资料题：Oceanographers have based their estimates of the thickness of the sediment layer on（　）。

The acquisition of new knowledge about a problem does not always help scientists solve it.Sometimes it complicates the original problem.A striking example of this occurred when thefindings of the international Indian Ocean Expedition were reported in 1962.Oceanographers(海洋学家)have long been perplexed by the fact that there is so little sediment(沉淀物)on the floors of the world's oceans.Their calculations, based on the estimated rate of sediment accumulation and on the time this process has had to work, indicate that there should be a layer of sand,rock particles, and organic matter from two to two and one-half miles thick over the bottom of the seas.Their findings of the actual thickness of the sediment layer, however, have indicated that it is not more than a fourth of this depth.(Some oceanographers believe he deposit of organic material alone, falling in what they call the"rain of death," would have accounted for this amount of sediment.)Then in 1962 the expedition scientists reported that they found the bottom of the Indian Ocean much rougher than had been thought.To the oceanographers, this means that their estimates of the actual thickness of the sediment layer have been too high.Thus it may be that aneven thinner layer of sediment exists than that which oceanographers had previously considered too small.