You can use a lamp viliqn combined with a heat pad on the side of your frogs tank. The heat mat/pad should either be on the back or side. Make sure the water dish is on the other end of the tank accross from the heat pad.

If you deside to use a lamp buy a Fluker's 5.5" dome lamp with a dimmer switch. The dimmer lamp will help you keep the temps up without having the bulb on high. You can adjust the amount of light and heat that comes from the bulb. For day time the Exo -Terra Daylight Basking Spotlight. Make sure to not go higher than a 50 watt bulb. Anything higher can harm your frog. For night time you can use the Exo Terra Heat Glo. Again no higher than 50 watts. Be sure to give your frog a place to hide from the light like they would naturally. A fake plant with large leaves for shade works well.

I hope this helps you deside which route to go. Like everyone said though, there should not be a heat source under or in the ground/Substrate.

PS. Do not appolgize for your english. I know plenty of people who's first language is english and do no speak it very well and especially can type it Lol!