Temperature contrasts in the water column inferred from amplitude‐versus‐offset analysis of acoustic reflections

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Abstract
We show that seismic amplitude‐versus‐offset (AVO) analysis can be used to remotely quantify the temperature contrasts responsible for acoustic reflections in the ocean. We inferred the sound speed and density contrasts associated with two reflections in the Norwegian Sea by comparing their AVO response to that predicted by the Zoeppritz equations. Estimates of temperature contrasts were calculated from sound speed contrasts using Wilson's equation and compared with in situ temperature measurements from coincident expendable bathythermographs (XBTs). A strong reflection from Line 9 is best explained by a −6 m/s step in sound speed, corresponding to a temperature decrease of ∼1.5°C. A weaker reflection from Line 11 yielded a sound speed contrast of −1.2 m/s, corresponding to a temperature decrease of ∼0.3°C. The inferred temperature contrasts match those found in XBT data remarkably well. L1‐norm misfits between predicted and calculated values indicate that the acoustic impedance contrasts are principally controlled by sound speed contrasts, rather than density contrasts. AVO analysis offers an accurate and robust technique for estimating sound speed (and therefore temperature) contrasts in the water column.

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