Spatial and temporal challenges
Seabed deformation can occur on a variety of spatial and temporal scales, with earthquake events characterising rapid local movement, whereas others slow slip events can occur over much longer timescales of months or even years. With the latter increasingly being regarded as precursors of the former, there is a need to measure these in order to understand how they relate to their catastrophic counterparts. Some of the areas of greatest interest are subduction zones where the oceanic crust collides with the adjacent continental crust and is forced beneath it forming an ocean trench, often in several thousand metres of water.
Long term observations
Implicit in the term ‘slow slip’ is the need to measure seabed deformation over periods of several years. At a fundamental level this requires low-power sensing technology and the ability to manage battery power, but it also means robust and reliable technology that isn’t susceptible to breakdown.
Central to these studies is the capability to measure centimetric or even millimetric changes in the seabed both in the vertical and horizontal planes. Commonly the former is measured with a pressure sensor, while the latter is measured by acoustic ranging between seabed transponders. To achieve this to the required resolution means that pressure sensors need to be calibrated to determine drift rates, but also that environmental conditions such as temperature that affect acoustic transmission need to be also accurately measured.
Seabed geodetic studies are by nature likely to be remote from land and often in deep water, so timely access to data can be challenging. At a minimum, data recovery by ship needs to be efficient and not waste costly ship time, however ideally more regular access to data harvested by long endurance unmanned surface vessels is highly desirable.