The term geothermal was originally coined to describe the heat that the earth produces from its molten core toward the surface of the earth. A few decades ago, it became the term used for the human created systems that capture this heat and use it for human involved processes- like heating a building or heating water that is used in a manufacturing process. True geothermal systems involve drilling holes several hundred feet straight down into the earth – far enough to start to tap into the heat provided by the earth’s core.
In more recent years the term geothermal has been used to refer to any heating or cooling system that involves digging a hole or trench to capture the heat (and/or cooling) at any depth over about 5 feet. It’s not exactly a correct translation, but as with the evolution of words in almost every society, the changes of definition are inevitable.
A more correct term for the “shallow” depth capture of heat or cool would be geosolar (aka annualized geosolar) – but that really only covers the “heating” part. The sun beats down on the earth and a few feet of the surface is heated by the sun,rather than by the heat of the earth itself. And one doesn’t have to dig too far down in most places to reach an almost year-round constant temperature. (Maybe a better term would be something like “geo-influenced” systems.)
In most populated areas, it is possible to use a these shallow depth systems of only about 5-20 feet. At this depth, the temperature hovers around 50-55 degrees Fahrenheit all year long due to a principle known as thermal inertia. In this instance, thermal inertia is the tendency of the mass of the earth at that depth to stay the same.