Seems like 100 seconds at ISO 400 ought not to be too dark in light like the picture.
Use tripod; a sturdy one. Lock up mirror or use liveview. Use a shutter release cable. Use low ISOs. I'd probably stay below 400. 100-320; some cameras actually have less noise at 320 than at 100 so you're experience may vary. I also set the C.Fn for long exposure noise reduction to 'on' or 'auto'. If you're having trouble focusing, bring a bright flashlight to help.
If you don't want to run long exposures only to find out it was off, try bumping ISO way up and/or opening aperture up and shoot test shots till exposure looks good. Don't worry about shaking or anything just the exposure. Then just adjust ISO down to 100 or whatever and reset aperture to where you need it for DOF and set shutter where it needs to go or use bulb if necessary. For instance, say you're set at ISO 3200 at f/2.8 and shutter is 1/8 and you get enough light in a test shot. Move ISO to 100 or down 5 stops, then lengthen the shutter by 5 stops to 4 seconds. If you need f/8 stop down to f/8 from f/2.8 or 3 stops, so then lengthen the shutter again by 3 stops to 30 seconds. You get yourself pretty close to the ballpark this way and then you can make slight adjustments later.
Hope this helps. Also FYI, it's not recommended to post other peoples' images in your post. Just post a link to them.