Earth’s surface using man-made seismic signals. In the first approach – reflection seismics – a multi-sensor streamer is towed behind the vessel which measures the man-made signals that travel near-vertically down into the sub-seabed and reflect from the boundaries between individual rocks layers due to their difference in density. The resulting images are in the two-way (there and back) travel time of the recorded reflections, and give a cross-sectional-like view of the sub-surface as if it was cut vertically through by a knife. Unfortunately, these images contain no information that allows them to be converted into true depth, so we cannot answer the question “How deep is this layer beneath the seabed?” or “How thick are these sediments?”
To answer these questions we need to know the speed, or velocity, at which each seismic signal travels through each layer, including the water layer. The water layer is a relatively easy velocity to measure using a sound velocity tool suspended from a wire lowered to near the seabed and back again. The velocities of rock layers are not so easy to measure. However, with these velocities we can convert the measured reflection times into distance much as you would use the speed limits on roads and the distances between two points to work out the time it would take to travel between A and B.
This is where an ocean-bottom seismograph (or 36, which is the maximum we have had deployed along any seismic line during this cruise at any one time) comes in useful and we use the seismic refraction approach. An example type of OBS from the UK national pool is shown below.
This is the primary sub-seabed imaging tool we have been using during the cruise. By synchronizing the OBS’s internal clock to the same clock against which we time the airgun shots, we can measure the time it takes for signals to travel from the airgun array to each OBS on the seabed, and if we know their distance away from the shots, we can work out the speed the signals travel through each sub-surface layer. We use GPS for this purpose as it can equally well provide an accurate time source as it can tell you where you are at any point.
The figure above shows how the method works and how it can be used in conjunction with reflection surveying “killing two birds with one stone” and making cost-effective use of the expensive ship time that that has been awarded for this project. So, we have also been towing a short hydrophone streamer to record reflections from any sediment layers.
We have been using a G-airgun array to generate the several thousand seismic signals that we have generated so far during the cruise, all of which the OBSs have recorded. In our work area, the crust is somewhere from a few km thick beneath the ocean, to up to 20 km thick or more as we approach the Honduran continental margin. The sea water throughout the work area is also very deep at more than 5000m in many places and so the crust-mantle boundary – or Moho as it is named after the eminent seismologist Mohorovicic – could be just anywhere from a few km below the sea surface at the mid-Cayman Spreading Centre, up to 25 or more beneath the Honduran continental margin. So to image the deepest part of this boundary, we need to propagate seismic signals to more then 25 km below the surface and to at least 75 km laterally to see these signals returning from depth where they have travelled through the mantle, to our instruments located on the seabed.
The photograph below shows the G-airgun array behind the FS Meteor’s stern, with the air bubbles for the sources just breaking the surface by their towing floats.
An example G-airgun source refraction data plot is shown below with an outline interpretation added.
Analysis of the OBS data allows us to build a model of the sub-surface in terms of the speed at which the seismic sound signals we generated travel through the rocks to the OBSs on the seabed. An example from an old piece of oceanic crust in the Atlantic is shown below. In the Cayman Trough the oceanic crust is thought to be very different from this, with very little or no gabbro. We will find out if this is true when we analyze the data back in the lab.
The OBSs also record large global earthquakes travelling through the work area as well as small local earthquakes originating along faults located at the Mid-Cayman Spreading Centre. We have recorded many of these small local events throughout the cruise with our OBSs, during both shooting and non-shooting periods as the examples show below with an earthquake recorded while airgun shooting shown at the top, and an earthquake that occurred during a non-shooting period shown at the bottom.
– Christine Peirce (Durham, UK)