Best way to drive long signal cables
I'd consider doing it digitally because the cable losses won't affect the analogue signal. You'd need 16 bit resolution - this gives a resolution of 15 \$\mu V\$ in 1 volt and you'd need to sample at 250k Sps to give your DAC filter a chance to reproduce 100 kHz.

Keeping it as tight as possible means probably a 20 bit word where the first 4 bits are a constant header so that you can recognize where the MSb is going to be. You could just about do it with 18 bits but 20 bits will play safe.

So it's 20 bits sent 250,000 times per second - that's a data rate of 5 Mbps - how far can you get this down decent coax? The sort of coax I've used in the past has transmitted 40M bpsec for 350m so maybe at one-eighth the data rate you'd get 8 x 350m = 2.8 km. The coax was 12mm diameter stuff and 75 ohms but I can't remember the part number - I know I went for the "fattest" coax I could fit in the cable glands and hoped it would be OK (and it was).

Looks like a couple of repeaters would help at 2km and 4km. Screened twisted pair might be even better than coax. Try looking up twinax.

Sending it analogue will be a problem unless you are prepared to suffer degradation of the signal in the frequencies above 100 kHz as the cable characteristic impedance changes from resistive-capacitive to purely resistive - you could try pre-emphasis and de-emphasis to help. Noise will surely be a problem over that distance and maybe someone else who has tried this can offer some advice. External pick-up is an unknown quantity but at least for digital you have a threshold that noise has to exceed.