One of the most common techniques I use in my work with field recordings is the use of EQ to create harmonic resonance, either emphasising a preexisting resonance or creating a new one. Using a large boost at a very narrow frequency, as long as some material exists within that frequency in the sound source, a resonance will become audible, effecting both the texture and the envelope of the sound.
These experiments began three years ago, with a recording of a street sign struck with the side of my fist. The resulting sound had various resonances, both low frequencies from the vibration of the sign, and high frequencies from the impact and attack. To accentuate these frequencies, I loaded an EQ plugin onto the track, and then created an 18dB boost, with a Q of 10. I swept the boost through the frequency range to find the point where the resonance was loudest, and then added extra boosts at harmonics above this. I repeated this process for the higher frequency range, using the partials from the impact.
Another technique I use involves recordings with a much higher noise content, such as waves on a beach or wind through branches. In these instances, boosting a range high frequencies can add a sense of space, and allow an otherwise neutral soundscape to function harmonically. Options include: emphasising a single, specific frequency; bolstering a frequency with partials following the harmonic series; creating tonal chords or intervals with resonances; and finally, boosting a group of unrelated frequencies, to produce an atonal cluster of tones. Depending on the strength and clarity of the frequencies boosted, they can be used to relate to other musical elements within the piece. By 'tuning' the recording to a particular scale degree, it can either be used to strengthen or weaken the tonic, or as a leading tone to propel harmonic movement.
A more complex combination of these techniques is to create chord progressions by automating the frequency of boosts. This allows the resonances to be adjusted in pitch as a piece progresses, and this can be used to create harmonic change as with a standard, polyphonic instrument. I have not yet used this technique in a piece, as it is very labour intensive without specific EQ plugins designed to facilitate such a technique.