Frequently Asked Questions:

What do IES, PIES and CPIES measure?

An IES measures vertical acoustic travel time (VATT), also called tau (τ), round trip from the sea floor to the sea surface.
A PIES adds bottom pressure measurements to an IES.
A CPIES adds current measurements (50 m above the bottom) to a PIES.

Example plot shows travel time concentrated in a narrow range of approximately 0.01 seconds, raw pressure dominated by the tidal signal, small temperature variations and small magnitude bottom currents.

IESs are configured to emit twenty-four 12-kHz pings per hour, in programmable bursts of 4, 8, 12 or 24 pings. A representative travel time for each hour is selected using a quartile method (Example plot: τ burst (blue), hourly τ (red)).

Because the speed of sound is approximately a linear function of temperature, an upward displacement of the main thermocline leads to proportionally less warm water and a lower average speed of sound. This increases τ (Powerpoint Animation: Click on sound source in either water column to begin). Watts and Rossby (1977) describe the first scientific use of IESs to acoustically monitor depth variations of the main thermocline during MODE.

A robust empirical relationship exists between τ and vertical profiles of temperature, salinity and density. These relationships are obtained using historical hydrographic measurements from the oceanic region of interest using the "Gravest Empirical Mode" or GEM method (Meinen and Watts, 2000; Watts et al., 2001b).

The flowchart identifies the following steps:
- Assemble regional hydrographic data set
- Choose reference τ _index and sort hydrographic data by τ _index (Example)
- Fit cubic splines to data (Example)
- Final GEM field is a lookup table based on τ _index (Example)

Measurements from two IESs can be used, through geostrophy, to determine the vertical profile of the component of horizontal velocity normal to the line between the two IESs. An L-shaped group of three IESs determines estimates of both velocity components. A 2-D array of appropriately spaced IESs can map out the 3-D structure of the horizontal velocity field. Additionally, deep pressure and current measurements can provide referencing to make the velocity profiles absolute.

Tidal response analysis (Munk and Cartwright, 1966) determines the tidal constituents for each instrument. Tides are then removed from the pressure records.

Leveling is the technique of referring all measurements onto a geopotential surface. Once leveled, pressure gradients can be used to calculate absolute geostrophic currents.

Watts and Kontoyiannis (1990) dedrifted bottom pressure by the subtraction of a linear plus exponential curve fitted to each time series. However, their method does not distinguish between instrumental pressure drift and real ocean signals. When deep current measurements are available the new technique described in Donohue et al. (2009) can be used. With this method, ocean signals determined with the measured currents are removed from the measured pressures, and curves are fitted only to the remaining instrumental drift. An advantage of this method is that it simultaneously dedrifts and levels pressure measurements. Figure 5 from Donohue et al. 2009 illustrates this new procedure.

Maps of the daily pressure and current fields are obtained by combining leveled pressure measurements and current measurements in a multivariate, nondivergent optimal interpolation (flowchart, Watts et al., 2001a).