ASCE has some guidelines on guyed towers, for design of anchorage and other considerations.
Your question is overly broad for the forum and a geotechnical report for one area doesn't necessarily apply to another area, even with similar soil classification or profile. Geotechnical considerations are site-specific. You list your category as civil/env....perhaps you should check with a local geotechnical engineer to get an idea of the depth he/she would recommend for that area and your tower loading considerations.
I responded to your question in the other forum thinking you were referring to a deadman for a retaining wall...apparently you were considering a tower, but that didn't seem clear in your question.
You are also unclear in your question this time. Your first point (depth of exploration) refers to a monopole, while your second point is for a guyed tower. Which do you want?
