"It has been relatively easy to follow southwards the wake left by this oil slick and observe its effects on sand and sea creatures."
This sentence is fine grammatically but maybe a bit clunky, you should have the directional adverb 'southwards' after the object. e.g. "follow the wake southwards".
Also, when you say "sand and sea creatures" I'm not sure if you're trying to say "sand creatures and sea creatures", or if you're really interested in how the oil is effecting the sand itself. At any rate, we don't really say 'sand creatures' in English, so if that is what you're trying to say, you should find a different way to say it.
I'd write this:
"It has been relatively easy to follow the oil slick's wake southwards and observe its effect on the environment and wildlife."
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"American *satellites* were used for that but found no trace. The oil slick seems to trick *satellites*."
Be careful, satellite not "satelite" in English! Also, no trace of what? "found no trace *of it*"!
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"Based on photos made available by Europeans, researchers at the University of Bahia saw what could be a 22-square-kilometer new oil slick off the Bahia coast. This possibility was ruled out."
Change the order of ‘new’ and ’22-square-kilometer’. Adjectives have a natural order, and normally size adjectives come before adjectives about the quality of it. So, it should be “new 22-square-kilometer oil slick”.
Also, these two sentences are related. One common problem people have when they translate is they miss out on all the small words that link sentences together. In this case, I would write:
“However, this possibility was ruled out.”
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“The oil was thought to *have* spread all over the surface.”
Spelling!
