Echo sounding

Obtaining an accurate and detailed depth-age relationship for an ice core is, of course, a necessary task for learning paleoclimate histories. Approximate time scales can be calculated using numerical models of ice and heat flow for the core site (Reeh, 1989), constrained by estimates of the modem accumulation rate and by measurements of ice thickness from radio-echo-sounding surveys. 

Robin, G. deQ., S. Evans, and J. T. Bailey, 1969. Interpretation of radio echo sounding in polar ice sheets, Philos. Trans. R. Soc. Lond., A265, 437-505. 

The speed of sound in water and sediments is assmned to be the same and constant. In reality, this is never true however, assuming it is true is good for a first approximation for shallow subbottom depths. Most recorders are set assimiing a sound speed of 1.5 km/s. If more accuracy is needed, you can refer fo Matthew s tables (1927) for water and Hamilton (1974) for sediments. Most charted ocean depths are based on echo sounding, which use an average or standard seawater sound speeds. 

The radar-echo surveys of the Antarctic ice sheet by Drewry (1975) provided information about the sub-ice topography but did not detect mineral deposits. More recently, Behrendt et al. (1980) combined the radar-echo sounding technique of Drewry (1975) with an airborne magnetometer survey which revealed that a laige part of the Dufek intrusion in the Pensacola Mountains is covered by the East Antarctic ice sheet. The enlargement of the area of this intrusion increased the amount of potential ore of iron, titanium, and vanadium in this differentiated body of gabbro, pyroxenite, and anorthosite. 

Drewry DJ (1975) Radio-echo sounding map of Antarctica ( 90°E-180°). Polar Record 17(109) 359-374 Drewry DJ (1976) Comparison of electromagnetic and seismic-gravity ice thickness measurements in East Antarctica, J Glaciol. 15(73) 137-150... 

DeUsle G, Thierbach R (1989) Radio echo sounding and geothermal investigations in the crater area of Mt. Melbourne, north Victoria Land, Antarctica. Geologisches Jahrbuch, Reihe E 38 435 53... 

Drewry DJ (1972) The contribution of radio-echo sounding to the investigation of Cenozoic tectonics and glaciation in Antarctica. In Price RJ, Sugden DE (eds) Polar geomorphology. Inst Brit Geol Spec Publ 4 43—58 Drewry DJ (1973) Sub-ice relief and geology of East Antarctica. 

The best method of monitoring is a multibeam echo sounding (Hermans Degens, 2011). The program of inspections includes periodical. 

The cost of hydrographical survey due to the technological development of echo sounding survey systems is still getting cheaper. Since last 10 years the cost dropped down about six times. 

All soundings shall be carried out by means of a recording trace dual frequency echo-sounder with sufficient sensitivity to permit measurement of the bed levels to an accuracy of +/-100 mm. The echo sounding equipment shall be regularly calibrated by means of a sounding plate. 

Echo sounding is carried out with acoustic equipment normally mounted underneath a ship. The two kinds of systems widely used in bathymetrical measurements are ... 

Depending on the aim of the bathymetric survey, the equipment and line density is chosen. For an overall bathymetric survey that is carried out for example on the North Sea, a line density of one hne every 25 or 50 metres using a single beam echo sounder can be sufficient. On the other hand, in case of monitoring the transport behaviour of so-called sand waves much more detail is needed and hence the use of multibeam echo sounding to obtain a full coverage of the sea bottom is advised. 

The multi beam echo sounder is used in coastal areas where water depths range from about 2 metres below the ship up to about 1000 metres or more. Deep echo sounding with a multi beam system is possible deeper than 1000 metres, but the data coverage will decrease due to the travel time of the acoustic waves. This causes a loss of sea bottom detail (an important aspect of multi beam). For deep echo sounding, a single beam echo sounder is generally preferred because it is easier to use than multi-beam echo sounders. 

Objects and obstacles located on the seabed give a sharp reflection of these pulses and create an acoustic shadow on their lee side. Thus, an image is obtained similar to an aerial photo at sunset, where shadows are accentuated, so that protrusions can be detected and an estimation of their height can be made. An absolute height measurement cannot be performed with the side scan sonar, for this echo sounding or multi-beam techniques are required. 

---