In general, the simple present tense is used to describe an action that that occurs now or on a regular basis. In scientific reports or papers, this tense is used to describe a generally accepted scientific fact or a personal statement. Therefore, it is typically applied in the introduction section, in which you give a literature review or statements of main ideas.

For example: “This thesis examines the first approach”

Simple present is also used to refer to figures and tables in your text, for example:

“Figure 3 shows that…”

The past tense emphasises the completed nature of a past activity or event. This tense is used to describe such past activities or events, for example:

“The drawdown of the Mediterranean water level caused erosion and deposition of nonmarine sediments”

More importantly, past tense is used to describe the methods, results, or conclusions of past research, including your own study that you are reporting on. For example:

“Vissers et al. (2013) found that the maximum extension in the eastern end of the Piemonte Ligurian Ocean was ~315 km”

or

“The measured piezometric levels were interpolated to a groundwater table for the entire study area”

or

“We demonstrated that nitrate concentrations differ strongly between the major flow routes”

Note however that you descriptions of rocks and structures always need to be in the present tense.

Present perfect tense is used to describe unfinished actions that started in the past and continue to the present. In scientific texts, the use of this tense is usually limited to the introduction section (literature review) to indicate that research in the area is still continuing or still has immediate relevance. For example:

“Recently, there has been a strong debate about the melt rate of Himalayan glaciers”

or

“This method has been effectively used in snow cover mapping”