Undisturbed temperature

Using the drop-down menu, three methods are provided to identify the undisturbed ground temperature T_g, and one of them also evaluates the mean annual ground temperature T_a (see Vertical profile). Whichever method is selected, the T_g it produces is the one handed to the first-order approximation and used as the starting position of the T_g slider of the short-term g-function.

T_g and T_a are two different temperatures - The panel reports both, and they are easy to confuse, so it is worth being precise about what each one is.

• T_g, the undisturbed ground temperature, is the temperature of the ground as it stands immediately before the heat injection starts, averaged over the whole height of the borehole. Both quantifiers matter: it is read at one moment in time, not over a year, and it describes the entire column the borehole exchanges heat with, not one depth. It is the initial condition both interpretation models assume, and everything the test measures afterwards is a departure from it.
• T_a, the mean annual ground temperature, is what the ground averages over a full year. It is obtained by keeping only the part of the ground that seasonal variations no longer reach, that is, by discarding the near-surface zone whose temperature swings with the calendar. It is T_a, not T_g, that should be carried into the sizing of a system: a borehole field is sized over years of operation, so the temperature it works against is the annual average of the ground, not the value the ground happened to hold on the day of the test.

In practice T_a comes from a vertical profile, and the work consists in identifying the section of that profile which the seasonal signal no longer disturbs. This is why T_a appears only when a profile has been read, and why the two methods that work from the test data alone cannot produce it.

The three options read the ground in three different ways. A depth-resolved temperature log read down a borehole (Vertical profile) gives temperature against depth, and the mean annual temperature T_a with it. Circulation phase reads the temperature of the fluid circulated through the loop without heating. Manual input takes a value established outside the test, from a nearby log or a regional dataset. Which of the three to trust is a question of field practice, taken up at the end of this page. Loading a dataset selects Circulation phase automatically, since that method reads T_g from the measurements themselves.

Manual input - This method allows the user to directly provide a value for T_g, from -10 to 40 °C.

Circulation phase - This method gives the average over the whole height by construction: the fluid has just travelled down and up the entire loop, so the temperature it comes back at is already the mixture of everything it met on the way. If a circulation phase was identified in the previous step, the dual-range slider allows selection of the data deemed representative of T_g. To refine your selection, click the slider, then press the Q (←) or E (→) keys to move the start of the window and the A (←) or D (→) keys to move its end. If the test carries no circulation phase, this method has nothing to read and the value is reported as a dash.

Selecting this method draws the circulation phase alone in the panel, inlet and outlet temperatures against time, with the two window boundaries marked and the resulting mean drawn as a horizontal orange segment between them. That segment is the T_g the console reports for this method, so its placement against the measured curves shows whether the window sits where you meant it to.

Vertical profile - By selecting this option from the drop-down menu, you can load a vertical temperature profile using the import button . The file must be a comma-separated values (.csv) file with 2 columns and with a full point as the decimal separator. The first column is the depth in meters (m) and the second column is the temperature in degrees Celsius (°C). A profile must hold at least two measurements, carry no negative depth, and its depths must increase strictly from one line to the next: the weighting that turns the readings into T_g and T_a gives each measurement the thickness of its own slice, so a profile listed from the bottom up, or holding two readings at the same depth, would produce a meaningless average with nothing to show for it. A file that breaks one of these rules is refused outright and the offending line is named. The table can also be edited in place, and the same rules apply cell by cell: a value that would break one of them is rejected and the previous one restored. The profile is then drawn in the panel, temperature on the horizontal axis placed at the top and depth increasing downward, so the chart reads like the borehole itself. The two dual-range sliders standing beside it select the depths that produce each temperature, and the two windows are not chosen the same way. The blue one, for T_g, should span the whole height of the borehole, from the surface down to its base, since that is the average both models expect. This is where the profile differs from the circulation phase, which delivers that average whether you want it or not: here it is yours to get right, and a window that stops short of the bottom, or that starts below the surface, returns the mean of a portion of the column rather than of the column. The red one, for T_a, must be confined to the section the seasonal signal no longer reaches, which the profile itself reveals: near the surface the readings follow the season, and below a certain depth they settle. Placing the red window in that settled section is what determines T_a. To refine an interval, click its slider, then press the Q (←), E (→), A (←) or D (→) keys. The four labels beside the sliders report the depth in metres of each boundary, not a row number, so the window can be read directly against the profile; the same four rows are tinted in the table, blue for T_g and red for T_a. A profile can equally be pasted from a spreadsheet with the paste button or Ctrl+V, in the same two columns, and the loaded profile copied back out with the copy button or Ctrl+C.

Summary & charts: the chart of the selected method and the temperatures in the console

Comparing the methods - The first page of the console, in the block below the phase statistics, reports T_g for all three determination methods side by side, manual entry, circulation phase and vertical profile, whichever one is currently selected here, plus T_a where a profile has been read. A fourth column shows the T_g the short-term g-function currently carries on its own slider, which it calibrates rather than determines. Comparing the columns is only meaningful when the profile window spans the full height of the borehole; short of that, the circulation phase and the profile are not measuring quite the same average, and a gap between the two says more about the window than about the ground.

TRT in practice: why measuring the undisturbed ground temperature accurately matters

Why it matters. T_g is not a peripheral input read once and set aside: it is the baseline every subsequent number is read against. It enters the first-order approximation as the reference the regression is built from, sets the starting position of the short-term g-function's T_g slider, and, once the interpretation is complete, becomes the boundary condition the borehole field is sized against for decades of operation. An error here does not stay contained to one figure: it moves λ_s and R_b^* together with the sizing that follows from them. How much it costs follows from the sizing equation itself: in a design driven by the heating peak, the borehole length is inversely proportional to the difference between T_g and the minimum entering fluid temperature the design allows, so an error on T_g is an error of the same absolute size on that difference. A 1 °C error is therefore worth about 10% of the total GHE length where the design works against a 10 °C approach — T_g at 10 °C and a minimum EFT near 0 °C — and about 7% where an antifreeze mixture allows the loop down to −4 °C.

Which of the three methods to trust. They are not equally reliable, and the trade-offs between them are examined in detail by Gehlin and Nordell (2003). A logged vertical profile is the most informative, because it shows how the temperature varies with depth and yields T_a as well. The circulation phase is the most common field practice, but it asks for a circulation long enough that the fluid and the rig have come into equilibrium with the ground, and it remains exposed to the heat the circulation pump adds to the loop. Manual input is best reserved for a value obtained independently, from a nearby log or a regional dataset.

Reading the circulation phase. The circulation must run long enough to be worth reading. A test unit arrives empty and is filled on site with water that is not at T_g, warmer in summer and colder in winter, and its above-ground pipework starts at the ambient temperature as well. What the first minutes of circulation measure is therefore the water that was put in and the rig that holds it, not the ground. Stability can take several hours. Place the window on the plateau the record settles onto: late enough for the heat or cold introduced on site to have dissipated, and short of the point where the accumulated pump work has begun to lift the curve. Gehlin and Nordell (2003) approach the same problem from the other end, logging the flow temperature at a short sampling interval so that the first pass of fluid through the loop can be read before the pump has had time to add anything to it. Either way the quantity being avoided is the same, and the record itself decides which end of the phase is cleaner: take the plateau when the fill water dominates the first hours, and the first pass when the pump drift never lets a plateau form. The shape of the phase tells the two apart. A drift over the first hours is the fill water and the pipework coming to the temperature of the ground, downward in summer and upward in winter; a slow rise that continues once that has settled is the pump adding heat to the loop.

Log a profile when you can. A profile is worth more than the two numbers this panel takes from it: it is informative well beyond T_g, a case made from field practice by Holmberg et al. (2018). Two instruments can produce it: a fibre-optic system logged along the borehole, or a submersible temperature sensor fitted with a clock or a pressure sensor, the pressure reading converted into depth. With a submersible sensor, hold each depth long enough for the reading to stabilise before moving to the next; a value taken before the sensor has equilibrated with its surroundings is not representative of that depth. Two ways are open for the measurement itself. The probe can be lowered straight into the borehole while it still stands full of water, before the grout is placed, which reads the ground before the installation has disturbed it at all. Or it can be lowered down the pipes of the completed loop, filled with water or with the antifreeze mix, provided at least a week has passed since the installation so that the heat released by the drilling and by the setting grout has dissipated. Where a layered ground is suspected, log the profile even if T_g is to be read from the circulation phase, and keep the log with the test record.

Timing matters too. In some regions T_g and the mean annual temperature T_a differ substantially, and a test run at the end of summer or the end of winter can return a near-surface temperature that is not representative of the ground the borehole field will actually see over the year. Where the two diverge, record both and note which one the downstream sizing and design should use.