# Gather Types

There are three types of gathers:

1. Type 1 (Regular offset) — Gathers that have regular offsets for every CMP (Type 1 = same, same).
2. Type 2 (Irregular offset) — Gathers that have irregular offsets, but the same irregularity for every CMP (Type 2 = different, same).
3. Type 3 (Irregular offset, and location-dependent) — Gathers that have irregular offsets, and the offset irregularity is different for every CMP (Type 3 = different, different).

## Examples

### Type 1 example:

CMP1: Trace 1 offset 75m, Trace 2 offset 150, Trace 3 offset 225m, etc (regular 75m increment)

### Type 2 example:

CMP1: Trace 1 offset 75m, Trace 2 offset 155m, Trace 3 offset 240m, etc (no regular increment)

CMP2: Trace 1 offset 75m, Trace 2 offset 155m, Trace 3 offset 240m, etc (but the irregularity is the same for every CMP)

### Type 3 example

CMP1: Trace 1 offset 75m, Trace 2 offset 155m, Trace 3 offset 240m, etc (no regular increment)

CMP2: Trace 1 offset 80m, Trace 2 offset 165m, Trace 3 offset 245m, etc (and irregularity between CMPs).

## General Discussion

Most gathers are typically Type 1.

Depending on the acquisition geometry and processing, some gather datasets may be Type 2 and, even less commonly, Type 3. The most common use for Type 2 and Type 3 is when the gather dimension is not offset (i.e. frequency).

Type 2 gathers have the following properties:

• low fold data
• acquired in feet, offsets converted to metres (rounding errors give increments of 24, 25, 24, 25, 25, 24)
• spectral decomposition frequency (i.e. many lower frequencies, but more sparse in higher frequencies).

Type 3 gathers are mostly used for shot gathers, channel gathers and other similar raw data. It treats each gather like a bucket of traces and has to sort through them when it gets there.