Product Focus: Pressure Inverted Echo Sounder (PIES)
Sonardyne’s track record in the development of wirelessly connected subsea instruments that position, monitor and communicate, meant the creation of an instrument to assist with reducing seismic variability and better understand the physical processes occurring in our oceans was a natural progression. We take a closer look at PIES and explains how measuring water velocity with a high degree of accuracy and repeatability sheds new light on survey data.
Offshore exploration is heading into ever deeper water as many shallow reservoirs gradually move towards the end of their useful production life cycles. Many deep water reservoirs are also geologically complex and require an increasing number of high quality seismic surveys to provide information on the changing structure and, ultimately, to maximise productivity.
Marine seismic acquisition is typically conducted using either several kilometre long streamers towed behind a surface vessel or by using seabed cables or nodes which are physically placed above the reservoir before shooting commences. These surveys can last from a few weeks to a few months and are often repeated as part of a reservoir surveillance programme at intervals ranging from several months to a number of years depending on the level of detail required.
Repeatability, repeatability, repeatability
Obtaining high quality images is often hampered by variations in the process of repeating the same seismic acquisition multiple times. This variability comes in many forms including changes to the geometry between the source events (shots) and the receiver positions (streamers, cables or nodes). Environmental differences such as the weather, currents and tides also affect the speed at which seismic pressure waves propagate through the water column.
This speed is known as water velocity by geophysicists who go to great lengths to ensure it is well characterised during each acquisition and across multiple surveys to improve the quality of the surveillance imagery and minimise uncertainty in the reservoir description.
The traditional method for measuring water velocity is to use a sound velocity probe repeatedly deployed through the water column from a surface vessel. This method is often considered suboptimal due to the variable deployment locations, the daily (or longer) intervals between measurements and that the probe does not always travel through the whole water column all the way to the seabed. A further limitation is the requirement for the vessel to remain stationary whilst each data set is collected. If repeated many times, this becomes a time consuming and costly exercise.
With these limitations in mind, in 2010 Shell geophysicists Paul Hatchell and Kanglin Wang approached Sonardyne with the challenge to develop a self-contained instrument that could be deployed on the seabed and left to autonomously measure the average water velocity over several months or even longer without any intervention. The result is PIES – Pressure Inverted Echo Sounder.
Measuring the water velocity using PIES involves sending an acoustic signal from the seabed which is reflected off the water-air interface (the waves act as multiple scatterers) and receiving the energy back again at the seabed. Special embedded signal processing detects the energy coming back and estimates the total travel time, measures the depth i.e. distance (using highly precise depth sensors) and so calculates the average water velocity of the entire water column as it varies with time.
PIES can be placed on the seabed by ROV or freefall deployed from a surface vessel. Once in position, it autonomously measures water velocity at defined intervals and records this data internally for wireless upload to an acoustic transceiver deployed over the side of a vessel. At the end of the survey, each PIES unit is collected from the seabed by ROV or it can make a floating ascent to the surface by activating its integral acoustic release mechanism.
Once processed, the user has access to a continuous and highly sampled time history of average water velocity as well as tidal history throughout each survey or even across multiple surveys, supporting the ideals of seismic surveillance. “
The ability to capture a complete time history of water velocity variations during our surveys is incredibly beneficial when it comes to minimising the variability in Shell’s reservoir surveillance activities,” observes Paul Hatchell. “PIES now plays an integral part in many of our surveys as the data it gathers helps us to obtain better seismic images which leads to a better understanding of our reservoirs.”
The benefits offered by PIES are now being recognised by other leading oil majors who are evaluating units deployed in the North Sea and West Africa.
Understanding our oceans – Subsea sound Speed
Supporting seismic surveys is not the only application for PIES. The continuous measurement of average water velocity and its inherent variability provides information that helps oceanographers to better understand the physical processes that occur in the deep ocean. This can range from the way in which sea currents transport energy around the planet and the nature of underwater weather fronts through to the dynamics of the atmosphere ocean coupling.
For this, Sonardyne has developed a long endurance version of PIES which, depending on the measurement interval configured, can remain on the seabed for up to five years. The novel design of the instrument ensures that it always lands on the seabed in an upright position, making it quick and easy for users to deploy off a small boat without the involvement or expense associated with a survey vessel and ROV.
Data can be uploaded on demand via a built-in acoustic modem or at the end of the survey period when the unit floats back up to the surface and is recovered.