The phrase "could not be observed" is correct and can be used in written English.
For example, "The migratory patterns of the birds could not be observed due to the clouds in the sky."
This re-orientation could not be observed for Source.
Inclusions and granular facet areas could not be observed.
Nothing mattered, in their view, that could not be observed and measured.
These properties could not be observed in TA and D populations.
Nevertheless, such reversibility could not be observed under anhydrous methanol or under the hydrocarbon vapors.
Cathodic redeposition at the ring cathode could not be observed in any case.
For HDPE, any indication of a glass transition could not be observed.
Both the scramble/NHDF and NHDF cells did not survive, and clonal expansion could not be observed later.
A systematic thickness dependence of the linewidth at zero magnetic field could not be observed for the samples investigated.
Consequently, stable oscillations could not be observed.
Differences between perfluoroalkyl carboxylic acids (PFCAs) could not be observed.
