Method	Strength	Weakness
Ground Penetrating Radar (GPR)	High resolution. Generates most "image-like" quality of any geophysical method; Rapid and inexpensive to deploy in flat, uncluttered topography. Careful acquisition can lead to good constraints on radar (electromagnetic) velocities, followed by spatial migration of "arrivals" to proper position in subsurface. The radar wave velocity is a good indicator of the dielectric constant which is controlled largely by the presence of water moisture.	Difficult to assign unique physical properties to specific "layered" features. Need corroborating complementary data. Not effective in rough, cluttered terrain. Costly lab time to extract section velocities. Often benefits from DC resistivity and/or seismic data to identify specific material.
Seismic Refraction Conventional reversed profiles: Delay time or time term method:	Classical engineering method. Reveals general layering in the subsurface. Relatively short processing time in lab compared to other seismic methods. Excellent complement to DC resistivity and GPR surveys for depth to the watertable or depth to bedrock. In principle, since each geophone or shot position can lead to an estimate of the local depth to the refractor beneath the respective "station", this procedure can result in a highly detailed configuration of the refracting surface, e.g. bedrock.	If structure is not dipping planar, then more refined methods should be (have been) used, such as the delay time method (below). Somewhat more time intensive in field than above conventional refraction method, and considerably more time intensive in lab. General to both refraction methods: While a layer may be detected at depth, its seismic velocity may not be an unambiguous discriminator between bedrock and the groundwater table. Low velocity layers, sandwiched between higher velocities, are masked. and total depths are biased. Complementary data may be required.
Seismic Reflection	Often good resolution of layering from depths of 20 m to more than several hundred meters. For decades, the favored tool of the oil industry for deep exploration, but has fewer proponents or service providers for shallow (<100 m) investigations.	Labor intensive both in the field and by skilled interpreters in the lab. Cost is usually justified when information from depths of 100 m or so is needed. Extracting precise interval velocities from multilayered media is sometimes difficult.
Gravity	The method "weighs" the earth directly beneath a field site. Because of the density difference between unconsolidated sediments and surrounding bedrock in a sediment-filled valley, for example, the technique is very effective for gross characterization of depth to bedrock beneath 10's to 100's of meters of sediments. Effective in culturally developed areas where buried water mains and utility lines, overhead wires, pavement, etc., may "mask" other geophysical signatures.	Relatively time intensive for a trained interpreter in the field and lab. Needs to be "calibrated" at one or more control points using complementary well data or other geophysics (e.g. seismic refraction/reflection).
DC Resistivity (aka four electrode "sounding" or "profiling" methods: Wenner, Schlumberger, Dipole-dipole.)	Direct indicator of electrical conductivity, which, in turn, is an indirect indicator of soil moisture or percent saturation. Good for determining depth to the watertable. Good indicator of bedrock at depth. Very effective complement to seismic refraction or GPR data.	Vertical resolution in the best of cases is somewhat coarse. Method needs special care when lateral features are encountered, adding greatly to acquisition cost and interpretation. Requires a lateral clearance for a horizontal array that may be 10 times the depth of resolution.
Electromagnetic - "Active" Sources: Frequency domain (FDEM) Time domain (TDEM)	Very effective for the rapid reconnaissance of an area for mapping depth to bedrock, depth to watertable, detecting clay lenses. Also effective for mapping infrastructure hazardous to drilling or excavation. Several light-weight FDEM and TDEM units are available for use by single operators for rapid reconnaissance. Active source (horizontal loop) methods can be specially deployed to map features at several 100 m's depth. TDEM appears to have good potential for vertically probing in areas of restricted horizontal access. Whereas TDEM has been used by the mineral industry for deep exploration for many years, it has few service providers for shallow (<100 m) investigations.	Topography can be a problem in interpreting FDEM data. TDEM is not widely used for shallow studies (less than 20 m) in resistive terranes, except for shallow (1 or 2 m) metallic infrastructure investigations. In some cases, source effects can be significant.
Electromagnetic - "Distant" or "Passive" Sources: VLF (Very Low Frequency) Method(s) Controlled Source Audio Frequency Magnetotellurics Magnetotellurics Tellurics Magnetic variations	Particular methods are effective for mapping depth to bedrock, depth to watertable, detecting clay lenses, and for delineating fracture patterns in bedrock, sometimes to may 10's of meters.. Generally, these methods are very cost effective for large scale reconnaissance studies, or in areas of rugged terrain. Also effective for mapping infrastructure hazardous to drilling or excavation. Various methods can be used in "profiling" or in "sounding" modes, and in some cases lead to direct estimates of the subsurface resistivity. Distant source methods - whether the signal source is controlled, uncontrolled, or natural - have the advantage that the roving field instrumentation is light-weight and portable, and that the interpretation of field data is, in ideal cases, relatively free of source effects.	Conventional VLF sometimes has difficulty receiving multiple stations. In some cases, interpretations of local structure are distorted by 2D and 3D effects beyond the immediate survey area. While VLF instruments are relatively inexpensive, the technique is not highly developed. Other instrumentation is relatively expensive, and requires some expertise in interpretation.
Magnetic	Often useful to delineate geologic features related to hydrogeology, e.g. bedrock lineations, intrusives, geologic contacts, etc. Very effective for identifying infrastructure hazardous to drilling or excavation. One of the most cost-effective techniques for screening an area for steel USTs, buried drums, water mains, etc.	Local traffic and other magnetic disturbances often require gradient (two sensor) techniques. Lower cost proton precession units tend to be more unstable than more expensive alkali-vapor units in the presence of high magnetic field gradients typical of many industrial sites. Depth resolution is poor except for small, shallow targets.