I’m going to attempt to answer all three of your questions (this is a long answer ~2500 words, so be prepared). Both Colony and Emergency Scenarios.
(1) Basic overview of Remote Sensing techniques
(2) You found your PlanetZ, now what! How to find a suitable location for a settlement taking into account biomass and mining options from orbit? (I assume you mean mining as you mentioned locating metals and metal ores).
REMOTE SENSING. It’s a real existing thing. Here is a very basic overview of the very basics.
One of many possible links
We have been measuring our planet from orbit from about the 1970’s. We have about 50 years of experience and we still have a lot of trouble with it. Equipment is expensive, and as soon as you get it upstairs it is most likely out-of-date. We end up having to rely on out-of-date tech that is in need of constant repair and if we had our way, replacement. Add to that, the atmosphere keeps reflecting, scattering and absorbing most of our satellite signals making it hard to focus on the ‘smaller’ planet surface area.
Remote sensing applications use the interaction of the atmosphere and earth systems (in your case planetZ systems) with the electromagnetic (EM) spectrum. Our satellites measure the EM responses in the microwave, infra-red, visible light, and UV spectrum. E.g. Snow scatters visible light, water vapour absorbs Infra-red and hail scatters microwave radiation and we have that pesky ozone hole letting in all the cancerous UV.
Our satellites have passive sensors, which measures using the EM from solar radiation, and we have active sensors, which sends an EM signal of its own and measures the response.
Data received is also reliant on what orbit your satellite is in. We have many satellites in orbit around the equator that are great at capturing images for large areas at an almost constant and regular rate but because they are quite far from the planet the resolution scale is quite large and we can’t see much below 1km. If you have your satellite orbit over the poles, you will get a much finer resolution image, but the temporal coverage is not great. Each location will only get a measurement once a day or even every 3-7 days! We use a combination of several orbits, and sensors.
Just some of the many satellites upstairs.
Plants use incoming radiation to photosynthesis and also reflect the radiation back. We use satellites to measure the reflected light wavelength from plants in very narrow wavelength ‘windows’. While green plants do reflect slightly differently than other pigmented plants, all plants reflect differently to other features. These other features, the ground, rocks, metals, sand, freshly-tilled earth, artificial surfaces, buildings, animals are all reflecting light back into space, as well as the water bodies, atmosphere chemicals, aerosols and atmospheric moisture. So your dry green desert sand would have a different reflection wavelength than your ‘green’ plants.
What does this mean? It means you first have to get through the atmosphere and any cloud cover before you can view the surface features. Different atmosphere compositions absorbs, reflects, scatters incoming and outgoing radiation in different ways. This is one of the (if not the only) reasons why we can tell the composition of far-distant atmospheres other than Earth but why we needed a rover to explore Mar’s surface. We use the reflected light from these planets that we receive from various satellites and compare it to known reflection patterns on Earth and make educated guesswork.
Temporal and seasonal changes need to be taken into account as well. Moist soil reflects differently than dry soil. Plants in dry soil reflect differently than plants in wet soils. Plants ‘breathe’ in Carbon Dioxide and ‘transpire’ Oxygen at different times of the day and night affecting the chemical composition of the nearby atmosphere and how it scatters/absorbs/reflects any light. Any Instruments in orbit would receive it all. It would have to be collaborated to remove the noise (the unwanted reflection signatures). That is why we currently rely so heavily on ground-truthing.
We have had to use ground-truthing over the last 50 years to figure out what each signal means, what is the best narrow wavelength ‘window’ and what is the best temporal ‘window’ to measure each object. For your planet colonisation, along with a massive multi-satellite project, your colonisation planning should involve a large amount of pre-arrival planetary exploration.
I would suggest a large portion of your planning funding be allocated to landing an armada of rovers full to the brim with equipment all over your planet. The rover option has a large unplanned failure potential. I think some manned missions to PlanetZ would be in order too. If this was an emergency situation, as in my related question, you wouldn’t have the ground-truthing data available to filter out the noise. You can possibly get this after you crash-land/evacuate your ship but that will be a bit late which is why I was focussed more on the larger atmospheric and oceanic features which my survivors can use to work out just what is in store for them and try and prepare.
I could go on and on about remote sensing but let me try focus on your title question, with some focus on your inbody questions - based on orbit only (no ground truthing).
Let’s assume you have found another goldilocks type PlanetZ with water, plants and animals (land is optional). Lucky you!
You will hopefully have more than enough time/just enough time to figure out the orbit of your planet, angle of inclination, distance to the sun/s etc. From this you will be able to tell the length of the days for any particular time of the year. From that you should be able to deduce your basic season characteristics (for ease I’m just using earthlike but it could be anything…). Colonisation Scenario could provide accurate temperature maps of the entire planet for the entire year (this can be measured by the amount of IR, near IR (NIR) and thermal IR (heat) each location reflects along with other sensors). Emergency scenario would be trickier. Based on current temperature readings you are able to determine the temperature at both the poles and equator and make deductions about possible temperature ranges of said seasons and latitudes.
From this initial detailed/basic planetary system survey combined with your temperature maps you will have your preferred ‘goldilocks’ latitude to search for a location of your colony/emergency settlements.
Next you look for water. You can’t have biomass without water (well, any known biomass. Maybe your PlanetZ has developed an entire plant and animal system that draws water straight from the atmosphere. Weird!).
If your planet is a water world, great! It should be fairly simple to spot the water!
If has got a reasonable amount of land, and you don’t want to live on the ocean, you have to determine where your world’s water actually flows from and to. You have 3 main areas to look at. High Atmosphere, near surface atmosphere, and surface water. The surface water will flow from higher mountains areas, through your midlands and out the lowlands into your oceans (with the odd stopover in a lake or glacier). I don’t think you could work out groundwater flow just from orbit but you might be able to calculate it based on surface features (if there is an oasis in the middle of a desert, it’s a sure sign of groundwater).
High up in the Atmosphere: Water Vapour reflects visible light in the 6.7 μm IR wavelength. The more water vapour in the upper atmosphere, the less surface can be seen. But that is not too bad, as you can track the direction your water vapour is flowing and determine upper atmosphere circulation patterns (wind can also be measured with satellite based doppler. Colonisation Scenario – detailed upper atmosphere patterns e.g. Jet streams, warm gulf stream, Emergency Scenario – snapshot. Deductions made off guesswork e.g. local weather report.
Low near the surface:
Now estimates of specific humidity are made directly from the
satellite microwave sensors such as the Defense Meteorological
Satellite Program (DMSP) Special Sensor Microwave Imager (SSM/I)
observations and the NOAA Advanced Microwave Sounding Unit B (AMSU-B).
Colonisation Scenario – You get near surface humidity reading and possibly can track low lying clouds and windflow. Coupled with your high atmosphere patterns you can deduce your complete atmospheric circulation system. Emergency Scenario – snapshot. Deductions made off guesswork.
Multi-angle Imaging Spectro-Radiometer (MISR), which was launched on the NASA >Terra satellite in 1999, views clouds from nine different angles and locates >them in three dimensions. It scans at blue, green, red, and near-IR >wavelengths (0.446, 0.558, 0.672, 0.866 µm, respectively), has spatial >resolution of 275 m across a 360-km swath, and achieves global coverage about >once every 9 days at the equator.
If you can track cloud movements over a long time, an entire year or several years you can deduce cloud forms and wind circulation. Are those thunderheads you are tracking or just dreary old clouds that never let down the rain? Is that the Hadley, Ferrel, and Polar cells forming that air convection and torrential downpour? Colonisation Scenario – you can track long term cloud patterns and calculate where the majority of rain falls and any seasonal patterns like monsoons as well as climate regions which would influence your biomass. Emergency Scenario – I’m still waiting for more answers.
Surface Water: You have two features of interest here, Sea-level, which is the ‘same’ throughout the world (It is not even close but a range of ~20m is fairly small considering you measuring an entire planet) and surface water on land. Surfacewind patterns over waterbodies can be worked out by measuring surface wave height with Doppler, Lidar or Radar. Surface water is largely dependent on your topography. You can measure both sea-level and topography with various satellite based sensors using lidar, radar, etc. Once you have your topographic measurements you can create a detailed/basic Digital Elevation Model (DEM).
On your DEM you can work out what altitudes are out of bounds. Try not set up your colony in the high reaches of mountain ranges (these tend to be slightly cold and have less breathable oxygen content) or in the sunken depressions below sea level (these tend to be arid, salty regions with possibility of severe flooding). Unless of course this region has some feature of extreme interest and is why you came to this planet in the first place (such as mining) or you had no choice in the matter, and are just glad to be alive.
Surface water has its own spectral reflection signature. However be aware that sediment and organics will alter the range of the signals. E.g. you can see areas of upwelling in the ocean not just from the cold sea surface temperatures but also from the chlorophyll NIR reflection signature. You will be looking for an area to settle preferably close to a water source but not at risk of flooding. If there is flooding risk, a semi-predictable event calendar would be nice; like the annual Nile flood in Egypt before they built Aswan Dam. You can figure out where surface water flows by looking at your DEM and then focussing on any likely water signatures. If it is particularly detailed you would be able to see erosion features, like V shaped valleys (U shaped are Glaciers), gully’s, etc etc. If it is a basic DEM, you can use rough guestimates on valley shapes, and water flowing from highland to lowlands.
Here is where your biomass comes into play.
What techniques could be used to identify large amounts of biomass from orbit?
Read this journal paper. It’s old, but it explains measuring plants reflection signatures in a lot more detail.
Currently we measure this in the visible and Near Infra-red light spectrums. We measure very narrow wavelength ‘windows’ and analyse the spectral signature of the biomass in question – normally coupled with groundtruthing data and inaccurate algorithms. These narrow ‘windows’ remove noise from neighbouring features and the atmosphere.
A Colonisation project would have a lot of planning time so you would have ‘enough’ time to figure out the atmospheric content and reflective, absorption and scattering properties. You could then try filter this out and focus on the ground. An emergency situation would have no forward panning time, so the best choice is to look at your spaceship viewscreen. Where you see green, dive dive dive! You can position your equipment (if you have time) to focus on a particular area that appears feasible from space. Focus in on the ‘green plantlife’ wavelength window rather than the ‘green colour’ wavelength window to pick a particularly vibrant spot.
Obviously without going down to the planet surface you can’t tell how useful said biomass is going to be, but generally biomass grows near water and in fertile soils. Fertile soils are transported by river (see watersource!) from the eroded mountains down to the lowlands (see topography!). An earth-like world would have a lot more rainfall in the equatorial region and in the higher latitudes as snowfall, than the mid – latitudes (see AtmosphericCirculation!). When the snow and ice melt the water normally flows down into the mid latitudes and lower altitudes (see DEM!), as seasonal spring melt allowing seasonal savannahs and swamps to form such as the Okavango delta.
What I believe you are looking for is a river delta. This is where the river deposits the majority of its fine sediment load which is excellent for farming. Plenty of ‘fresh water’ and it is also abundant in animal life as a result, so you can hunt for meat …and be hunted as meat.
If you are not wanting the flood risk of a delta, you are looking for a midland area still in the tropics. Slightly elevated altitude but not high mountains. This would give you either rolling hills (still good for farming) or possibly Mesas and buttes which would provide excellent positions for hillforts. These don’t necessarily have to be in dry, arid regions such as the Grand Canyon but can be covered in plantlife such as the, um. It can exist! I just drawing a blank right now. Again all this can be found using your DEM, water locations, atmospheric circulation models. You just have to decide which features are of interest to you.
Is there any way to identify metals or metal ores near the surface?
Simple answer yes. There would be some ground-truthing needed for alien compositions but any known earth-like absorption signatures should be fine.
The Morenci satellite image above is an open-pit copper mine in southeast Arizona is North America's leading producer of copper. This processed and interpreted ASTER image used short wavelength infrared bands to highlight in bright pink the altered rocks in the Morenci pit associated with copper mineralization.
Satellite images can also benefit geologists, scientists, and exploration managers due to the multiple bands that the satellites carry which allow them to interpret wavelengths that cannot be seen by the human eye. Near infrared, short wave infrared, and thermal infrared can be used to identify the difference in structural features of the earth's surface.
Multispectral imaging and thematic mapping allows researchers to collect reflection data and absorption properties of soils, rock, and vegetation. This data could be utilized by trained photogeologists to interpret surface lithologies, identify clays, oxides, and soil types from satellite imagery.
Colonisation Scenario – plan to settle in location with all desired features, water, biomass, and minerals. Or bring along technology to provide all the water conversion and biomass replicators you could need for your inhospitable mining site. Emergency Scenario – you’d be lucky if you got all one, let alone all three, water, food, and metal, in one go. But you’ll be able to gather plantstuffs (hunter -gatherer style), prospect for metals, follow hints to water, on the ground. Depending on your emergency situation you could have some basic to detailed geological maps along with your DEM. You could have launched small satellites in orbit providing you with basic real time data which could point you in the right direction. Etc etc. It all depends on your story requirements.
HOPE I COVERED EVERYTHING!
(1) All quotes (except mineral related) and First Image Some Satellites- http://www.goes-r.gov/users/comet/tropical/textbook_2nd_edition/print_2.htm#page_3.0.0
(2) Second Image Hadley Cell - https://askabiologist.asu.edu/explore/desert
(3) Mineral related Quote and Third Image - http://www.satimagingcorp.com/applications/energy/mining/