"were commonly observed" is correct and usable in written English.
You can use it when talking about something that was seen frequently in the past. For example: "In the 19th century, the customs and traditions of the culture were commonly observed."
While no population estimates were made, earthworms were commonly observed during sampling on the Argonne.
Overall, carbamate insecticide and herbicide poisonings were commonly observed after organophosphate poisoning.
The current study intended to simulate conditions that were commonly observed in practical settings.
Better yields were commonly observed while a phase-transfer agent TBAB was participated in the reactions.
Tilted trees and meter-scale local landslides triggered by fault slip were commonly observed along the rupture zone.
From the analyses results, typical forms of pressure distribution shown in Fig. 4 were commonly observed to all specimens.
Furthermore, a double difference dataset was created by linking events that were less than 5 km apart and were commonly observed at least at six stations.
Using log-normal distributions of Ksat resulted in mean relative differences distributions that were commonly observed in experimental studies of soil water content variability.
Clay lamellae were commonly observed at the interface of sand and gravel, with the exception of a soil profile directly above a drain pipe.
According to the veterinary practitioners, skin lesions, lymphadenomegaly and weight loss were commonly observed, although these signs are not specific to CanL.
For three ZnO-deposited samples (−2, −2.4, and −2.8 V), the dominant ZnO (002) peaks were commonly observed, indicating that the ZnO was preferentially grown along the c-axis.
