I live in southern nj,this is my first winter with a geo system.I have had problems with the units running all night,during the day they run for 12 to 15 mins and then shut down for 4 or 5 min.The installer checked the brine temp,which is in the high 30s.He said this is normal for this time of the year,because we have been pulling heat out all winter.I thought the ground temp should remain around 50 degrees,am i wrong?

High 30's at this point in the winter is fine . and does not indicate a problem . Long run periods are also fine as long as the temp is reaching its set point . I would like to see longer off cycles 4-5 min is short adjusting stat sensitivity or the time delay on the system may need to be increased or installed but this also doesnt indicate a loop issue .
how have the electric use been and use of aux heat ?
air temps in and out of the system ?

Ace, I'm not a pro but I've learned a lot on here. Loop temps in the 30s are normal in the winter. The deep ground temps will stay around 50 in a lot of the US but the ground around your loops will cool from the extracted heat and the water temps in your loop will never quite warm up to the ground temp due to the efficiency of the heat transfer through the tubing. But all that is normal and you are still extracting a lot of heat.

If you're still reaching your thermostat set point at night it's probably fine that the unit runs all night at the coldest time of the year. That just probably means your unit is sized just right.

4-5 minutes is too short an off cycle. OK every now and then, but not on a continuous basis. Consider reducing CPH setting on thermostat if available. 3 CPH is a good starting point. 2 CPH is even easier on the system but may cause intervals of discomfort in a low thermal mass house

I'm not a pro. I would even try 1 CPH. On our 2 stage compressor, I have 1st stage set at 1CPH and 2nd stage set at 1CPH. Last month I decided to time cycle times. I was surprised to see (but maybe shouldn't have been) the system running 2 CPH in 1st stage. This is with a Honeywell thermostat.

I think this was happening, but I'm not positive. The first CPH ran as expected in 1st stage. After that 1st stage CPH was used, the thermostat had an additional CPH to use in 2nd stage. But, 2nd stage on our thermostat doesn't engage until 1st stage reaches 90% capacity, so 1st stage engaged a 2nd time during the same hour. Based on conditions, 2nd stage never engaged. This resulted in 2 CPH in 1st stage even though 1st stage was set for 1 CPH.

Anyway, I wouldn't be afraid to try lower CPH, even as low as 1 to reduce start/stop cycles (and wear and tear) on your equipment. Stages can always be increased if you don't like the temperature swing.

I have a high mass house, so 2 CPH works for us. I think that folks in frame houses in cold climates, or where there is significant infiltration, might notice the temperature swings likely at 1 CPH

I am a homeowner with geo and live in Upstate NY. My system was designed to run pretty much 100% of the time when it gets below freezing. As the season progresses, my entering water temp (you call your brine) drops from in the 40's to 30 (sometimes even a bit lower). My system uses the electric coil backup for a small percentage of the time... I chose this system rather than paying significantly more for a larger geo system.

To give you an idea of the cost of running 100% of the time when the ambient temp is below freezing here in upstate NY was about $800 last heating season. Now, we keep our tstate at 64 which is cold for most. We have an older house with 1950 sq ft. Our windows are updated and we have blown in cellulose in the attic, but our doors are old and leaky and we have rock wool in our walls.

The reason I'm sharing this is so you don't panic about your geo running a lot. Even running consistently, in my experience, it still does not cost that much and gives you a nice even temperature. Since I'm not a pro, I can't comment on the cycling off for only 5 minutes.

We have a conventional 18 year old stick frame house. Probably rates a 6 on a scale of 1-10 on tightness (with 5 being average). Nothing special as far as insulation.

With our CPH set to 1 for first stage and 1 for second stage, we still get 2 stages per hour. Maybe someone else can verify my findings that has 2 stage equipment on a cold night? While we have a Honeywell thermostat, it would be interesting to see what other thermostats do with the same CPH settings of 1 and 1 (with no time delay).

We previously used large setbacks with our old gas furnace, so a 1-2f temperature swing is not very noticeable to us. I find more frequent on/off cycles more annoying. :-)